Community-Acquired Pneumonia

Clinical Snapshot

Community-acquired pneumonia (CAP) is an acute infection of the lung parenchyma acquired outside a healthcare facility, producing cough (usually productive), dyspnea, pleuritic chest pain, and fever, with a new infiltrate on chest imaging confirming the diagnosis clinically suspected on history and exam. It is caused by a wide range of typical bacteria, atypical bacteria, and respiratory viruses, and in roughly 60% of cases no causative organism is ever identified.^[1] CAP spans a genuine severity spectrum — most patients are managed safely as outpatients, but hospitalized and especially ICU-level disease carries substantial mortality — which is why a validated severity score, not gestalt alone, should drive the site-of-care decision that determines everything downstream: which empiric regimen to start, how aggressively to investigate, and how long to treat.

Recognize the Condition

Typical presentation

Cough (usually productive of purulent sputum), dyspnea, pleuritic chest pain, fever, chills, myalgia, headache, and arthralgia. New-onset confusion may be the dominant presenting feature in older adults and can occur without prominent respiratory symptoms. Examination may show tachypnea, dullness to percussion, increased tactile/vocal fremitus, bronchial breath sounds, whispered pectoriloquy, crackles, or a pleural friction rub over the affected lobe.^[1]

Typical vs. atypical clinical patterns

Typical (classic bacterial) pattern

Abrupt onset, high fever, rigors, productive cough with purulent sputum, pleuritic chest pain, and focal findings of consolidation on exam and imaging (lobar pattern). Most consistent with Streptococcus pneumoniae, Haemophilus influenzae, or other typical bacteria.

Atypical pattern

More gradual onset, lower-grade fever, prominent extrapulmonary symptoms (headache, myalgia, sore throat, GI upset), a dry or minimally productive cough, and a chest exam that can be surprisingly unremarkable relative to the degree of infiltrate seen on imaging. Suggests Mycoplasma pneumoniae, Chlamydophila pneumoniae, or Legionella species. This clinical pattern alone is not reliable enough to select therapy — empiric regimens are chosen by severity/site of care, not by guessing typical vs. atypical from the history.

Common causative organisms

Infection may be mixed or polymicrobial (e.g., viral plus bacterial co-infection).^[1]

Typical bacteria	Atypical bacteria	Respiratory viruses	Other
Streptococcus pneumoniae Haemophilus influenzae Moraxella catarrhalis Staphylococcus aureus (MSSA/MRSA) Gram-negative bacilli (E. coli, Klebsiella spp., Enterobacter spp., Serratia spp., Pseudomonas aeruginosa)	Mycoplasma pneumoniae Chlamydophila pneumoniae Legionella species Coxiella burnetii (Q fever)	Influenza A and B SARS-CoV-2 and other coronaviruses Respiratory syncytial virus Human metapneumovirus Parainfluenza Adenovirus Human rhinovirus	Mycobacterium tuberculosis (uncommon) Fungi, e.g., Blastomyces, Histoplasma, Cryptococcus (uncommon, geographic/exposure-dependent)

When to consider tuberculosis

Suspect M. tuberculosis specifically in nonresolving pneumonia, patients born outside Canada, patients with HIV or other immunodeficiency, long-term care residents, First Nations communities, patients with poor housing/crowded living conditions or limited access to nutrition, and any patient who fails to respond to standard CAP therapy.^[1]

Risk Factors

Age under 2 years or over 65 years.
Tobacco smoking, including exposure to second-hand smoke.
Excessive alcohol use or alcohol use disorder.
Cardiopulmonary comorbidity — COPD, asthma, heart failure, coronary artery disease.
Immunodeficiency, including HIV, active malignancy, and immunosuppressive medications.
Recent loss of consciousness or altered level of consciousness (aspiration risk).
Comorbid illness that may also prolong recovery once infection occurs.
Occupational/exposure risk factors: removal of wild rodent excrement (hantavirus), exploring caves (histoplasmosis), exposure to cats or animal farms (Q fever), recent travel to a region with pathogens causing severe acute respiratory infections.
Risk factors specifically for drug-resistant S. pneumoniae and other resistant pathogens: antibiotic use within the prior 3 months, exposure to a child in daycare, a recent episode of CAP, or prior treatment failure/early recurrence — these factors change the empiric regimen, not just the risk of disease.

Red Flags & Refer

Escalate immediately to emergency assessment or hospital admission for any of the following:

Respiratory failure or a clinical trajectory suggesting imminent need for mechanical ventilation.
Septic shock or hemodynamic instability requiring vasopressor support.
Multilobar infiltrates on chest imaging.
Altered mental status or new confusion in the setting of suspected CAP.
Oxygen saturation persistently below 90%, or a PaO2/FiO2 ratio ≤250.
Respiratory rate ≥30 breaths/minute — the single most sensitive and specific bedside sign of severe pneumonia in adults (≥25 breaths/minute in patients under 50).^[1]
Suspected empyema, lung abscess, or a parapneumonic effusion large enough to require drainage.
Suspected endocarditis in a patient with bacteremic S. aureus pneumonia — particularly if multiple rounded opacities (septic emboli) are seen on chest imaging.
Failure to improve, or clinical deterioration, 48–72 hours into appropriate empiric therapy.
Suspicion of active pulmonary tuberculosis — needs airborne isolation and public health notification, not routine CAP management.

