Skin abscess

A skin abscess is a localised collection of pus in the dermis or subcutaneous tissue, usually caused by Staphylococcus aureus. Incision and drainage is the primary treatment, and antibiotics are added only for defined indications.

Types. A furuncle (boil) is a single infected hair follicle extending into the dermis. A carbuncle is a group of coalescent furuncles with multiple drainage points, typically on the neck, back or thigh. A cutaneous abscess is a deeper collection with a fluctuant centre.

Severity. Mild disease has no systemic features. Moderate disease has systemic signs (temperature over 38 C, heart rate over 90, respiratory rate over 20, white count over 12 or under 4 x 10^9/L) or extensive cellulitis. Severe disease is sepsis, necrotising infection, failure of drainage and oral therapy, or an immunocompromised host.

Recurrent abscess. Two or more in 6 months, or 3 or more in 12 months. Causes include staphylococcal carriage (nose, perineum), Panton-Valentine leukocidin positive strains, hidradenitis suppurativa, diabetes, obesity, HIV, neutrophil defects (hyper IgE syndrome, chronic granulomatous disease), injection drug use and household transmission.

Microbiology. Community acquired MRSA is common in many regions. Streptococcus pyogenes, anaerobes (perianal and groin abscesses), Gram negative bacilli and polymicrobial infection occur at particular sites.

Key distinction. Cellulitis is a diffuse spreading infection without a collection, and antibiotics alone treat it. An abscess needs drainage.

History

• Duration, pain, size progression, fever, rigors, drainage.
• Previous abscesses, household or team contacts with similar lesions, recent hospital stay, trauma, shaving, waxing, bites, injection drug use, tattoos, surgery.
• Diabetes, HIV, steroids, chemotherapy, cirrhosis, kidney failure, malnutrition, skin disease.
• Site specific symptoms: pain on defecation (perianal), lactation (breast), facial swelling and eye symptoms (face), hand stiffness (hand).
• Travel history and exposure to flies in tropical regions (furuncular myiasis), sand fleas and soil (tungiasis), and tuberculosis exposure.

Examination

• Vital signs, perfusion and mental state.
• Lesion size (measure and mark the margin of erythema), induration, fluctuance, pointing, tenderness, regional lymphadenopathy, skin necrosis, bullae, crepitus.
• Perianal and perineal examination for deep or fistulating disease.
• Hand: flexor tendon sheath signs (flexed posture, fusiform swelling, tenderness along the sheath, pain on passive extension).
• Face: orbital swelling, proptosis, ophthalmoplegia, headache.
• Ultrasound at the bedside: a hypoechoic collection with cobblestoning confirms an abscess and improves accuracy over examination alone.

Red flags

• Pain out of proportion, rapid spread, bullae, necrosis, crepitus or hypotension (necrotising infection).
• Abscess of the face, nose or upper lip (risk of cavernous sinus thrombosis).
• Hand abscess with tendon sheath signs.
• Perianal or perineal abscess with swelling and sepsis (Fournier gangrene).
• Deep neck infection with trismus, stridor or drooling.
• Back pain with fever and neurological signs (epidural abscess).
• Over a joint or prosthesis.
• Injection site infection, diabetes with sepsis, neutropenia, neonate.
• Failure to improve after drainage.

Special groups

• Pregnancy: avoid doxycycline, fluoroquinolones and co-trimoxazole near term.
• Breastfeeding: breast abscess needs drainage with continued milk removal.
• Children: avoid doxycycline under 12 years.
• Region specific: furuncular myiasis (tumbu fly), tungiasis and pyomyositis (often HIV associated).

Infective collections

• Furuncle and carbuncle.
• Cutaneous abscess (staphylococcal, streptococcal, polymicrobial).
• Perianal and ischiorectal abscess.
• Pilonidal abscess.
• Bartholin abscess.
• Breast abscess and lactational mastitis.
• Suppurative lymphadenitis.
• Pyomyositis.
• Infected epidermoid or pilar cyst.
• Injection site abscess.
• Hidradenitis suppurativa.
• Folliculitis decalvans and dissecting cellulitis of the scalp.

