Definition and epidemiology
An abscess is a discrete collection of pus (dead neutrophils, bacteria, and liquefied tissue debris) surrounded by a fibrous or granulation tissue capsule, formed as the body's attempt to contain an infection and prevent its spread.
- Cutaneous and soft tissue abscesses are extremely common, accounting for a large share of emergency department visits for skin and soft tissue infections.
- Incidence has risen over recent decades largely because of the spread of community-associated MRSA (CA-MRSA).
- Can occur at any age; risk is increased in people who inject drugs, patients with diabetes, obesity, or immunosuppression, and in areas of skin trauma or occlusion (axilla, groin, buttocks).
- Internal abscesses (hepatic, pelvic, intra-abdominal, perianal, dental, brain) are less common but carry higher morbidity and are often missed without imaging.
- I&D (incision and drainage) is the primary treatment for a simple abscess; antibiotics alone are usually inadequate.
- S. aureus, including CA-MRSA, is the most common cause of cutaneous abscesses.
- Fluctuance on exam is the key clinical sign distinguishing abscess from cellulitis.
- Bedside ultrasound helps differentiate abscess from cellulitis when exam is equivocal.
- Add systemic antibiotics after I&D if systemic signs, immunosuppression, or extensive disease are present.
- Perianal and deep abscesses often need imaging (CT/MRI) and may require surgical or image-guided drainage.
- Culture pus in patients with risk factors for resistant organisms or treatment failure.
- Recurrent abscesses should prompt evaluation for diabetes, hidradenitis suppurativa, or MRSA colonization.
Etiology and risk factors
Common organisms
- Staphylococcus aureus (including MRSA) is the leading cause of cutaneous abscesses.
- Streptococcus pyogenes and other streptococci, often in mixed or polymicrobial infections.
- Anaerobes and gram-negative enterics predominate in perianal, perirectal, intra-abdominal, and bite-wound abscesses (polymicrobial, gut flora).
- Oral anaerobes (e.g., Fusobacterium, Peptostreptococcus) in dental and peritonsillar abscesses.
Risk factors
- Skin trauma, insect bites, shaving, or follicular occlusion
- Injection drug use (skin popping, needle sharing)
- Diabetes mellitus and obesity
- Immunosuppression (HIV, chemotherapy, chronic steroid use)
- Poor hygiene, overcrowding, close contact (sports teams, military barracks) for CA-MRSA
- Foreign body or retained material (sutures, splinters)
- Hidradenitis suppurativa (recurrent axillary/inguinal abscesses)
Pathophysiology
Bacterial invasion through a break in the skin or mucosal barrier triggers an acute inflammatory response. Neutrophils are recruited and phagocytose bacteria, but in the process many die, releasing lysosomal enzymes that liquefy surrounding tissue. This creates a central cavity of pus (necrotic debris, dead neutrophils, bacteria, and proteinaceous fluid).
The surrounding tissue reacts by laying down a wall of fibrin and granulation tissue, which helps localize the infection but also impedes antibiotic penetration into the abscess cavity. This is the key physiologic reason antibiotics alone are usually insufficient: the fibrous capsule and necrotic, poorly vascularized center limit drug delivery, so the pus must be mechanically evacuated.
Clinical features
Cutaneous abscess
- Painful, tender, fluctuant, erythematous nodule or mass
- May enlarge over days and "point" (develop a central area of thinning skin or spontaneous drainage)
- Surrounding induration and warmth; may have overlying pustule
- Fever and systemic symptoms are variable; often absent in simple, localized abscesses
Deep or internal abscess (site-dependent)
- Fever, chills, malaise, localized pain
- Hepatic abscess: right upper quadrant pain, fever, hepatomegaly
- Pelvic/tubo-ovarian abscess: lower abdominal pain, fever, vaginal discharge
- Perianal/perirectal abscess: severe perianal pain, swelling, fever
- Peritonsillar abscess: sore throat, trismus, "hot potato" voice, uvular deviation
- Brain abscess: headache, focal neurologic deficits, fever (fever may be absent)
Diagnosis
Diagnosis of a superficial cutaneous abscess is usually clinical, based on inspection and palpation for fluctuance.
- Bedside ultrasound is useful when cellulitis and abscess are hard to distinguish clinically; it shows a hypoechoic fluid collection and can guide drainage.
- Culture of the pus (obtained at drainage) is recommended, especially in patients with risk factors for resistant organisms, treatment failure, recurrent abscesses, or systemic illness, to guide antibiotic choice if needed.
- Laboratory tests (CBC, inflammatory markers) are not required for simple abscesses but help assess severity if systemic illness is suspected.
- CT or MRI is used for suspected deep-seated abscesses (intra-abdominal, perirectal, intracranial, hepatic) where physical exam is unreliable.
- Blood cultures are reserved for patients with signs of systemic sepsis or suspected bacteremia.
Differential diagnosis
| Condition | Distinguishing feature |
|---|---|
| Cellulitis | Diffuse erythema and warmth, no fluctuant fluid collection |
| Furuncle/carbuncle | Follicular origin; carbuncle is coalescence of multiple furuncles |
| Epidermal inclusion cyst (infected) | History of a pre-existing cyst, central punctum |
| Hidradenitis suppurativa | Recurrent, chronic abscesses in axilla/groin with sinus tracts |
| Necrotizing fasciitis | Severe pain out of proportion, rapid progression, systemic toxicity, crepitus |
| Lymphadenitis | Tender lymph node, not a fluctuant skin mass |
| Sebaceous cyst (uninfected) | Non-tender, slow-growing, no erythema |
Management
General principle
Incision and drainage (I&D) is the primary and most important treatment for a simple, localized abscess. Evacuating the pus removes the bacterial burden and necrotic material that antibiotics cannot adequately reach.