Assessment & Diagnosis

Differentiating CAP from its common mimics

Feature	Community-acquired pneumonia	Acute bronchitis	COPD exacerbation	Acute decompensated heart failure
Fever	Common	Uncommon/low-grade	Uncommon	Absent
Chest imaging	New infiltrate/consolidation — required for diagnosis	Clear, or no imaging needed	Usually clear (hyperinflation only)	Bilateral perihilar/interstitial edema, effusions, cardiomegaly
Focal exam findings	Often present — dullness, crackles, bronchial breathing over one area	Absent or diffuse wheeze only	Diffuse wheeze/prolonged expiration, not focal	Bibasilar crackles, often with elevated JVP and edema
Sputum	Often purulent	May be purulent even without bacterial infection	Increased volume/purulence during exacerbation	Pink, frothy (pulmonary edema) rather than purulent
Key distinguishing clue	Focal exam findings + fever + a new infiltrate together	Normal chest exam and imaging despite cough	Known COPD, wheeze-predominant, responds to bronchodilators/steroids	Orthopnea, weight gain, elevated natriuretic peptide, responds to diuresis

History

Characterize cough, sputum, dyspnea, pleuritic pain, hemoptysis, fever, chills, myalgia, headache, arthralgia, and new-onset confusion (may be the dominant finding in older adults).
Ask specifically about recent travel, tobacco/second-hand smoke exposure, alcohol use, rodent excrement exposure, cave exploration, animal/farm exposure, recent loss of consciousness, and comorbid illness — several of these both raise risk and can prolong recovery.^[1]

Physical examination

General appearance and degree of respiratory distress; auscultation for crackles, wheezes, and signs of consolidation (dullness to percussion, increased tactile/vocal fremitus, bronchial breathing, whispered pectoriloquy); pleural friction rub; mental status.

Objective measurements

Respiratory rate ≥30 breaths/minute is the most sensitive and specific sign of severe pneumonia in adults (≥25 in those under 50).
Check ongoing fever (oral temperature >37.5°C), slow resolution, or progression of symptoms with a repeat chest radiograph to look for empyema.
Measure oxygen saturation in every patient presenting to the emergency department; obtain an arterial blood gas if saturation is <92% in a patient with COPD.
Chest radiograph (PA and lateral); consider CT chest if the radiograph is negative but clinical suspicion for pneumonia remains high.
Consider a follow-up chest radiograph 6–12 weeks after presentation in patients over 50, particularly if they smoke, specifically to screen for an underlying lung malignancy masked by the pneumonic opacity; pursue bronchoscopy if the opacity has not resolved. Repeat imaging is not needed if there is clinical resolution and the patient is under 50.^[1]

Laboratory testing — routine for hospitalized patients

Electrolytes, glucose, urea, creatinine, CBC with differential.
Sputum and blood cultures — limit to patients with severe CAP, or those with risk factors for MRSA or Pseudomonas; not routinely needed for milder inpatient disease.
An MRSA nasal PCR swab can be used to assess colonization status; given its high negative predictive value, a negative swab supports stopping empiric vancomycin/linezolid started for MRSA coverage.^[2]
Pneumococcal urinary antigen testing — limit to severe CAP.
Legionella urinary antigen (detects serogroup 1 only) or PCR on respiratory secretions if there is high clinical suspicion or the patient requires ICU admission for progressive pneumonia.
Consider HIV testing where risk is increased.
Consider RT-PCR for influenza and other circulating respiratory viral pathogens, and NAAT testing for atypical pathogens (e.g., suspected M. pneumoniae) as dictated by the clinical setting.
Special cultures (tuberculosis, Legionella, fungi such as Blastomyces or Cryptococcus) are directed by the clinical picture — involve microbiology/infectious diseases rather than ordering reflexively.^[1]

Severity assessment tools — CURB-65 and the Pneumonia Severity Index (PSI/PORT)

A validated severity score should supplement, not replace, clinical judgment for the site-of-care decision — clinician judgment always overrides a scoring system when the two disagree.^[1] The 2019 ATS/IDSA guideline preferentially recommends the PSI over CURB-65 based on higher discriminative power and a lower false-positive rate, though CURB-65 remains widely used at the bedside for its simplicity.

CURB-65 — 1 point for each of the following present

Criterion	Points
Confusion (new onset)	1
Urea >7 mmol/L	1
Respiratory rate ≥30 breaths/minute	1
Blood pressure — systolic <90 mmHg or diastolic ≤60 mmHg	1
Age ≥ 65 years	1

Interpretation: score 0–1 = low risk (mortality under roughly 3%) — outpatient treatment is generally appropriate; score 2 = intermediate risk — a short, closely supervised inpatient stay or hospital-supervised outpatient management is generally appropriate; score 3–5 = high risk (mortality rises steeply toward 57% at a score of 5) — hospitalize and evaluate for ICU-level care.^[3] The CRB-65 variant (omitting urea) is used where a lab value is not readily available, e.g., primary care.

Pneumonia Severity Index (PSI/PORT score)

A larger 20-item point system incorporating age directly as points (age in years for men, age minus 10 for women), nursing-home residence, comorbid illness, physical exam findings, and laboratory/radiographic findings, stratifying patients into 5 risk classes.^[4]

Category	Patient characteristic	Points assigned
Demographic factors	Male	Age in years
	Female	Age in years minus 10
	Nursing home resident	+10
Comorbid illness	Neoplastic disease	+30
	Liver disease	+20
	Heart failure	+10
	Cerebrovascular disease	+10
	Renal disease	+10
Physical exam findings	Altered mental status	+20
	Respiratory rate ≥30/min	+20
	Systolic BP <90 mmHg	+20
	Temperature <35°C or ≥40°C	+15
	Pulse ≥125/min	+10
Laboratory/imaging findings	Arterial pH <7.35	+30
	Urea >11 mmol/L	+20
	Sodium <130 mmol/L	+20
	Glucose ≥14 mmol/L	+10
	Hematocrit <30%	+10
	PaO2 <60 mmHg, or pleural effusion	+10 each

Risk classes: Class I (assigned directly, without points, to patients under 50 with none of the listed comorbidities/exam findings) and Class II (≤70 points) are low risk, treated as outpatients; Class III (71–90 points) can be managed as an outpatient or with brief observation; Class IV (91–130 points) and Class V (>130 points) are progressively higher risk and warrant inpatient treatment. In practice, a total score ≤90 supports outpatient treatment and ≥91 supports inpatient treatment.

