Cellulitis is an infection of the deep dermis and subcutaneous tissue, presenting with expanding erythema, warmth, tenderness, and swelling. It accounts for an estimated 14.5 million cases annually in the United States and over 650,000 admissions per year. [1] The majority of cases are caused by β-hemolytic streptococci and Staphylococcus aureus (MSSA), and diagnosis is clinical — there is no gold standard diagnostic test. [1-2]
1. History
- Onset and progression: Acute onset (hours to days) of unilateral erythema, warmth, swelling, and pain — typically on the lower extremities [1]
- Portal of entry: Ask about recent skin breaks — wounds, abrasions, insect bites, surgical sites, IV drug use, tinea pedis (present in ~50% of cases as a portal of entry) [1]
- Preceding trauma or immersion injury: Penetrating trauma, animal/human bites, freshwater/saltwater exposure change the likely pathogen [3]
- Prior episodes: Recurrence is common (22–49% report at least one prior episode); recurrences tend to occur at the same site [1]
- Systemic symptoms: Fever, chills, malaise, rigors — presence predicts failure of outpatient therapy [1]
- Immunosuppression: Chemotherapy, organ transplant, HIV, chronic liver/kidney disease [3]
- MRSA risk factors: Prior MRSA infection/colonization, IVDU, incarceration, athletes, long-term care facility residents, men who have sex with men [1]
2. Alarm Features
- Pain out of proportion to exam findings → necrotizing fasciitis [1]
- Rapid progression of erythema despite antibiotics
- Crepitus, skin necrosis, bullae, cutaneous anesthesia → necrotizing fasciitis [1]
- Hypotension, SIRS criteria (T >38°C or <36°C, HR >90, RR >20, WBC >12k or <4k) → severe cellulitis requiring IV antibiotics and possible admission [1]
- Signs of septic shock → penicillin G + clindamycin for possible streptococcal toxic shock syndrome [1]
- Periorbital/orbital involvement → risk of intracranial extension
- Bilateral presentation → extremely rare for true cellulitis; strongly consider pseudocellulitis (especially stasis dermatitis) [1]
3. Medications
Outpatient (Mild — no SIRS criteria, nonpurulent)
- Cephalexin 500 mg PO QID (first-line) [1]
- Dicloxacillin 250–500 mg PO QID [1]
- Amoxicillin/clavulanate 875 mg PO BID [1]
- Penicillin VK 250–500 mg PO Q6H [1]
- Clindamycin 300–450 mg PO QID (penicillin allergy) [1]
If MRSA suspected (purulent, IVDU, prior MRSA)
- TMP-SMX 1–2 DS tabs PO BID (add cephalexin for streptococcal coverage) [1]
- Doxycycline 100 mg PO BID [1]
Inpatient (Moderate-Severe — ≥2 SIRS criteria or failed oral therapy):
- Cefazolin 1 g IV Q8H or ceftriaxone 1–2 g IV daily [1]
- Vancomycin 15 mg/kg IV Q12H (if MRSA concern or severe) [1]
- Severe/immunocompromised: Vancomycin + piperacillin-tazobactam or meropenem [1][3]
Duration: 5 days per IDSA guidelines; extend if no improvement by day 5. [3-4] The ACP similarly recommends 5–6 days. [5]
Contraindicated/Cautions
- Avoid TMP-SMX or doxycycline as monotherapy for nonpurulent cellulitis (poor streptococcal coverage) [1]
- Clindamycin carries risk of C. difficile infection [1]
The following table from the JAMA review summarizes standard antimicrobial dosing:
4. Diet
- No specific dietary triggers or restrictions for cellulitis
- Adequate hydration is important, especially in febrile patients
- Long-term: Weight management is relevant — obesity is a risk factor for recurrence [1][6]
5. Review of Systems
- Constitutional: Fever, chills, rigors, malaise, fatigue
- Skin: Spreading redness, warmth, swelling, drainage (purulent vs. non-purulent), bullae, skin breakdown
- Musculoskeletal: Joint pain/swelling overlying erythema (consider gout, septic arthritis) [1][7]
- Vascular: Leg swelling, varicosities, history of DVT (consider DVT as mimic) [8]
- Lymphatic: Lymphadenopathy, lymphangitic streaking [1]
6. Collateral History and Family History
