Purulent Skin and Soft Tissue Infections

Clinical Snapshot

Purulent skin and soft-tissue infection is primarily a S. aureus problem, including both methicillin-susceptible (MSSA) and methicillin-resistant (MRSA) strains. A furuncle originates in a hair follicle; a carbuncle is interconnected furuncles; an abscess is a localized pus collection in dermal/subcutaneous tissue.^[1] Drainage is the treatment for drainable pus; antibiotics are added for host risk, surrounding cellulitis, systemic illness, a difficult location, or inadequate drainage.

Recognize the Condition

A tender, erythematous, fluctuant furuncle (boil) centred on a hair follicle

Furuncle (boil) — a deeper follicular S. aureus infection with suppuration into the dermis and subcutaneous tissue. (CNX OpenStax, CC BY 4.0, Wikimedia Commons)

Focal painful, tender, raised lesion with fluctuance or purulent drainage.

The purulent SSTI spectrum

Furuncle

Deeper follicular nodule/boil.

Carbuncle

Cluster of interconnected draining furuncles.

Abscess

Localized collection of pus without required follicular origin.

Purulent cellulitis

Cellulitis with purulent drainage/exudate, even without an obvious discrete abscess.

Risk Factors

Prior MRSA infection or colonization.
Skin trauma and close-contact transmission.
Crowded living conditions, contact sports, incarceration, or injection drug use.
Diabetes or immunosuppression.
Recurrent disease, hidradenitis suppurativa, pilonidal disease, or a retained foreign body.

Red Flags & Refer

Urgent referral/hospital assessment for any of the following:

Severe systemic illness.
Rapid progression.
Deep-space infection.
Facial/periorbital lesions not safely drainable.
Hand infection with functional compromise.
Necrosis.
Crepitus.
Severe pain out of proportion.
Immunocompromise with major infection.
Failure after adequate drainage plus therapy.

Assessment & Diagnosis

Examine for fluctuance, spontaneous drainage, surrounding cellulitis, lymphangitis, and systemic toxicity. Ultrasound is useful when a deep or occult abscess is uncertain.

When to cultureCulture pus in recurrent disease, severe infection, MRSA-risk cases, prior antibiotic exposure, or treatment failure. Definitive antibiotic selection should be narrowed based on culture results.^[1]

Recurrent abscess at the same site

Do not repeatedly prescribe antibiotics without looking for hidradenitis suppurativa, a pilonidal cyst, a foreign body, or another local structural cause.

Treatment Targets

Achieve source control by draining pus.
Stop surrounding cellulitis and systemic spread.
Use culture results to narrow therapy.
Prevent recurrence by correcting colonization or structural causes when relevant.

Choose Initial Treatment

Direct treatment rule

Drainable abscess

Incision and drainage first.

Add antibiotics only when systemic symptoms, extensive surrounding cellulitis, a difficult/nondrainable site, immunocompromise, or recurrent disease is present.^[2]

When antibiotics are indicated and MRSA risk is low

1st line: cephalexin.

When MRSA risk is meaningful (systemic symptoms, prior MRSA, high local prevalence, immunocompromise)

1st line: trimethoprim-sulfamethoxazole or doxycycline.

Add streptococcal coverage if substantial surrounding nonpurulent cellulitis is also present.

Severe purulent infection / systemic illness

1st line (IV): vancomycin.

Reserve-list alternatives (specialist-directed): linezolid, dalbavancin, or daptomycin.^[3]

Non-Pharmacologic Treatment

Warm compresses for small early furuncles that have not yet organized into a drainable abscess.
Do not squeeze boils.
Cover draining wounds and use meticulous hand hygiene.
Routine packing after uncomplicated drainage is optional rather than mandatory — it does not appear to influence clinical cure rates.^[4]
For selected recurrent S. aureus infection, consider a decolonization course of intranasal mupirocin plus chlorhexidine body cleansing.^[5]