Procedure
- Incise over the point of maximal fluctuance, break up loculations, and irrigate the cavity.
- Packing is traditionally used for larger cavities, though evidence suggests it may not be necessary for small abscesses and can increase pain.
- Loop drain placement is an option for larger or deeper abscesses to allow continued drainage with less packing-related discomfort.
Role of antibiotics
Guideline consensus (IDSA and others) favors adding systemic antibiotics (with MRSA coverage, e.g., trimethoprim-sulfamethoxazole, clindamycin, or doxycycline) after I&D in patients with:
- Systemic signs of infection (fever, tachycardia)
- Severe or extensive disease, or rapid progression
- Immunosuppression or significant comorbidities (diabetes)
- Abscess in a difficult-to-drain area (face, hands, genitalia)
- Associated cellulitis
- Failed initial drainage, or multiple abscesses
- Extremes of age
For small, uncomplicated abscesses without these features, I&D alone is often sufficient, though some guidelines increasingly favor adjunctive antibiotics even for simple cases given data showing modestly higher cure rates; practice varies by region and this is an area of evolving consensus.
Deep/internal abscesses
- Often require image-guided percutaneous drainage (e.g., hepatic, intra-abdominal) or surgical drainage, combined with targeted antibiotics covering likely polymicrobial/anaerobic flora.
- Perianal abscesses require prompt surgical drainage; underlying fistula evaluation may follow.
- Peritonsillar abscess: needle aspiration or incision/drainage plus antibiotics.
Supportive care
- Analgesia, warm compresses, wound care, and follow-up to ensure resolution.
- Recurrent abscesses warrant evaluation for underlying cause (diabetes, hidradenitis suppurativa, nasal MRSA colonization) and consideration of decolonization protocols.
Complications and prognosis
- Most simple cutaneous abscesses resolve fully after adequate drainage, with excellent prognosis.
- Local spread: cellulitis, lymphangitis, lymphadenitis.
- Systemic spread: bacteremia, sepsis, endocarditis (especially with S. aureus).
- Recurrence: common, particularly with MRSA colonization, hidradenitis suppurativa, or inadequate initial drainage.
- Necrotizing fasciitis: rare but life-threatening progression requiring emergent surgical debridement.
- Deep abscesses (brain, hepatic, intra-abdominal) carry higher morbidity and mortality if diagnosis or drainage is delayed.
- Scarring at the drainage site is common but usually of minor cosmetic concern.
"Pus must come OUT": Open it (incise), Un-roof loculations, Tidy irrigation, then treat systemically only if sick.
Practice questions
1. A 28-year-old man presents with a painful, fluctuant, erythematous 3 cm mass on his thigh. He is afebrile and has no significant medical history. There is no surrounding cellulitis. What is the most appropriate next step in management?
2. A 45-year-old woman with poorly controlled diabetes presents with a fluctuant perianal mass, fever of 38.9°C, and severe pain. What is the most appropriate immediate management?
3. A 19-year-old college athlete develops a recurrent, tender, fluctuant abscess on his forearm, his third episode in the past year. Culture from a prior drainage grew MRSA. What is the most appropriate next step to reduce recurrence?
Frequently asked questions
What causes an abscess?
An abscess is most often caused by bacterial infection, typically Staphylococcus aureus (including MRSA) entering through a break in the skin. The body's neutrophil response liquefies tissue and bacteria into pus, which becomes walled off by inflamed, fibrous tissue. Risk factors include skin trauma, diabetes, immunosuppression, and injection drug use.
What are the first symptoms of an abscess?
The first symptoms are usually localized pain, redness, and swelling at a site, which progresses to a tender, fluctuant lump as pus accumulates. The skin over it may feel warm and the lump can enlarge over several days, sometimes developing a point of spontaneous drainage. Fever and systemic illness are variable and often absent in simple cases.
How is an abscess diagnosed?
Diagnosis is usually clinical, based on finding a tender, fluctuant, erythematous mass on physical exam. Bedside ultrasound can confirm a fluid collection when exam findings are unclear or to distinguish it from cellulitis. Deeper abscesses require CT or MRI, and pus culture at the time of drainage helps guide antibiotic choice if needed.
Is an abscess curable?
Yes, most simple abscesses are fully curable with incision and drainage, which removes the pus and resolves the infection. Antibiotics are added when there are systemic signs, immunosuppression, or extensive disease. Recurrence can occur, especially with MRSA colonization or underlying conditions like hidradenitis suppurativa, but each episode is generally treatable.
Abscess vs cellulitis — what is the difference?
An abscess is a discrete, fluctuant collection of pus that typically requires drainage, while cellulitis is a diffuse skin and soft tissue infection without a defined pus pocket, treated with antibiotics alone. Clinically, an abscess feels like a soft, movable fluid-filled lump, whereas cellulitis presents as spreading, non-fluctuant erythema and warmth. The two can coexist, and ultrasound helps distinguish them when exam is unclear.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: L02
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.