- Collateral: Confirm timeline of symptom onset, prior antibiotic use, recent hospitalizations, living conditions (shelters, correctional facilities → MRSA risk) [1]
- Social context: IVDU, homelessness, athletic participation (contact sports) [1][3]
- Family history: Generally not contributory; rare hereditary conditions (e.g., primary lymphedema syndromes) may predispose to recurrent cellulitis
7. Risk Factors
- Edema/lymphedema — the most commonly associated risk factor [1][6]
- Tinea pedis/onychomycosis — portal of entry in ~50% of cases [1][6]
- Prior cellulitis — strongest predictor of recurrence [3][9]
- Venous insufficiency and chronic leg ulcers [6][10]
- Obesity (BMI ≥33 predicts prophylaxis failure) [1]
- Skin disruption: Wounds, eczema, dermatitis, surgical sites, insect bites [9]
- IVDU [1]
- Immunosuppression: Chemotherapy, organ transplant, chronic liver/kidney disease [3]
- Notably, diabetes and smoking were not independently associated with increased risk of acute cellulitis in one prospective study, though they are associated with poor treatment response [1][10]
8. Differential Diagnosis
Misdiagnosis rates are estimated at ~30% in hospitalized patients and up to 74% in those requiring dermatology consultation. [1]
Common mimics (pseudocellulitis)
- Stasis dermatitis — most common mimic; often bilateral, chronic, with hemosiderin staining [1]
- Contact dermatitis — pruritic, well-demarcated, exposure history [2]
- Hematoma — trauma/anticoagulation history; confirm with ultrasound [1]
- Gout — erythema overlying a joint, may have fever/leukocytosis; joint aspiration is diagnostic [1]
- Lymphedema — chronic, non-tender, bilateral or unilateral [8]
Dangerous cannot-miss diagnoses
- Necrotizing fasciitis — pain out of proportion, rapid progression, crepitus, skin necrosis, bullae, cutaneous numbness [1]
- Deep venous thrombosis — unilateral leg swelling, calf tenderness [7-8]
- Septic arthritis/bursitis — erythema overlying a joint, limited ROM [7]
- Erythema migrans (Lyme) — well-demarcated, often annular, tick exposure [1]
- Calciphylaxis — severe pain out of proportion, ESRD/dialysis patients [1]
Key distinguishing features of true cellulitis: Acute onset, unilateral, poorly demarcated, expanding erythema with warmth and tenderness. [1][8]
9. Past Medical History
- Prior episodes of cellulitis (location, frequency, treatment)
- Chronic venous insufficiency, lymphedema, peripheral vascular disease
- Diabetes, chronic kidney/liver disease, immunosuppressive conditions
- History of saphenous vein harvesting, lymph node dissection, radiation
- Skin conditions: Eczema, psoriasis, tinea pedis
- Prosthetic joints or indwelling hardware near the affected area
10. Physical Exam
- Vitals: Temperature, heart rate, blood pressure, respiratory rate — assess for SIRS criteria [1]
- Skin: Unilateral, poorly demarcated erythema with warmth, edema, tenderness; mark the borders with a skin marker to track progression [1]
- Peau d'orange appearance (lymphatic involvement) [1]
- Lymphangitic streaking (linear erythematous streaks proximal to the lesion) [1]
- Regional lymphadenopathy [1]
- Fluctuance → suggests underlying abscess; use POCUS if uncertain [11]
- Crepitus → concerning for necrotizing fasciitis or gas gangrene [1]
- Interdigital spaces: Examine for tinea pedis, maceration, fissuring (portal of entry) [1]
- Bilateral exam: Compare both extremities — bilateral involvement strongly suggests an alternative diagnosis [1]
11. Lab Studies
Routine uncomplicated cellulitis: Labs are generally not required. [1][3]
When to obtain labs
- CBC with differential — if systemic signs present; WBC elevated in 34–50% [1]
- CRP — elevated in 77–97% of cases; useful for trending response [1]
- Blood cultures — only in immunocompromised, malignancy, severe systemic features (high fever, hypotension), immersion injuries, animal bites [3]
- BMP — if concern for renal impairment (vancomycin dosing) or sepsis
- Lactate — if sepsis suspected