Pharmacologic Treatment Options

Drug / intervention	Role	Dose / use	Important adverse effects	Teaching points
Incision and drainage Preferred	Primary treatment for any fluctuant abscess, furuncle, or carbuncle with drainable pus	Procedure, not a drug	Procedural risk only	Send purulent material for culture in recurrent, severe, MRSA-risk, or treatment-failure cases.
Cephalexin Preferred	MSSA when MRSA is unlikely, after drainage	500 mg PO QID for about 5-7 days, adjusted to response	GI upset, hypersensitivity	Use when local MRSA risk is low.
Sulfamethoxazole/Trimethoprim Preferred	MRSA risk, culture-confirmed MRSA, or systemic symptoms	1-2 DS tablets PO BID	Rash, hyperkalemia, renal effects, cytopenias	Add streptococcal coverage when surrounding nonpurulent cellulitis is significant.
Doxycycline Preferred	Alternative MRSA-active oral therapy	100 mg PO BID	Photosensitivity, esophagitis	Avoid in pregnancy.
Clindamycin Alternative	MRSA + streptococcal coverage when susceptible	300-450 mg PO TID	Diarrhea, C. difficile infection	Inducible resistance can occur — confirm susceptibility.
Vancomycin Special situation	Severe purulent infection / systemic illness	Weight-based IV dosing with institutional AUC monitoring	Nephrotoxicity, infusion reaction	Hospital/specialist use for severe disease.
Linezolid Special situation	MRSA when standard agents cannot be used or resistance is documented	600 mg PO/IV Q12H	Myelosuppression, serotonin syndrome	Reserve-listed — infectious-disease guidance recommended.<cite data-reference-id="dd6d638d-375a-4623-95b4-3fec02e9eccd"></cite>

Special Populations

Pregnancy

Drainage remains the primary intervention regardless of trimester. When antibiotics are needed, choose pregnancy-compatible agents guided by organism and susceptibility; avoid doxycycline, and generally avoid trimethoprim-sulfamethoxazole in the first trimester (neural tube defect risk) and near term (kernicterus risk) when a safer effective option exists.

Immunocompromised patients

Use a lower threshold for systemic therapy, culture, imaging, and referral because source control alone may be insufficient.

Monitoring & Follow-Up

After drainage, pain and pressure should fall quickly. Reassess within 24-48 hours when disease is extensive, packing was used, systemic therapy was started, or host risk is present. Worsening pain, spreading erythema, or new systemic symptoms requires urgent reassessment.

Treatment Failure / Next Step

Confirm adequate drainage/source control.
Review culture and susceptibility.
Look for an undrained loculation or deeper collection.
Assess adherence and whether the chosen therapy covers MRSA and streptococci as required.
Investigate hidradenitis, pilonidal disease, or foreign body in recurrent same-site abscess.

Common Errors to Avoid

Giving antibiotics without draining a clearly drainable abscess.
Assuming every abscess needs antibiotics.
Using trimethoprim-sulfamethoxazole alone when extensive nonpurulent streptococcal cellulitis also needs coverage.
Repeatedly treating recurrent axillary/groin abscesses without considering hidradenitis suppurativa.
Failing to culture severe or recurrent purulent infection.

Clinical Decision Algorithm

Algorithm A — Furuncle / carbuncle / abscess

Painful fluctuant nodule or collection of pus

Assess size/extent, location, systemic symptoms, immunocompromise, and MRSA history

Drainable and patient systemically well

Incision and drainage is the primary treatment

No antibiotic if small/uncomplicated with no surrounding cellulitis or host-risk factor

Systemic symptoms, extensive surrounding cellulitis, difficult/nondrainable site, immunocompromise, recurrent disease

Drain if possible + culture + systemic anti-staphylococcal therapy; include MRSA coverage (TMP-SMX/doxycycline) when indicated

Algorithm B — Purulent cellulitis without a clear drainable collection

Purulent drainage/exudate with cellulitis but no obvious abscess

Ultrasound or further examination if occult abscess is possible

MRSA risk low / culture suggests MSSA

Use MSSA-active therapy such as cephalexin; culture purulent material

MRSA risk, systemic illness, prior MRSA, beta-lactam failure

Use MRSA-active therapy (TMP-SMX/doxycycline/clindamycin; severe → IV vancomycin) and tailor to culture

Drug Tables

How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

MSSA-directed oral therapy

Drug	Dosage	Adverse effects	Comments
Cephalexin	Adults: 500 mg PO QID for about 5-7 days, adjusted to response Children: 25-50 mg/kg/day divided Q6H PO	GI upset, hypersensitivity	Use after drainage when MSSA is expected.