ATS/IDSA criteria for severe CAP (ICU-level care)

Distinct from the site-of-care scores above — this determines the need for ICU rather than ward admission. Severe CAP is defined by either 1 major criterion, or ≥3 of the 9 minor criteria.^[1]

Major criteria (either alone indicates severe CAP)	Minor criteria (≥3 of 9 indicates severe CAP)
Septic shock requiring vasopressor support	Respiratory rate ≥30 breaths/minute
Respiratory failure requiring mechanical ventilation	PaO2/FiO2 ratio ≤250
	Multilobar infiltrates
	Confusion/disorientation
	Uremia (urea ≥7 mmol/L)
	Leukopenia (white blood cell count <4 ×10⁹/L)
	Thrombocytopenia (platelets <100 ×10⁹/L)
	Hypothermia (core temperature <36°C)
	Hypotension requiring aggressive fluid resuscitation

Treatment Targets

Correctly assess severity to select the appropriate initial treatment setting — outpatient, inpatient ward, or ICU.
Relieve symptoms — cough, pleuritic chest pain, sputum production, dyspnea.
Prevent morbidity: respiratory failure, pleural effusion, empyema, pulmonary abscess, pneumothorax, sepsis/septic shock, syndrome of inappropriate antidiuretic hormone secretion, and immobility-related complications.
Start an appropriate empiric antibiotic regimen promptly, matched to site of care and any Pseudomonas/MRSA risk factors.
Prevent transmission to others.
Prevent recurrence, including addressing modifiable risk factors (smoking, aspiration risk, vaccination status).

Choose Initial Treatment

Direct treatment rule

Site-of-care assessment

Use CURB-65 or the PSI, together with clinical judgment, to select outpatient vs. inpatient-ward vs. ICU-level care — a low score with a clinically unwell-appearing patient (or vice versa) should be resolved in favour of clinical judgment, never the score alone.^[1]

Outpatient empiric therapy

Healthy outpatient, no comorbidities amoxicillin 1 g PO TID, or doxycycline 100 mg PO BID as an equally preferred alternative. A macrolide (azithromycin or clarithromycin) is only appropriate as outpatient monotherapy when local pneumococcal macrolide resistance is confirmed to be under 25% — it is no longer a default first-line choice given rising resistance.^[1]

Outpatient with comorbidities (chronic heart/lung/liver/renal disease, diabetes, alcoholism, malignancy, asplenia, immunosuppression) or risk factors for drug-resistant pathogens use a beta-lactam (amoxicillin/clavulanate or a second-generation cephalosporin) combined with a macrolide or doxycycline, or a respiratory fluoroquinolone as monotherapy. Amoxicillin or ampicillin alone is not adequate in this group since it does not reliably cover beta-lactamase-producing H. influenzae, M. catarrhalis, and MSSA.

Inpatient empiric therapy

Inpatient ward, non-severe, no MRSA/Pseudomonas risk factors a beta-lactam (ceftriaxone or cefotaxime) plus a macrolide (azithromycin preferred), or a respiratory fluoroquinolone (levofloxacin or moxifloxacin) as monotherapy.^[1]

ICU/severe CAP, no MRSA/Pseudomonas risk factors a beta-lactam (ceftriaxone or cefotaxime) plus either azithromycin or a respiratory fluoroquinolone — combination therapy, not monotherapy, is standard at this severity level.

Pseudomonas risk factors present (prior respiratory isolation of P. aeruginosa, or recent hospitalization with parenteral antibiotics within the past 90 days) — switch the beta-lactam backbone to an antipseudomonal agent (piperacillin/tazobactam, cefepime, or meropenem) combined with azithromycin or an antipseudomonal fluoroquinolone (ciprofloxacin or levofloxacin).^[1]

MRSA risk factors present (prior respiratory isolation of MRSA, or recent hospitalization with IV antibiotics, or locally validated risk factors) — add vancomycin or linezolid to the backbone regimen and obtain an MRSA nasal PCR swab; de-escalate and stop MRSA coverage if the swab is negative, given its high negative predictive value.^[2]

Duration of therapy

For outpatients and inpatients with non-severe CAP who reach clinical stability, a minimum of 3 days of therapy is now supported, with most courses completed by day 5 — this supersedes the older flat "minimum 5 days" recommendation. For severe/ICU CAP that reaches clinical stability, continue for a minimum of 5 days.^[5] Specific pathogens or complications may require longer courses regardless of severity: severe legionnaires disease (21 days), bacteremic gram-negative bacilli or P. aeruginosa (7–10 days), empyema with drainage (14 days or longer), and lung abscess (until the cavity closes on imaging, which may take 12–16 weeks).^[1]

Respiratory virus testing and the antibiotic decision

A positive respiratory virus test does not reliably exclude bacterial co-infection in a hospitalized patient or an outpatient with comorbidities — continue guideline-directed antibiotics in these groups rather than withholding them on the strength of the viral result alone, while weighing symptom duration, severity, and radiographic findings if genuinely reconsidering the need for antibiotics on a case-by-case basis.^[5]

