Abscesses require drainage. Cellulitis requires antibiotics. The distinction determines whether the patient needs a procedure, a prescription, or both — and getting it wrong means treatment failure.
CLINICAL PRINTABLE
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- Abscess: a walled-off collection of pus in the dermis or subcutaneous tissue. Presents with a fluctuant, tender, erythematous nodule or mass — often with a central pustule or point of fluctuance. Fluctuance (a wave-like sensation on palpation) is the key clinical finding.
- Cellulitis: diffuse bacterial infection of the dermis and subcutaneous tissue without a discrete fluid collection. Presents with spreading erythema, warmth, swelling, and tenderness — without fluctuance. No palpable fluid collection.
- Purulent cellulitis: cellulitis with purulent drainage or exudate but without a discrete abscess cavity. MRSA is the most common cause. Requires antibiotics with MRSA coverage.
- Furuncle (boil): abscess involving a hair follicle. Carbuncle: multiple interconnected furuncles. Both are caused by Staphylococcus aureus (often MRSA). Require incision and drainage.
- Pilonidal abscess: abscess in the sacrococcygeal area, often associated with ingrown hairs. Presents with pain, swelling, and fluctuance in the gluteal cleft. Requires incision and drainage.
- Treating an abscess with antibiotics alone: antibiotics do not penetrate the avascular pus-filled cavity adequately. Incision and drainage is the definitive treatment. Antibiotics are adjunctive — not a substitute for drainage.
- Missing a deep abscess without fluctuance: deep abscesses (perirectal, psoas, deep neck) may not be fluctuant on surface palpation. Ultrasound or CT is required when a deep abscess is suspected. Point-of-care ultrasound (POCUS) can identify occult fluid collections at the bedside.
- Confusing an inflamed epidermoid cyst with an abscess: inflamed epidermoid cysts are tender, erythematous, and may have a central punctum — but they contain keratin, not pus. Incision and drainage of an inflamed cyst is appropriate for symptom relief, but complete excision of the cyst wall is required to prevent recurrence.
- Missing necrotizing fasciitis in a patient with apparent abscess or cellulitis: pain out of proportion to appearance, rapid spread, skin necrosis, crepitus, or systemic toxicity should prompt immediate surgical consultation. CT can show gas in the soft tissues but should not delay surgery when clinical suspicion is high.
- Inadequate drainage: incomplete drainage (too small an incision, inadequate packing, failure to break up loculations) leads to treatment failure and recurrence. The incision must be large enough to allow complete drainage and packing.
- Clinical diagnosis in most cases — fluctuance on palpation confirms abscess. Ultrasound (POCUS) is useful when fluctuance is uncertain, the abscess is deep, or the diagnosis is unclear.
- Point-of-care ultrasound (POCUS): identifies fluid collections, distinguishes abscess from cellulitis, guides drainage, and identifies deep abscesses not palpable on surface examination.
- Wound culture: send pus from incision and drainage for culture and sensitivity — especially important for MRSA surveillance and treatment failure. Routine culture of uncomplicated abscesses is not universally required but is recommended in immunocompromised patients and treatment failures.
- CBC and blood cultures: for patients with systemic signs (fever, tachycardia, leukocytosis), immunocompromised hosts, or suspected bacteremia.
- CT soft tissue: for suspected deep abscesses (perirectal, psoas, deep neck, breast), necrotizing fasciitis, or when the extent of infection is unclear.
Abscess — incision and drainage
Incision and drainage (I&D) is the definitive treatment. Adequate anesthesia (local infiltration or procedural sedation). Incision large enough for complete drainage. Break up loculations with a hemostat. Irrigate the cavity. Pack with gauze (or leave open for small abscesses). Follow-up in 24–48 hours for wound check and repacking.
Antibiotics after I&D
Antibiotics after I&D of uncomplicated abscesses reduce treatment failure and recurrence. Trimethoprim-sulfamethoxazole (TMP-SMX) or doxycycline for MRSA coverage. Duration: 5–7 days. Consider adjunct antibiotics based on systemic signs, host factors, severity, and current local guidance; do not use a >2 cm cutoff as the sole indication. Antibiotics are particularly important for: multiple abscesses, surrounding cellulitis, systemic signs, immunocompromised patients, or high-risk locations (face, hands, genitalia).