MRSA-active oral therapy

Drug	Dosage	Adverse effects	Comments
Sulfamethoxazole/Trimethoprim	Adults: 1-2 DS tablets PO BID Infants (2 months+), children, adolescents: 8-12 mg/kg/day (TMP component) divided Q12H PO	Rash, hyperkalemia, renal effects, cytopenias	Add streptococcal coverage when surrounding nonpurulent cellulitis is significant.
Doxycycline	Adults: 100 mg PO BID Children over 8 years: 2-4 mg/kg/day divided Q8-12H PO (max 100 mg/dose)	Photosensitivity, esophagitis, chelation interactions	Add streptococcal coverage when needed. Avoid in pregnancy.
Clindamycin	Adults: 300-450 mg PO TID Children over 1 month: 25-40 mg/kg/day divided Q8H PO (max 450 mg/dose)	Diarrhea, C. difficile infection; inducible resistance can occur	Use when local isolate is susceptible.

Severe / parenteral therapy

Reserve-list agents require infectious-disease guidance and should be used only when resistance has been established or other agents are contraindicated.

Drug	Dosage	Adverse effects	Comments
Vancomycin	Adults: 1 g Q12H (or 15 mg/kg Q8-12H) IV, institutional AUC monitoring Children: 45-60 mg/kg/day divided Q6-8H IV	Nephrotoxicity, infusion reaction	Severe purulent SSTI/systemic illness.
Linezolid	Adults and children 12+: 600 mg PO/IV Q12H Infants and children under 12: 10 mg/kg/dose Q8H PO/IV (max 600 mg/dose)	Myelosuppression (>2 wk use), peripheral/optic neuropathy, serotonin syndrome	Reserve-listed; contraindicated with MAOIs.<cite data-reference-id="dd6d638d-375a-4623-95b4-3fec02e9eccd"></cite>
Dalbavancin	Adults: 1.5 g IV once, or 1 g then 500 mg one week later Children 6-17 years: 18 mg/kg IV once (max 1.5 g) Children 3 months-5 years: 22.5 mg/kg IV once (max 1.5 g)	GI effects, infusion reactions, headache	Long-acting specialist/ambulatory infectious-disease option.
Daptomycin	Adults: 4-6 mg/kg IV Q24H Children 12-17 years: 5 mg/kg IV Q24H Children 7-11 years: 7 mg/kg IV Q24H Children 2-6 years: 9 mg/kg IV Q24H Children 1 year: 10 mg/kg IV Q24H	Myopathy/rhabdomyolysis; monitor CPK	Last-line for resistant Gram-positive infection; statins increase muscle-toxicity risk.<cite data-reference-id="dd6d638d-375a-4623-95b4-3fec02e9eccd"></cite>

References

Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the Infectious Diseases Society of America — Clinical Infectious Diseases (Stevens DL, Bisno AL, Chambers HF, et al.). Clin Infect Dis 2014;59(2):e10-e52
Clinical practice guidelines by the Infectious Diseases Society of America for the treatment of methicillin-resistant Staphylococcus aureus infections in adults and children — Clinical Infectious Diseases (Liu C, Bayer A, Cosgrove SE, et al.). Clin Infect Dis 2011;52(3):e18-e55
Reserve list for antimicrobial drugs — Public Health Agency of Canada. Updated November 2024
Treatment of skin abscesses: a review of wound packing and post-procedural antibiotics — South Dakota Medicine (List M, Headlee D, Kondratuk K). S D Med 2016;69(3):113-119
Staphylococcus aureus decolonization for recurrent skin and soft tissue infections in children — Canadian Family Physician (Smith CH, Goldman RD). Can Fam Physician 2012;58(12):1350-1352

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