Non-Pharmacologic Treatment

Encourage adequate oral hydration and rest, with early, gradual mobilization once tolerated — trial evidence supports early mobilization in reducing length of hospital stay.
Supplemental oxygen for documented hypoxemia; titrate to a saturation target appropriate to the patient (generally ≥92% on room air, adjusted for baseline in COPD).
Smoking cessation counselling at every encounter — tobacco smoking roughly doubles the risk of invasive pneumococcal pneumonia, and second-hand smoke exposure is itself a risk factor.^[1]
Aspiration-prevention measures for at-risk patients: a "chin-down" swallowing posture, oral/dental hygiene maintained by caregivers after meals, and elevation of the head of the bed.
Review pneumococcal and influenza vaccination status at every visit and immunize if indicated (see Drug Tables).
For hospitalized patients on IV therapy, switch to oral antibiotics once all of the following are met: the GI tract is functioning normally (no vomiting, diarrhea, or malabsorption), the patient is hemodynamically stable, 2 temperature readings are normal (oral temperature <37.5°C) over 16 hours, the white blood cell count has normalized, there is subjective improvement in cough/dyspnea, and the patient can take oral medications.^[1]
Use of a structured clinical pathway emphasizing early IV-to-oral switch and early mobilization reduces length of hospital stay.

Pharmacologic Treatment Options

Drug / intervention	Role	Dose / use	Important adverse effects	Teaching points
Amoxicillin Preferred	Outpatient, no comorbidities	1 g PO TID, for the duration determined by site-of-care/stability (see Choose Initial Treatment)	GI upset, rash, interstitial nephritis (rare)	First-line for previously healthy outpatients. This dose already reflects the high-dose strategy needed to cover possible resistant S. pneumoniae.
Doxycycline Preferred	Outpatient, no comorbidities (equally preferred alternative to amoxicillin)	100 mg PO BID on day 1, then 100 mg PO once daily	Photosensitivity, GI upset, esophageal irritation	Avoid in pregnancy. A reasonable alternative when penicillin allergy or intolerance is a concern.
Azithromycin Special situation	Outpatient monotherapy — only if local pneumococcal macrolide resistance is confirmed under 25%	500 mg PO on day 1, then 250 mg PO daily x4 days; or 500 mg PO daily x3 days	QTc prolongation, GI upset	No longer a default first-line agent given rising resistance — confirm local resistance data before using as monotherapy.
Amoxicillin/clavulanate + macrolide or doxycycline Preferred	Outpatient with comorbidities or drug-resistance risk factors	Combo: amoxicillin/clavulanate 875/125 mg PO BID + azithromycin 500 mg day 1 then 250 mg daily x4 days (or doxycycline 100 mg PO BID)	GI upset, diarrhea, QTc prolongation (with macrolide)	Preferred combination once comorbidities or resistance risk factors are present — amoxicillin/ampicillin alone is not adequate here.
Respiratory fluoroquinolone (levofloxacin or moxifloxacin) Alternative	Outpatient-with-comorbidities or inpatient monotherapy alternative	Levofloxacin 750 mg PO/IV daily x5 days; or moxifloxacin 400 mg PO/IV daily	QTc prolongation, tendinopathy, C. difficile-associated diarrhea, peripheral neuropathy	Reserve for beta-lactam allergy/intolerance rather than routine first-line use, given resistance and toxicity concerns.
Ceftriaxone + azithromycin Preferred	Inpatient ward and ICU/severe CAP backbone regimen	Combo: ceftriaxone 1-2 g IV once daily + azithromycin 500 mg IV/PO daily	GI upset, hypersensitivity, infusion-site reactions, QTc prolongation (azithromycin)	Standard inpatient combination across severity tiers; a respiratory fluoroquinolone may replace azithromycin.
Piperacillin/tazobactam + azithromycin or antipseudomonal fluoroquinolone Special situation	Pseudomonas risk factors present (prior respiratory isolation, or recent hospitalization + parenteral antibiotics within 90 days)	Combo: piperacillin/tazobactam 3 g/0.375 g IV Q6H + azithromycin 500 mg IV/PO daily (or ciprofloxacin 400 mg IV Q12H)	Nephrotoxicity, GI upset, hypersensitivity	Reserve for documented Pseudomonas risk factors — not routine severe CAP.
Vancomycin or linezolid Special situation	Added to the backbone regimen when MRSA risk factors are present	Vancomycin 15-20 mg/kg/dose IV Q8-12H (typically 1000 mg Q12H), trough/AUC-guided; or linezolid 600 mg PO/IV Q12H	Nephrotoxicity/ototoxicity (vancomycin); thrombocytopenia, peripheral/optic neuropathy, serotonin syndrome risk (linezolid)	Obtain an MRSA nasal PCR swab; de-escalate if negative. Linezolid requires infectious diseases consultation (Health Canada reserve antimicrobial).
Systemic corticosteroid (dexamethasone, prednisone, or methylprednisolone) Special situation	Severe CAP only — do not use in non-severe inpatient CAP, and do not use in influenza pneumonia	Dexamethasone 5 mg IV daily x4 days; or prednisone 50 mg PO daily x7 days; or methylprednisolone 0.5 mg/kg IV BID x5 days	Hyperglycemia, GI bleeding risk, mood/sleep disturbance, adrenal suppression with abrupt discontinuation	A strong recommendation exists AGAINST routine corticosteroids in non-severe inpatient CAP; only a conditional recommendation FOR them in severe CAP.
Daptomycin Avoid	Never appropriate for pneumonia of any cause, including MRSA pneumonia	Not applicable — do not use for pneumonia	n/a	Inactivated by pulmonary surfactant and clinically ineffective in the lung regardless of in-vitro susceptibility.

