Perianal abscess / perianal sepsis / Fournier gangrene (esp. diabetic / immunocompromised) (ASCRS 2023)

THE load-bearing colorectal-cancer rule (§5.5.2 LR+ ≥20) that OVERRIDES "it is just haemorrhoids." Painless bright-red blood that coats / drips and is clearly distal-anorectal (haemorrhoid pattern) carries a LOW likelihood ratio for CRC (binned LR+ ≈ 0.3–0.5 — it argues AWAY from cancer in an otherwise asymptomatic screened patient). The pivot to colonoscopy fires when bleeding is mixed with stool / dark / altered OR co-occurs with an alarm feature. Reference-standard = colonoscopy + histology; designs = primary-care prospective/population cohorts + SR. Pre-test priors (cohort): unselected rectal bleeding in primary care PPV(CRC) ≈ 2.2–8% (Astin/Hamilton SR pooled PPV 8.1%, 95% CI 6.0–11 at age ≥50 — PMID 21619747; du Toit/Wauters BMJ 2006 PMID 16790459 ~5.7% CRC, 4.9% adenoma in new-onset bleeding). The HIGHEST-LR finding mandating colonoscopy: rectal bleeding COMBINED WITH a change in bowel habit — Hamilton Br J Cancer 2006 (PMID 16882123) PPV rises into the ~double-digit range and the COMBINATION carries the dominant single-/combined-symptom likelihood ratio for CRC (binned LR+ ≈ 20+ vs the painless-pure-haemorrhoidal pattern in screened patients; rectal bleeding + change-in-bowel-habit is the diagnostic-accuracy SR top single combination — PMID 19935790, 37948886). Encoded pivot: PAINLESS-BRIGHT-RED-DISTAL → benign attribution permitted ONLY after the conditional checks below; ANY alarm feature, atypical/mixed bleeding, or iron-deficiency anaemia → CRC posterior dominates → mandatory colonoscopy BEFORE benign attribution (Siminoff PMID 26504796 — haemorrhoid attribution is the single most common missed CRC diagnostic opportunity, 36.5% missed-dx rate; ~80% of misattributions GI-GU incl. haemorrhoids). CONDITIONAL DEPENDENCIES (≥4, NOT independent): (1) CRC posterior PPV is conditional on AGE — same bleeding at <50 yr OR≈1 vs 50–69 OR≈5.1 vs ≥70 OR≈8.2 (PMID 16790459/16882123); the colonoscopy threshold drops at age ≥45 (current screening start). (2) Bleeding-attribution ERROR is conditional on ANOSCOPY/proctoscopy actually being performed AND a visible bleeding haemorrhoid identified — without endoscopic confirmation the "haemorrhoid" label is unsupported and the CRC posterior is NOT discounted (Siminoff PMID 26504796). (3) BLEEDING SEVERITY is conditional on anticoagulant/antiplatelet therapy — antithrombotic agents amplify both haemorrhoidal and occult-tumour bleeding, so heavy bleeding on anticoagulation is LESS discriminating for CRC and more discriminating for procedural risk (PMID 19095121 late post-band bleeding on antithrombotics; PMID 36048039 post-haemorrhoidectomy bleeding 5.92% on antithrombotic vs 2.66% not). (4) The lesion identity (haemorrhoid vs anorectal varix) is conditional on LIVER DISEASE / portal hypertension — in cirrhosis a dilated anorectal vessel is a VARIX, the haemorrhoid prior collapses, and banding/injection is contraindicated (route gi.cirrhosis.core.v1 / gi.variceal_bleed.v1). INDEPENDENCE NOTE: age and family-history-CRC contribute approximately independently to the pre-test prior (multiply); bowel-habit change and bleeding are NOT independent of an underlying tumour (their co-occurrence is the high-LR constellation, not a simple product). Threshold rule: colonoscopy mandated when (age ≥45) OR (any alarm feature) OR (iron-deficiency anaemia) OR (atypical/persistent/mixed bleeding) OR (no anoscopic haemorrhoidal source identified) OR (incomplete prior screening) — benign attribution is permitted ONLY when ALL are absent (Siminoff PMID 26504796; ASCRS 2024 PMID 38294832).

Rectal bleeding WITH any alarm feature — age ≥45 / per current screening, change in bowel habit, weight loss, iron-deficiency anaemia, family history CRC/IBD, bleeding atypical for haemorrhoids, incomplete prior screening, persistent/atypical bleeding (Siminoff 2015 PMID 26504796; ASCRS 2024)

Symptomatic / significant iron-deficiency anaemia attributable to chronic anorectal bleeding (ASCRS 2024)

Acutely thrombosed external haemorrhoid with severe pain, onset within ~72 h (Greenspon 2004 PMID 15486746)

Grade III–IV haemorrhoids or grade I–III refractory to conservative + office-procedure therapy (ASCRS 2024)

Planned rubber-band ligation / sclerotherapy / haemorrhoidectomy in a patient on anticoagulant or antiplatelet therapy (ASCRS 2024)