Community-Acquired Pneumonia

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.

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]

Common causative organisms

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

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]

• 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.

Differentiating CAP from its common mimics

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

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]

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]

• 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).

• 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.

Pregnancy

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.

• 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.

1. Confirm the diagnosis, the correct drug/dose/route was actually given, and adherence — before assuming the regimen has failed.
2. 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.
3. 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.
4. Reconsider noninfectious mimics that can look like nonresolving pneumonia: pulmonary embolism, heart failure, malignancy, or organizing pneumonia.
5. Broaden or change empiric coverage based on culture results, local resistance patterns, and specialist input rather than empirically escalating without new information.

• 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.

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

Cephalosporins

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.

Macrolides

Tetracyclines

Fluoroquinolones (respiratory)

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

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.

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.

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.

Antivirals for influenza pneumonia

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

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

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]