Special Populations

Pregnancy

CAP accounts for a meaningful share of antepartum hospitalizations for non-obstetric complications and should be diagnosed and treated promptly — untreated pneumonia in pregnancy risks preterm birth and low birth weight. Chest radiography is still indicated when CAP is clinically suspected. In a healthy pregnant patient with no recent antibiotic exposure, high-dose amoxicillin is recommended. For severe CAP in pregnancy, use a beta-lactam plus a macrolide: amoxicillin is the preferred beta-lactam (ceftriaxone an alternative), and azithromycin is the preferred macrolide (erythromycin is classically preferred but is not currently available in Canada). Involve infectious diseases if a resistant pathogen is suspected. Expect clinical improvement within 48–72 hours; do not change the regimen in the first 72 hours unless there is marked deterioration. Fever typically resolves in 2–4 days and cough in 7–10 days; radiologic abnormalities can persist up to 6 weeks, so continued hospitalization solely to await radiologic clearing is not required. Treat for a minimum of 5 days for uncomplicated CAP in pregnancy.^[1]

Breastfeeding

The antimicrobials recommended above for pregnancy, including agents used for MRSA coverage, are considered compatible with breastfeeding.^[1]

Pediatrics

Pediatric CAP is managed under separate pediatric-specific protocols with different pathogen distributions, dosing, and severity criteria — it is outside the scope of this adult-focused chapter and should not be extrapolated from the adult dosing above.

Older adults

New-onset confusion may be the dominant presenting feature and can occur with minimal respiratory symptoms. The respiratory-rate threshold for severity (≥30/min) remains sensitive and specific in this group; nursing-home residence itself adds significantly to PSI risk scoring. Consider tuberculosis in long-term care residents with nonresolving pneumonia.

Renal and hepatic impairment

Several key CAP antibiotics require renal dose adjustment — including piperacillin/tazobactam, vancomycin, levofloxacin, and cefepime — confirm creatinine clearance before finalizing the regimen in any patient with renal impairment. Cefotaxime is a reasonable choice in hepatobiliary disease given non-renal elimination characteristics relative to some alternatives.

Immunocompromised patients

Broaden the differential (fungal, tuberculous, Pneumocystis, or unusual viral causes) and lower the threshold for specialist involvement and additional diagnostic testing; consider HIV testing where risk is increased and not already known.

Monitoring & Follow-Up

Reassess clinical response at 48–72 hours — most patients improve within this window on appropriate empiric therapy.
Switch IV to oral therapy once the patient is hemodynamically stable, GI tract functioning normally, 2 temperature readings are normal over 16 hours, WBC has normalized, there is subjective symptomatic improvement, and the patient can take oral medications.^[1]
Discharge once IV-to-oral switch criteria are met, plus: no complications from the pneumonia itself (e.g., empyema), no complications from comorbid illness (e.g., MI), no complications from treatment (e.g., a severe adverse drug reaction), and physiologic stability — oxygen saturation ≥92% on room air (or a return to baseline in COPD), pulse <100 beats/minute, and respiratory rate ≤24 breaths/minute.
If blood cultures are positive, let the duration of IV therapy be dictated by the organism recovered rather than a fixed default.
De-escalate empiric MRSA or Pseudomonas coverage by 48 hours if cultures (and, for MRSA, a nasal PCR swab) are negative and the patient is improving.
Arrange a follow-up chest radiograph 6–12 weeks after presentation in patients over 50 (particularly smokers) specifically to screen for an underlying malignancy — pursue bronchoscopy if the opacity persists. Repeat imaging is not needed if the patient is under 50 and has resolved clinically.
Confirm pneumococcal and influenza vaccination status at follow-up and immunize if indicated.
For patients ≥65 years or with recurrent pneumonia, use a structured checklist to identify and address causes of aspiration and other modifiable recurrence risk.

Treatment Failure / Next Step

Confirm the diagnosis, the correct drug/dose/route was actually given, and adherence — before assuming the regimen has failed.
Reassess for a local complication (empyema, lung abscess, parapneumonic effusion) with repeat imaging, since these change management and may need drainage rather than a different antibiotic.
Broaden microbiologic testing to consider a resistant, unusual, or initially uncovered pathogen — tuberculosis, fungal infection, Legionella, or a respiratory virus not covered by the empiric regimen — and obtain infectious diseases input.
Reconsider noninfectious mimics that can look like nonresolving pneumonia: pulmonary embolism, heart failure, malignancy, or organizing pneumonia.
Broaden or change empiric coverage based on culture results, local resistance patterns, and specialist input rather than empirically escalating without new information.

Common Errors to Avoid

Using macrolide monotherapy for outpatient CAP without confirming local pneumococcal resistance is under 25% — it is no longer a default first-line choice.
Using amoxicillin or ampicillin monotherapy in a patient with comorbidities — it does not reliably cover beta-lactamase-producing H. influenzae, M. catarrhalis, or MSSA in this group.
Using daptomycin for MRSA pneumonia — it is inactivated by pulmonary surfactant and will not work regardless of in-vitro susceptibility.^[6]
Starting empiric Pseudomonas or MRSA coverage without actually checking for the specific risk factors that justify it, and then failing to de-escalate at 48 hours when cultures/nasal swab are negative.
Ordering a repeat chest radiograph before 6 weeks in a patient who is improving clinically and under 50 — not indicated.
Adding routine anaerobic coverage for aspiration pneumonia without a suspected lung abscess or empyema — not indicated for uncomplicated aspiration pneumonia.
Treating aspiration pneumonitis (sterile chemical injury from gastric contents) with antibiotics — it does not require antimicrobial therapy.
Withholding indicated antibiotics from a hospitalized or comorbid outpatient solely because a respiratory virus test is positive, without also excluding bacterial co-infection.
Giving corticosteroids in influenza pneumonia, or giving them routinely in non-severe inpatient CAP where there is a strong recommendation against doing so.
Missing the opportunity to review and update pneumococcal/influenza vaccination status at every CAP-related encounter.

