Reasoning GP · Algorithm · 1-page summary
Unilateral leg swelling
For: adults with one swollen leg in primary care. Not for: bilateral swelling (ankle swelling algorithm); pregnancy or postpartum (same-day maternity/medical pathway); children.
Core rule: one swollen leg is a DVT until excluded — Wells score, D-dimer and a scan within 4 hours, or interim anticoagulation and a scan within 24 hours. Ask about the chest every time.
0Stop — call 999 now
- Breathless, pleuritic pain, haemoptysis or collapse (PE) → 999
- Massive blue or white leg (phlegmasia) or cold pulseless limb → 999
- Pain out of proportion: compartment syndrome (pain on passive stretch) or necrotising infection → 999
- High-risk sepsis with spreading cellulitis → 999
1Triage — two-level Wells (NG158)
| Wells | Action |
|---|---|
| 2 or more | Scan within 4 h; else D-dimer + interim anticoagulation + scan within 24 h Same-day |
| 1 or less | D-dimer within 4 h; positive → scan as above; negative → other cause |
| Scan neg, D-dimer pos | Stop interim anticoagulation; repeat scan 6–8 days (Wells 2 or more) |
| Pregnant / postpartum | Same-day maternity pathway; LMWH not DOAC |
2Screen
- Cellulitis with moderate sepsis risk or Eron III–IV → Same-day
- Superficial thrombosis in the thigh or near a junction → Same-day scan
- Hot swollen foot in diabetic neuropathy (Charcot) → Same-day foot team
- Pelvic mass, groin nodes, unexplained lymphadenopathy → Suspected cancer pathway per NG12
3Assess
☐ Obs incl. SpO₂ ☐ Both calves measured (3 cm or more scores) ☐ Pitting, warmth, edge marked, entry wound ☐ Knee (Baker's), compartments ☐ Pulses before compression ☐ Abdomen, pelvis, groin nodes ☐ Before anticoagulation: FBC, U&E (Cockcroft–Gault), LFT, PT, APTT — do not wait
Avoid: Homan's sign; treating a "Baker's cyst" before a scan.
4Classify
- DVT — scan positive → DOAC at least 3 months.
- Cellulitis — acute spreading redness, fever, portal of entry.
- Baker's cyst / calf tear — knee arthritis, sudden pain; scan shows it.
- Venous / post-thrombotic — gradual, skin changes, old DVT.
- Lymphoedema or extrinsic compression — non-pitting, Stemmer, pelvic mass.
- Uncertain — scan negative: re-examine in 1–2 weeks; image abdomen/pelvis if unexplained.
5Treat today — doses: NICE/SPC, BNF, local formulary
- Interim or confirmed DVT: apixaban or rivaroxaban, dose per SPC; specialist if cancer, APS, severe renal impairment.
- Cellulitis: flucloxacillin 500 mg–1 g four times a day 5–7 days; allergy: clarithromycin 500 mg twice a day.
- Baker's / tear (after scan): analgesia, elevation.
- Venous: compression only after pulses ± ABPI.
6Refer routinely
- Suspected lymphoedema (after DVT and pelvic cause excluded) → lymphoedema service.
- Venous skin changes or ulcer → vascular / leg-ulcer pathway.
7Never reassure by
- A negative Homan's sign.
- No risk factors — many DVTs are unprovoked.
- "Baker's cyst" before an ultrasound.
- Redness — cellulitis and DVT coexist.
- A raised D-dimer "because they are old" without a scan.
- "Lymphoedema" without examining abdomen, pelvis and groin.
8Safety-net — say it, then document it
999 “Call 999 for sudden breathlessness, chest pain, coughing blood or collapse, or if the leg turns cold, white or blue.” Same-day “Contact us today if the redness spreads past the line, you get a fever, or the swelling or pain keeps getting worse.” Confirm the scan within 24 h.
✓Safe to leave primary care only if — all five, documented
No block 0–2 row applies
Wells recorded; PE symptoms asked
Scan booked within NG158 times or interim anticoagulation
Abdomen, pelvis, groin examined
Result owner named; safety-net given
Summary of the full Reasoning GP unilateral leg swelling algorithm, which holds the detail, examination, investigations and evidence. Sources: NICE NG158, NG141, NG253, NG12 (updated April 2026); RCOG GTG 37b; product SPCs. Clinical reference — follow local pathways; doses: NICE/SPC, BNF, local formulary.