Reasoning GP · Algorithm · 1-page summary
Anogenital ulcers
For: sexually active adults with a new genital or perianal ulcer, in primary care. Not for: children, or a confirmed diagnosis already under care (use the genital herpes, HIV and chlamydia protocols). Pregnancy, immunosuppression and HIV exposure ≤72 h go straight to block 2.
Core rule: a painless, indurated genital ulcer is syphilis until serology proves otherwise. Test for every STI at the same visit (co-infection is common), and send a persistent or unexplained ulcer for specialist assessment without delaying STI testing.
0Stop — call 999 now
- Rapidly spreading painful ulceration, black eschar, crepitus or perineal spread with sepsis (Fournier's, necrotising fasciitis), or progressive ulceration with systemic collapse → 999
- Hospital gives IV antibiotics and immediate debridement: transfer is the treatment; do not wait for swabs or bloods.
1Triage by pattern — pain, number, systemic features
| Pattern | Think → action |
|---|---|
| Painless, indurated | Primary syphilis → serology + RPR, Urgent GUM |
| Painful, multiple | Herpes (or chancroid after travel) → HSV PCR, start aciclovir, Urgent GUM |
| Umbilicated, proctitis | Mpox (fever, nodes) → swabs via sexual-health/ID pathway; not routine HSV |
| Rash on palms/soles | Secondary syphilis or HIV seroconversion → Same-day GUM |
| Persistent | Months, warty or plaque-like → NG12 (block 2) |
2Screen — one row is enough
- Urinary retention, parenteral-level pain, severe systemic illness, progressive necrosis, ocular or neurological symptoms → Same-day hospital
- Pregnant with suspected primary herpes or syphilis → Same-day hospital with obstetric input. First-episode herpes in the third trimester → Same-day obstetric contact
- HIV exposure ≤72 h → PEP, ideally within 24 h → Same-day sexual health / ED
- HIV-positive or immunocompromised, or ulcer persisting or unexplained → Urgent GUM reassessment
- Behçet's with painful red eye, photophobia or visual change → Urgent ophthalmology
- NG12: unexplained vulval lump, ulceration or bleeding; penile mass or ulcer once STI excluded or persisting after STI treatment; unexplained anal mass or ulceration → consider Suspected cancer pathway
3Assess — chaperone, good light
☐ Sexual history (anal sex, travel, skin-to-skin contact, exposure ≤72 h), pregnancy, immunosuppression, medicines, eye/joint/bowel symptoms ☐ Ulcer edge, base, number; inguinal nodes; anus, mouth, palms and soles
☐ HSV PCR with typing from a fresh lesion base ☐ Syphilis serology + quantitative RPR ☐ 4th-generation HIV test ☐ Gonorrhoea/chlamydia NAAT by site ☐ Mpox/VZV PCR if features ☐ Hepatitis B/C where indicated. Not routine: ANA, HLA-B51, Paul-Bunnell.
4Classify — only if the screen is negative
- Genital herpes — painful vesicles → shallow ulcers, tender nodes; first episode may be systemic → treat, GUM.
- Primary syphilis — single painless indurated ulcer, clean base, non-tender rubbery node; may be hidden on cervix or rectum → urgent GUM.
- LGV — proctitis or buboes in MSM → rectal NAAT + LGV genotyping via GUM; not standard chlamydia treatment.
- Non-STI — Lipschütz ulcer (exclusion, after STI tests), aphthae, fixed drug eruption, lichen sclerosus/planus, Crohn's, Behçet's, trauma or assault.
- Uncertain — say so: full STI screen, urgent GUM, review until healed or explained.
5Treat today — doses: NICE, SPC and local formulary
- First-episode herpes (within 5 days or new lesions forming): aciclovir 400 mg TDS or valaciclovir 500 mg BD for 5–10 days; review at day 5.
- Symptom relief: saline bathing, paracetamol + ibuprofen, lidocaine 5% gel; pass urine in a warm bath.
- Known recurrent herpes: aciclovir 800 mg TDS for 2 days; consider suppression (400 mg BD) if painful, distressing, disruptive or transmission concern; review yearly.
- Not GP prescribing: syphilis, LGV, PEP, HIV → GUM/HIV services. Fixed drug eruption → withdraw the drug.
6Refer routinely
- Partner notification for syphilis, HIV, LGV, gonorrhoea, chlamydia → sexual-health service.
- Recurrent or multisystem Behçet's → rheumatology/dermatology. Atypical perianal fissures or fistulae → gastroenterology/colorectal.
- Recurrent aphthae → ferritin, B12, folate, coeliac screen. Ulcerated, indurated or irregular wart → specialist.
- HSV distress → NHS Talking Therapies; HSV history → tell midwife at booking.
7Never reassure by
- A painless ulcer — syphilis and early cancer are painless.
- Spontaneous healing — a chancre heals in 3–6 weeks untreated.
- Negative syphilis serology very early — repeat if suspicion remains.
- Negative HSV PCR from an old or crusted lesion.
- Negative HIV test within 45 days of exposure — repeat.
- One positive result or a Lipschütz label — test for every STI first.
8Safety-net — say it, then document it
999 “Call 999 if the sore spreads fast, the skin turns black, or you feel very unwell or collapse.” Same day: “Contact us today if you cannot pass urine, have severe pain, eye pain or vision change, a severe headache or new weakness, or you are pregnant.”
Remote or OOH: an ulcer is not assessed by phone or photo alone — arrange face-to-face examination or direct urgent sexual-health access. If the ulcer persists or stays unexplained, reassess urgently. Before syphilis treatment, warn about the Jarisch-Herxheimer reaction (usually within 24 h); in pregnancy it needs obstetric advice.
✓Safe to leave primary care only if — all five, documented
No block 0–2 row applies
Pregnancy, immunosuppression and exposure ≤72 h asked
HSV PCR, syphilis, HIV and STI screen sent today
GUM referral, results owner and partner notification set
Safety-net given; review until healed or explained
Summary of the full Reasoning GP anogenital ulcers algorithm, which holds the detail, examination, PEP/PrEP, pregnancy and follow-up. Sources: BASHH syphilis 2024, anogenital herpes 2024, HSV in pregnancy (with RCOG) 2024, chlamydia 2026, STI testing 2023, PEP 2021; BHIVA/BASHH/BIA HIV testing 2020; NICE NG60, NG12 (updated April 2026). Clinical reference — follow local sexual-health pathways and check doses: NICE, SPC and local formulary.