Serious infections that mimic or complicate

• Necrotising fasciitis.
• Cellulitis without collection.
• Septic arthritis, osteomyelitis and septic bursitis.
• Epidural and deep neck abscess.
• Fournier gangrene.
• Cutaneous anthrax.
• Melioidosis.

Specific organisms and tropical causes

• Cutaneous tuberculosis and scrofuloderma.
• Nontuberculous mycobacterial infection and Buruli ulcer.
• Actinomycosis and nocardiosis.
• Mycetoma.
• Deep fungal infection (sporotrichosis, chromoblastomycosis).
• Furuncular myiasis.
• Tungiasis with secondary infection.
• Cat scratch disease.
• Amoebic abscess.
• Hydatid cyst.

Noninfective

• Haematoma and ruptured cyst.
• Foreign body granuloma.
• Gouty tophus.
• Ruptured Baker cyst.
• Pyoderma gangrenosum.
• Necrotic tumour (lymphoma, sarcoma, metastasis).
• Insect or spider bite with necrosis.

Step 1. Triage. Look for sepsis, necrotising infection, facial or hand involvement and deep or perianal collections.

Step 2. Confirm collection. Clinical examination is often adequate. Bedside ultrasound when fluctuance is uncertain, in children, and in obese patients, and to separate abscess from cellulitis.

Step 3. Microbiology. Send pus for Gram stain and culture, including MRSA testing, after incision in: systemic illness, immunocompromise, recurrence, treatment failure, hospital acquired infection, large or multiple abscesses, outbreaks and suspicion of unusual organisms. Blood cultures if febrile. Request mycobacterial and fungal culture when the course is subacute or the patient is immunosuppressed.

Step 4. Blood tests when systemically unwell or high risk. Full blood count, CRP, urea, creatinine, glucose or HbA1c, liver tests and lactate. HIV test in recurrent or unusual abscess.

Step 5. Deeper imaging. CT or MRI for suspected deep, perianal, neck, spinal or pyomyositis collections, or osteomyelitis.

Step 6. Recurrent abscess. Nasal and perineal swabs for carriage, Panton-Valentine leukocidin gene testing, glucose, HIV, immunoglobulins and neutrophil function tests in children with unexplained recurrent deep infection, and clinical assessment for hidradenitis suppurativa.

Limits. Culture can be negative in patients already on antibiotics. Ultrasound cannot always exclude necrotising infection, which needs surgical exploration.

Immediate. For sepsis or necrotising infection, give intravenous fluids, blood cultures, broad spectrum antibiotics within 1 hour and urgent surgery. Piperacillin with tazobactam 4.5 g intravenously every 6 hours (adjust for kidney function) plus clindamycin 600 to 900 mg intravenously every 8 hours, with vancomycin 15 to 20 mg/kg intravenously every 8 to 12 hours when MRSA is likely.

Incision and drainage

• Use local anaesthesia: lidocaine 1% (10 mg/mL) maximum 4.5 mg/kg plain (300 mg) or 7 mg/kg with adrenaline (500 mg). Warm and buffer the solution to reduce pain. Avoid adrenaline in end arteries of the hand, penis and ear. Procedural sedation for large or deep abscesses.
• Make a single incision along skin tension lines over the point of maximal fluctuance, break loculations, and irrigate. Packing is not routine for small abscesses. Loose packing or a drain may be placed in large abscesses. Do not squeeze lesions on the face.
• Cover with a dry absorbent dressing and review at 24 to 48 hours for packing removal and wound check. Do not close the wound with sutures.
• Small furuncles under 2 cm may drain with warm compresses without incision.

Antibiotics. Give antibiotics when there is cellulitis extending over 5 cm, systemic signs, immunocompromise, extremes of age, difficult sites (face, hand, genitals, perianal), multiple lesions, incomplete drainage, prosthetic material or poor response to drainage.