Non-purulent cellulitis
Beta-hemolytic streptococci are the most common cause. Cephalexin or dicloxacillin for mild-moderate disease. IV cefazolin for severe disease. MRSA coverage is not routinely required for non-purulent cellulitis without risk factors.
Purulent cellulitis (without discrete abscess)
MRSA is the most common cause. TMP-SMX or doxycycline for outpatient treatment. IV vancomycin for severe disease or failed oral therapy. Duration: 5–7 days.
Recurrent abscesses
Decolonization for recurrent MRSA abscesses: mupirocin nasal ointment BID for 5 days + chlorhexidine body wash daily for 5–14 days. Household decontamination (laundry, shared items). Screen household contacts. Consider infectious disease consultation for recurrent or refractory cases.
- Uncomplicated abscess after adequate I&D: most resolve within 1–2 weeks. Follow-up at 24–48 hours for wound check and repacking is standard.
- Failure to improve after I&D: consider inadequate drainage (incomplete, loculated), resistant organism, deep extension, or necrotizing fasciitis. Repeat imaging and surgical consultation.
- Uncomplicated cellulitis: improvement within 24–72 hours of appropriate antibiotics. Mark the border at presentation to monitor progression.
- Recurrent abscesses: MRSA decolonization and household decontamination reduce recurrence. Recurrent abscesses in the same location may indicate a retained foreign body, pilonidal sinus, or hidradenitis suppurativa.
- Abscess with systemic toxicity, rapidly spreading cellulitis, or failure to improve after I&D — IV antibiotics, repeat imaging, surgical consultation.
- Pain out of proportion, rapid spread, skin necrosis, crepitus — necrotizing fasciitis. Immediate surgical consultation.
- Perirectal abscess — surgical consultation for I&D. Do not attempt outpatient drainage of perirectal abscesses — they require examination under anesthesia to exclude fistula-in-ano.
- Facial abscess (especially periorbital, nasal, or upper lip) — high risk of cavernous sinus thrombosis. IV antibiotics and surgical consultation.
CASE 1
A 28-year-old presents with a 3 cm fluctuant, tender nodule on the right thigh. No fever, no surrounding cellulitis. Abscess is diagnosed. I&D is performed under local anesthesia — 5 mL of purulent material is drained. TMP-SMX is prescribed for 5 days for MRSA coverage. Follow-up in 48 hours for wound check.
CASE 2
A 45-year-old presents with a 4 cm area of erythema, warmth, and tenderness on the left forearm without fluctuance. No purulent drainage. Non-purulent cellulitis is diagnosed. Cephalexin is prescribed for 5 days. The border is marked. She returns in 48 hours — the erythema has not spread beyond the mark and she is improving.
NOW CHANGE ONE DETAIL
Same patient as Case 1, but at 48-hour follow-up the wound is not draining and there is a new area of fluctuance adjacent to the original site. Inadequate drainage with a second loculation is suspected. POCUS confirms a second fluid collection. Repeat I&D is performed — complete drainage is achieved.
Bottom Line
Abscesses require drainage — antibiotics alone will not resolve a walled-off collection of pus. When in doubt about fluctuance, use ultrasound. After I&D, consider adjunct antibiotics based on systemic signs, host factors, severity, and current local guidance; do not use a >2 cm cutoff as the sole indication.
EVIDENCE & REFERENCES
- Stevens DL, et al. Practice Guidelines for the Diagnosis and Management of Skin and Soft Tissue Infections: 2014 Update by the IDSA. Clin Infect Dis. 2014;59(2):e10–52. doi:10.1093/cid/ciu444
- Talan DA, et al. Trimethoprim-Sulfamethoxazole versus Placebo for Uncomplicated Skin Abscess. N Engl J Med. 2017;376(26):2545–2555. doi:10.1056/NEJMoa1701789
- Moran GJ, et al. Methicillin-Resistant S. aureus Infections among Patients in the Emergency Department. N Engl J Med. 2006;355(7):666–674. doi:10.1056/NEJMoa055356