- Wound/aspirate cultures — low yield in typical cellulitis; consider in immunocompromised or atypical presentations [3]
12. Imaging
- First-line: Point-of-care ultrasound (POCUS) — useful to differentiate cellulitis from abscess (fluid collection), identify foreign bodies, and assess for DVT [11]
- CT with contrast: If concern for necrotizing fasciitis (fascial thickening, gas tracking, fluid along fascial planes) — though imaging should not delay surgical consultation if nec fasc is suspected clinically [1]
- MRI: Most sensitive for deep soft tissue infection, osteomyelitis, or necrotizing fasciitis, but rarely needed acutely
- Plain radiographs: May show subcutaneous gas in necrotizing infections
- When imaging is unnecessary: Typical, uncomplicated cellulitis without fluctuance or concern for deeper infection [9]
13. Special Tests
- ALT-70 Score (Asymmetry, Leukocytosis, Tachycardia, Age ≥70): A validated clinical prediction rule for differentiating lower extremity cellulitis from pseudocellulitis. Score ≥3 has 97.8% sensitivity for cellulitis; score 0–2 has only 9.1% likelihood of cellulitis [8]
- LRINEC Score: Laboratory Risk Indicator for Necrotizing Fasciitis — can help risk-stratify if necrotizing infection is a concern
- Thermal imaging: Temperature difference ≥0.85°F between affected and unaffected areas is 100% sensitive for cellulitis, but requires specialized equipment [8]
- POCUS: For abscess detection — changes management in up to 50% of cases [11]
14. ECG
- Not routinely indicated for uncomplicated cellulitis
- Obtain ECG if:
- Sepsis or hemodynamic instability
- Tachycardia out of proportion to fever
- Pre-existing cardiac disease with systemic infection
- Planning vancomycin or other QT-prolonging agents
15. Assessment
Severity stratification per IDSA guidelines: [1][3]
- Mild: No systemic signs of infection → outpatient oral antibiotics
- Moderate: ≥1 SIRS criterion → can trial oral antibiotics if only 1 criterion; consider IV if ≥2 criteria or oral failure
- Severe: SIRS + hypotension, immunocompromise, or rapid progression → IV antibiotics, admission, consider broad-spectrum coverage
Typical presentation: Acute, unilateral, poorly demarcated erythema with warmth, swelling, and tenderness on the lower extremity. [1]
Atypical features warranting reassessment: Bilateral involvement, failure to respond to appropriate antibiotics, slowly progressive or chronic course, symmetric distribution, overlying a joint. [1]
Complications: Bacteremia, endocarditis, osteomyelitis, glomerulonephritis, toxic shock syndrome, lymphatic damage leading to chronic lymphedema and recurrent cellulitis. [1]
16. Treatment Plan
Initial stabilization (ED)
- IV access, fluid resuscitation if septic
- Mark borders of erythema with skin marker and note time
- Assess for abscess (POCUS) — I&D if present [11]
Antibiotic selection (see Medications section above for dosing):
- Mild nonpurulent: Cephalexin or dicloxacillin × 5 days [1][3]
- Moderate: Oral trial if 1 SIRS criterion; IV cefazolin/ceftriaxone if ≥2 SIRS criteria [1]
- Severe: IV vancomycin ± piperacillin-tazobactam or meropenem [1][3]
- Purulent: Culture drainage; empiric MRSA coverage (TMP-SMX or doxycycline + β-lactam for strep coverage) [1]
Adjunctive measures
- Elevation of the affected extremity [3]
- Treat predisposing factors: Tinea pedis (topical antifungals), edema (compression), skin care [1][3]
- NSAIDs/acetaminophen for pain and inflammation — a small RCT showed no adverse sequelae with anti-inflammatory use [1]
- IV antibiotics should be transitioned to oral after 48 hours of apyrexia and regression of inflammation [1]
Recurrence prevention (for ≥3–4 episodes/year despite risk factor management):
- Prophylactic penicillin VK 250 mg PO BID or erythromycin 250 mg PO BID for 4–52 weeks [1][3]
- Prophylaxis reduces recurrence (RR 0.46) but effect diminishes after discontinuation [1]
17. Disposition
Discharge criteria (outpatient management)