• Cefalexin 500 mg orally 4 times daily for 5 to 7 days (reduce the dose when creatinine clearance is under 40 mL/min).
• Flucloxacillin 500 mg to 1 g orally 4 times daily for 5 to 7 days (reduce in severe kidney impairment; monitor liver tests with prolonged use because cholestatic hepatitis can occur).
• Clindamycin 300 to 450 mg orally every 6 to 8 hours for 5 to 7 days, maximum 1.8 g daily. Monitor for diarrhoea and Clostridioides difficile infection.
• When MRSA is likely: co-trimoxazole 960 mg orally twice daily for 5 to 7 days (halve the dose when creatinine clearance is 15 to 30 mL/min; avoid under 15 mL/min; monitor potassium and creatinine; avoid in G6PD deficiency and late pregnancy; interacts with warfarin, methotrexate and ACE inhibitors) or doxycycline 100 mg orally twice daily for 5 to 7 days. Neither covers Streptococcus pyogenes reliably, so add a beta lactam when cellulitis is prominent.
• Severe disease or MRSA with systemic signs: vancomycin 15 to 20 mg/kg intravenously every 8 to 12 hours (loading dose 25 to 30 mg/kg in critical illness), adjusted to levels and renal function. Alternatives: linezolid 600 mg intravenously or orally every 12 hours (monitor platelet count weekly; serotonin syndrome risk with serotonergic drugs; limit to under 14 days) or daptomycin 4 mg/kg intravenously once daily (check creatine kinase weekly; every 48 hours when creatinine clearance is under 30 mL/min).
• Children: flucloxacillin 25 to 50 mg/kg daily in 4 doses; clindamycin 10 to 13 mg/kg each dose every 6 to 8 hours (maximum 40 mg/kg daily); co-trimoxazole trimethoprim component 8 to 12 mg/kg daily in 2 doses (not under 6 weeks).

Analgesia. Paracetamol 1 g orally every 6 hours (maximum 4 g daily; 3 g daily in hepatic impairment or weight under 50 kg). Ibuprofen 400 mg orally every 8 hours with food (maximum 1.2 g daily without specialist advice) when no contraindication exists.

Site specific measures

• Face (danger area): admit for intravenous antibiotics and surgical review; watch for cavernous sinus thrombosis (headache, orbital pain, proptosis, ophthalmoplegia).
• Hand: urgent hand surgery.
• Perianal and perineal: surgical drainage and examination for fistula.
• Breast: needle aspiration under ultrasound control every 1 to 3 days or incision, with flucloxacillin and continued breast emptying.
• Pilonidal: surgical drainage.
• Furuncular myiasis: apply petroleum jelly to occlude the punctum and extract the larva when it surfaces, or remove surgically.
• Tungiasis: sterile flea extraction and tetanus check.
• Check tetanus immunisation.

Recurrent abscess

• Decolonisation: mupirocin 2% ointment to the nostrils twice daily for 5 days plus chlorhexidine 4% body wash once daily for 5 to 7 days. Treat household contacts if recurrence continues.
• Hygiene: separate towels and razors, launder bedding and clothing at 60 C, keep wounds covered.
• Treat diabetes, obesity, eczema and underlying immunodeficiency.
• Hidradenitis suppurativa: dermatology for long term medical or surgical treatment.

Monitoring. Check temperature, pain, wound drainage and erythema at 24 to 48 hours and at 5 to 7 days. Reassess for an undrained collection, resistant organism or wrong diagnosis if there is no improvement.

Referral and escalation. Surgery for deep, large, facial, perianal, hand, breast and necrotising abscesses. Infectious diseases for recurrent disease, MRSA with complications, and immunocompromised hosts. Dermatology for hidradenitis suppurativa and folliculitis decalvans. Immunology for suspected immune defect.

Admission criteria. Sepsis, necrotising infection, immunocompromise, facial or deep space abscess, failed outpatient treatment, inability to take oral therapy, and uncontrolled diabetes.

Discharge and follow up. Safety net advice: return for fever, spreading redness, increasing pain, new swelling, black skin, or no improvement within 48 hours of drainage.