Clinical Decision Algorithm

Site-of-care assessment and empiric therapy selection

Suspected CAP: cough, dyspnea, fever, new infiltrate on chest imaging

Calculate CURB-65 or PSI; apply clinical judgment; assess for ATS/IDSA major/minor severe-CAP criteria

Low risk (CURB-65 0-1, PSI Class I-II)

Outpatient: amoxicillin 1 g PO TID, or doxycycline 100 mg PO BID

Comorbidities or resistance risk factors present?

Yes

Amoxicillin/clavulanate + macrolide or doxycycline, or respiratory fluoroquinolone monotherapy

Amoxicillin or doxycycline alone is sufficient

Intermediate/high risk, no severe-CAP criteria (CURB-65 2-3, PSI Class III-IV)

Inpatient ward: ceftriaxone or cefotaxime + azithromycin, or respiratory fluoroquinolone monotherapy

Pseudomonas or MRSA risk factors present? Modify regimen accordingly and obtain cultures/MRSA nasal swab

Severe CAP: 1 major or ≥3 minor ATS/IDSA criteria (CURB-65 ≥4, PSI Class V)

ICU: ceftriaxone or cefotaxime + azithromycin or respiratory fluoroquinolone

Add antipseudomonal agent and/or vancomycin/linezolid if risk factors present; consider corticosteroids (not if influenza)

Reassess clinical response at 48-72 hours

48-72 hour reassessment and treatment-failure workup

Reassessing response at 48-72 hours

Improving

Meets IV-to-oral switch criteria?

Switch to oral therapy; plan for minimum 3 days (non-severe) or 5 days (severe) total, individualized to stability

Discharge once physiologic stability and IV-to-oral criteria are both met

Not improving or worsening

Confirm diagnosis, dose, route, and adherence

Repeat imaging for empyema/abscess/effusion; broaden cultures for resistant/unusual pathogens

Reconsider noninfectious mimics (PE, heart failure, malignancy); escalate care and obtain infectious diseases input

Drug Tables

How to use this table: Doses are adult doses and require patient-specific confirmation, including renal/hepatic adjustment. Pediatric CAP is managed under separate pediatric-specific protocols and is not covered here. Select any drug name to open its complete Pharmasist Drug Database page. Evidence cross-checked against the 2019 ATS/IDSA guideline^[1] and, where noted, the 2025 ATS update.

Penicillins

Drug	Dosage	Adverse effects	Drug interactions	Comments
Penicillin V	Adults: 300 mg PO TID-QID	Hypersensitivity reactions, rash, GI upset, interstitial nephritis	Probenecid prolongs serum levels; possible increased INR with warfarin	Oral step-down once a penicillin-susceptible pathogen is confirmed.
Penicillin G	Adults: 2 million units IV Q4H	Hypersensitivity reactions, rash, GI upset, interstitial nephritis	Probenecid prolongs serum levels; possible increased INR with warfarin	IV therapy for confirmed penicillin-susceptible S. pneumoniae.
Amoxicillin	Adults: 1 g PO TID	Hypersensitivity reactions, rash, GI upset, interstitial nephritis	Probenecid prolongs serum levels; may increase methotrexate levels; may decrease mycophenolate levels; possible increased INR with warfarin	Standard outpatient empiric dose already reflects the high-dose strategy for possible resistant S. pneumoniae.
Amoxicillin/clavulanate	Adults, option 1: 500/125 mg PO TID Adults, option 2: 875/125 mg PO BID	Hypersensitivity reactions, rash, GI upset (more diarrhea than amoxicillin alone), interstitial nephritis	Same as amoxicillin	Preferred beta-lactam for outpatients with comorbidities or resistance risk factors.
Ampicillin	Adults: 1 g IV Q6H	Hypersensitivity reactions, rash, GI upset, interstitial nephritis	Same as amoxicillin	IV option when oral therapy is not feasible and the pathogen is penicillin-susceptible.
Cloxacillin	Adults: 1-2 g IV Q6H	Hypersensitivity reactions, rash, GI upset, interstitial nephritis	Same as amoxicillin	Alternative for methicillin-susceptible S. aureus (MSSA) pneumonia.

Cephalosporins

Drug	Dosage	Adverse effects	Drug interactions	Comments
Cefazolin	Adults: 1-2 g IV Q8H	GI upset, renal/hepatic dysfunction, hypersensitivity, injection-site phlebitis	Probenecid prolongs serum levels; possible increased INR with warfarin	First-generation; alternative for MSSA pneumonia.
Cefuroxime	Adults: 500 mg PO BID	GI upset, hypersensitivity	Antacids/H2 blockers may reduce absorption of the oral tablet; possible increased INR with warfarin	Second-generation oral option for outpatients with comorbidities requiring a beta-lactam.
Cefotaxime	Adults: 1-2 g IV Q8H	GI upset, renal/hepatic dysfunction, hypersensitivity, injection-site phlebitis	Probenecid prolongs serum levels; possible increased INR with warfarin	Third-generation; can be used preferentially in hepatobiliary disease.
Ceftriaxone	Adults: 1-2 g IV once daily	GI upset, renal/hepatic dysfunction, hypersensitivity, injection-site phlebitis	Probenecid prolongs serum levels; possible increased INR with warfarin; do not co-administer with IV calcium (insoluble precipitate risk)	Standard inpatient beta-lactam backbone; once-daily dosing simplifies administration.