- No SIRS criteria or only 1 SIRS criterion with ability to tolerate oral antibiotics
- No immunocompromise, hemodynamic instability, or rapid progression
- Reliable follow-up and ability to return if worsening [1][11]
Admission criteria
- ≥2 SIRS criteria or failed outpatient oral antibiotics [1]
- Hypotension, sepsis, or toxic appearance
- Immunocompromised patients with systemic signs
- Concern for necrotizing fasciitis or deep space infection
- Inability to tolerate oral medications
- Unreliable follow-up or social factors precluding safe discharge
Observation
Specialist consultation triggers
- Surgery: Suspected necrotizing fasciitis, abscess requiring operative drainage, crepitus [1]
- Dermatology: Diagnostic uncertainty, suspected pseudocellulitis, failure to respond to appropriate therapy (misdiagnosis rate up to 74% in consulted cases) [1]
- Infectious disease: Immunocompromised host, unusual organisms, recurrent cellulitis despite prophylaxis
18. Follow Up / Return Precautions
Follow-up timing
- 48–72 hours for reassessment — erythema may initially worsen in the first 24–48 hours even with appropriate therapy (due to bacterial lysis releasing inflammatory enzymes) [3]
- If no improvement by day 5, extend antibiotics and reassess for alternative diagnoses or resistant organisms [3]
Return precautions — instruct patients to return immediately for:
- Worsening redness, swelling, or pain despite antibiotics
- Expanding erythema beyond skin markings
- New fever, chills, or rigors
- Skin color changes (darkening, blistering, numbness)
- Inability to tolerate oral medications
Patient counseling
- Elevate the affected limb above heart level as much as possible
- Complete the full antibiotic course
- Treat tinea pedis and maintain skin integrity to prevent recurrence
- Manage edema with compression stockings (once acute infection resolves)
- Expected recovery: Improvement typically begins within 48–72 hours; residual erythema and swelling may persist for days to weeks after clinical cure [1][3]
References
1. Cellulitis: A Review. — Raff AB, Kroshinsky D. The Journal of the American Medical Association. 2016.
2. Cellulitis: A Review of Current Practice Guidelines and Differentiation From Pseudocellulitis. — Boettler MA, Kaffenberger BH, Chung CG. American Journal of Clinical Dermatology. 2022.
3. Practice Guidelines for the Diagnosis and Management of Skin and Soft Tissue Infections: 2014 Update by the Infectious Diseases Society of America. — Stevens DL, Bisno AL, Chambers HF, et al. Clinical Infectious Diseases : An Official Publication of the Infectious Diseases Society of America. 2014.
4. Appropriate Use of Short-Course Antibiotics in Common Infections: Best Practice Advice From the American College of Physicians. — Lee RA, Centor RM, Humphrey LL, et al. Annals of Internal Medicine. 2021.
5. Antibiotic Courses for Common Infections: Recommendations From the ACP. — Dakkak M, Sabharwal M. American Family Physician. 2022.
6. Interventions for the Prevention of Recurrent Erysipelas and Cellulitis. — Dalal A, Eskin-Schwartz M, Mimouni D, et al. The Cochrane Database of Systematic Reviews. 2017.
7. Clinical Mimics: An Emergency Medicine-Focused Review of Cellulitis Mimics. — Blumberg G, Long B, Koyfman A. The Journal of Emergency Medicine. 2017.
8. Applying a Clinical Prediction Rule to Distinguish Lower Extremity Cellulitis from Its Mimics. — Bruner LP. American Family Physician. 2021.
9. Cellulitis. — Bystritsky RJ. Infectious Disease Clinics of North America. 2021.
10. Cellulitis in Adult Patients: A Large, Multicenter, Observational, Prospective Study of 606 Episodes and Analysis of the Factors Related to the Response to Treatment. — Collazos J, de la Fuente B, García A, et al. PloS One. 2018.
11. Diagnosis and Management of Cellulitis and Abscess in the Emergency Department Setting: An Evidence-Based Review. — Long B, Gottlieb M. The Journal of Emergency Medicine. 2022.