AKT · Infections/Sepsis, fever & healthcare-associatedlow yield

Abscess

Localised collection of pus walled off by the body — commonly Staphylococcus aureus

Overview

A localised collection of pus walled off from surrounding tissue, most often staphylococcal. It can be superficial (skin/soft tissue) or deep (intra-abdominal, hepatic, perianal, psoas, dental, brain). The cardinal principle that the exam tests relentlessly: PUS MUST BE DRAINED — antibiotics alone do not clear an undrained abscess. Deep abscesses present as a focus of sepsis or PUO and need imaging to find and drain them.

Recognise

Superficial: a tender, warm, fluctuant, erythematous swelling ± systemic upset; deep abscess — fever/sepsis or PUO with localising features
Skin/soft tissue (Staph. aureus, incl. MRSA/PVL), perianal, dental, intra-abdominal/pelvic (post-surgery/diverticular/appendiceal), hepatic, psoas, lung, brain
Raised inflammatory markers; the pointer is a collection on imaging (ultrasound/CT)

Red flags

Deep/organ abscess with sepsis → drainage + antibiotics urgently
Brain abscess (focal neurology + fever), perianal abscess in diabetes/immunosuppression, or an abscess complicating necrotising infection → emergencies

Differentials & how to tell them apart

Cellulitis (no collection)diffuse erythema without a drainable collection — antibiotics, not drainage; image if abscess suspected

Other fluctuant swellingshaematoma, cyst — ultrasound distinguishes

Necrotising fasciitispain out of proportion, rapid spread, systemic toxicity — surgical emergency

Investigations

Examination (fluctuance) for superficial; ultrasound/CT to locate and characterise a deep collection; pus for microscopy/culture (and to guide antibiotics); blood cultures/inflammatory markers if systemic; identify the source.

Management

Incision & drainage (drain the pus) ± antibiotics; image and drain deep collections

1Confirm a collection (fluctuance, or ultrasound/CT for deep abscesses) and DRAIN it (incision & drainage, or image-guided). Send pus for culture.Gate: PUS MUST BE DRAINED — antibiotics alone will not clear an undrained abscess; a deep abscess presenting as sepsis/PUO must be located on imaging and drained.
2Antibiotics as an adjunct (or for cellulitis/systemic/immunocompromised cases), targeted to culture; treat the underlying source; specialist/IR drainage for intra-abdominal, hepatic or brain abscesses.

INCISION & DRAINAGE (or image-guided drainage) — the definitive treatment — pus must be drained; antibiotics alone fail an undrained abscess

Antibiotics — adjunct (or for surrounding cellulitis/systemic upset/immunocompromise); targeted to culture — cover Staph. aureus

Source control of the underlying cause — e.g. treat the diverticular/appendiceal/dental source; remove infected material

Specialist drainage for deep collections — interventional radiology/surgery for intra-abdominal/hepatic/brain abscesses

Key points

A walled-off collection of pus (usually Staph. aureus) → the rule is PUS MUST BE DRAINED (incision & drainage / image-guided); antibiotics alone fail an undrained abscess and are an adjunct. A deep abscess is a classic occult cause of sepsis/PUO — image (USS/CT) to find and drain it.

Monitor & prognosis

Resolution post-drainage, cultures, recurrence; the underlying source.

Excellent after adequate drainage; persists/recurs if undrained or the source untreated.

Source: Surgical/ID references; cross-ref dermatology, gastroenterology, neurology (brain abscess)