Antipseudomonal beta-lactams

Reserve for documented Pseudomonas risk factors — prior respiratory isolation of P. aeruginosa, or recent hospitalization with parenteral antibiotics within the past 90 days.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Piperacillin/tazobactam	Adults: 3 g/0.375 g IV Q6H	GI upset, renal/hepatic dysfunction, hypersensitivity	Probenecid prolongs serum levels; possible increased INR with warfarin	For severe CAP or documented Pseudomonas risk factors.
Cefepime	Adults: 2 g IV Q8H	GI upset, hypersensitivity, neurotoxicity/encephalopathy at high doses with renal impairment	None significant beyond additive nephrotoxicity with other nephrotoxic agents	Antipseudomonal alternative to piperacillin/tazobactam.
Meropenem	Adults: 1 g IV Q8H	GI upset, hypersensitivity, rare seizures at high doses/renal impairment	May reduce valproic acid levels significantly — avoid combination where possible	Reserve for multi-drug-resistant risk or beta-lactam allergy precluding other options; involve infectious diseases.

Macrolides

Drug	Dosage	Adverse effects	Drug interactions	Comments
Azithromycin	Adults, oral option 1: 500 mg day 1, then 250 mg daily x4 days Adults, oral option 2: 500 mg daily x3 days Adults, IV: 500 mg IV daily until able to tolerate oral therapy	Better tolerated than erythromycin; GI upset, rash, cholestatic hepatitis, QTc prolongation	Caution with other QTc-prolonging agents; possible increased INR with warfarin	A 3- or 5-day oral course is adequate for mild-to-moderate CAP; not recommended as monotherapy without confirming low local resistance.
Clarithromycin	Adults, regular-release: 500 mg PO BID Adults, extended-release: 1000 mg PO once daily	Better tolerated than erythromycin; GI upset, rash, cholestatic hepatitis, QTc prolongation	Potent CYP3A4 inhibitor — may increase levels of atorvastatin, carbamazepine, digoxin, simvastatin; rifampin may decrease clarithromycin levels	Alternative macrolide to azithromycin.

Tetracyclines

Drug	Dosage	Adverse effects	Drug interactions	Comments
Doxycycline	Adults: 100 mg PO BID on day 1, then 100 mg PO once daily	GI upset, photosensitivity	Iron or antacids decrease absorption; strong CYP3A4 inducers (phenobarbital, phenytoin, rifampin, carbamazepine) decrease drug levels	Equally preferred to amoxicillin for healthy outpatients; avoid in pregnancy.

Fluoroquinolones (respiratory)

Reserve for beta-lactam allergy/intolerance given resistance and toxicity concerns — not routine first-line therapy.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Levofloxacin	Adults, oral option 1: 500 mg PO/IV daily x10 days Adults, oral option 2: 750 mg PO/IV daily x5 days	GI upset, headache, dizziness, photosensitivity, QTc prolongation, hypoglycemia, C. difficile-associated diarrhea, tendinopathy, peripheral neuropathy, neuropsychiatric effects; avoid in children (cartilage toxicity)	Antacids/metal cations/sucralfate decrease absorption; possible increased INR with warfarin; avoid with other QTc-prolonging agents	750 mg daily x5 days is equivalent to 500 mg daily x10 days.
Moxifloxacin	Adults: 400 mg PO/IV once daily	Same class effects as levofloxacin	Same class effects as levofloxacin	Can switch IV to PO while maintaining serum levels.
Ciprofloxacin	Adults, oral: 500-750 mg PO BID Adults, IV: 400 mg IV Q12H	Same class effects as levofloxacin	Same class effects as levofloxacin; may decrease theophylline/cyclosporine elimination	Not first-line for CAP — used specifically for patients with Pseudomonas risk factors, combined with a beta-lactam.

MRSA-directed agents

Add to the backbone regimen only when MRSA risk factors are present; obtain a nasal PCR swab and de-escalate if negative. Never substitute daptomycin — it is inactivated by pulmonary surfactant.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Vancomycin	Adults: 15-20 mg/kg/dose IV Q8-12H, typically 500 mg Q6H or 1000 mg Q12H	Infusion-related flushing/hypotension with rapid infusion, nephrotoxicity, ototoxicity	Additive oto-/nephrotoxicity with other such agents	Trough or AUC-guided dosing per institutional protocol.
Linezolid	Adults: 600 mg PO/IV Q12H	GI upset, headache, dose/time-dependent myelosuppression; rare peripheral/optic neuropathy, lactic acidosis	MAOI interaction risk — avoid tyramine-rich foods and SSRIs (serotonin syndrome risk)	Requires infectious diseases consultation — Health Canada reserve antimicrobial. Monitor CBC at least weekly if used beyond 2 weeks; avoid use beyond 4 weeks where possible.

Anaerobic coverage (aspiration pneumonia with lung abscess or empyema only)

Not routine for uncomplicated aspiration pneumonia — add only when a lung abscess or empyema is suspected or confirmed.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Clindamycin	Adults, oral: 300-450 mg PO QID Adults, IV: 600 mg IV Q8H	GI upset, diarrhea, risk of C. difficile-associated colitis	May potentiate neuromuscular blocking agents	Anaerobic coverage option when abscess/empyema is present; amoxicillin/clavulanate is a reasonable alternative given its own anaerobic activity.

Corticosteroids (adjunctive, severe CAP only)

A strong recommendation exists AGAINST routine use in non-severe inpatient CAP. A conditional recommendation exists FOR use in severe CAP, excluding influenza pneumonia. The ideal specific regimen has not been established — the 3 below reflect the regimens studied in the supporting meta-analyses.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Prednisone	Adults: 50 mg PO daily x7 days	Hyperglycemia, GI upset/bleeding risk, mood/sleep disturbance, adrenal suppression with abrupt discontinuation	Additive hyperglycemia with other hyperglycemic drugs; reduced response to vaccines during immunosuppressive dosing	Oral option.
Dexamethasone	Adults: 5 mg IV daily x4 days	Same as prednisone	Same as prednisone	IV option.
Methylprednisolone	Adults: 0.5 mg/kg IV BID x5 days	Same as prednisone	Same as prednisone	IV option.

Antivirals for influenza pneumonia

Neuraminidase inhibitors for confirmed or strongly suspected influenza A/B; do not use corticosteroids for influenza pneumonia.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Oseltamivir	Adults: 75 mg PO BID x5 days	GI upset (nausea, vomiting), headache; rare neuropsychiatric events	None significant	Preferred oral neuraminidase inhibitor; start as early as possible after symptom onset.
Zanamivir	Adults: 10 mg (2 inhalations) BID x5 days	Bronchospasm risk — avoid in underlying airway disease (asthma, COPD)	None significant	Inhaled alternative when oseltamivir is not appropriate.

Prevention: pneumococcal vaccination

Current NACI guidance recommends Pneu-C-20 or Pneu-C-21 for all adults 65 years and older, and for adults 18-64 years with conditions that increase invasive pneumococcal disease risk (long-term care residency, immunocompromising conditions or medications, chronic lung/heart/liver/kidney disease, diabetes, cochlear implants, or tobacco/alcohol/substance use disorder). In patients previously vaccinated with Pneu-C-13, Pneu-C-15, or the older 23-valent polysaccharide vaccine, give a dose of Pneu-C-20 or Pneu-C-21 with at least a 1-year gap since the prior dose (a shorter interval of at least 8 weeks may be appropriate in some immunocompromised individuals). Pneu-C-10, Pneu-C-13, Pneu-C-15, and the 23-valent polysaccharide vaccine are no longer routinely recommended for adults, and are used now only when Pneu-C-20/21 are unavailable, per current series-specific guidance.^[7]

Pneumococcal conjugate vaccines

Drug	Dosage	Adverse effects	Drug interactions	Comments
Pneumococcal 20-valent conjugate vaccine	Adults: Single 0.5 mL dose IM	Injection-site erythema/swelling/pain, fever, fatigue, decreased appetite	None significant	Preferred single-dose option for eligible adults per current NACI guidance.
Pneumococcal 21-valent conjugate vaccine	Adults: Single 0.5 mL dose IM	Injection-site erythema/swelling/pain, fever, fatigue, decreased appetite	None significant	Co-preferred single-dose option alongside Pneu-C-20; program choice depends on local availability.
Pneumococcal 15-valent conjugate vaccine	Adults: Single 0.5 mL dose IM, followed by 23-valent polysaccharide vaccine 8 weeks later	Injection-site erythema/swelling/pain, fever, fatigue, decreased appetite	None significant	Not routinely recommended — use only when Pneu-C-20/21 are unavailable; requires a follow-up 23-valent polysaccharide dose 8 weeks later when used.

Prevention: influenza vaccination

Annual influenza vaccination is recommended for everyone over 6 months of age, especially those at high risk of influenza complications (older adults, pregnant patients, and those with chronic cardiopulmonary disease). Formulation and valency are updated each influenza season — confirm the current season's authorized product rather than relying on a fixed formulation reference.^[8]

References

Diagnosis and Treatment of Adults with Community-acquired Pneumonia. An Official Clinical Practice Guideline of the American Thoracic Society and Infectious Diseases Society of America — American Thoracic Society / Infectious Diseases Society of America (Metlay JP, Waterer GW, Long AC, et al.). Am J Respir Crit Care Med 2019;200(7):e45-e67
The clinical utility of methicillin-resistant Staphylococcus aureus (MRSA) nasal screening to rule out MRSA pneumonia: a diagnostic meta-analysis with antimicrobial stewardship implications — Clinical Infectious Diseases (Parente DM, Cunha CB, Mylonakis E, et al.). Clin Infect Dis 2018;67(1):1-7
Defining community acquired pneumonia severity on presentation to hospital: an international derivation and validation study — Thorax (Lim WS, van der Eerden MM, Laing R, et al.). Thorax 2003;58(5):377-382
A prediction rule to identify low-risk patients with community-acquired pneumonia — New England Journal of Medicine (Fine MJ, Auble TE, Yealy DM, et al.). N Engl J Med 1997;336(4):243-250
Diagnosis and Management of Community-acquired Pneumonia: An Official American Thoracic Society Clinical Practice Guideline — American Thoracic Society (Jones BE, Ramirez JA, Oren E, et al.) — reviewed and approved by the ATS; not endorsed by IDSA. Am J Respir Crit Care Med 2026;212(1):24-44 (published online July 18, 2025)
Inhibition of daptomycin by pulmonary surfactant: in vitro modeling and clinical impact — Journal of Infectious Diseases (Silverman JA, Mortin LI, Vanpraagh AD, et al.). J Infect Dis 2005;191(12):2149-2152
Recommendations on the use of pneumococcal vaccines in adults, including Pneu-C-21 — National Advisory Committee on Immunization, Public Health Agency of Canada. PHAC/NACI Statement, November 15, 2024
Canadian Immunization Guide: Part 4 — Immunizing Agents: Influenza Vaccines — Public Health Agency of Canada. Canadian Immunization Guide, updated July 25, 2024

Professional reference only. Apply clinical judgment and verify patient-specific decisions against current authoritative guidance and product information.