RCGP SCA Algorithm — UK Primary Care
One swollen leg is a DVT until excluded — and the conditions that kill or cost a limb (PE, phlegmasia, compartment syndrome, necrotising infection, acute ischaemia) are recognised on the first look. Bilateral swelling follows the ankle swelling algorithm.
DVT, ruptured Baker's cyst, cellulitis and calf muscle injury look alike, and clinical features alone cannot exclude a DVT — which is why NICE NG158 builds the assessment around a structured score, a D-dimer and imaging within a fixed time. PE presents as breathlessness or chest pain, so ask about the chest in every patient with a swollen leg.
Compartment syndrome and necrotising fasciitis share one early clue: pain out of proportion to what you can see. Both need surgery within hours.
VTE risk factors raise the pre-test probability but their absence does not exclude DVT — up to a third of DVTs are unprovoked. Baker's cyst rupture typically follows exertion in someone with knee arthritis and can cause calf swelling, tenderness and later a crescent of bruising below the malleolus; it can coexist with DVT, so imaging is still needed.
Examining both legs together makes asymmetry obvious. Do not use Homan's sign — it is neither sensitive nor specific. An unexplained unilateral swelling without DVT needs an abdominal, pelvic and groin examination, because extrinsic compression of the iliac veins or lymphatics (pelvic tumour, lymphadenopathy, May–Thurner anatomy) presents exactly like this.
NICE NG158 sets clear time limits: imaging within 4 hours, or a D-dimer and interim anticoagulation with imaging within 24 hours. A DOAC that can be continued avoids switching if the scan is positive. In people with a likely DVT but a negative proximal ultrasound and a positive D-dimer, a repeat scan at 6–8 days picks up calf clots that have propagated.
Superficial vein thrombosis near the saphenofemoral or saphenopopliteal junction can extend into the deep system, so a scan is needed when it is in the thigh or close to a junction (CKS).
When the scan excludes DVT, the differential is guided by onset and examination. In people with diabetes and neuropathy, a hot swollen foot with normal or minimal pain may be an acute Charcot foot, which needs same-day offloading and specialist foot team referral.
Most areas run an ambulatory DVT service that delivers the NG158 timeframes; refer there the same day rather than by routine letter. Lymphoedema referral should follow exclusion of DVT and of a pelvic or abdominal cause, because a new unilateral lymphoedema can be the first sign of malignancy.
Next steps: see Deep vein thrombosis · Pulmonary embolism · DOACs, antiplatelets & anticoagulation · Cellulitis & erysipelas · Varicose veins
NICE NG158 recommends interim therapeutic anticoagulation when imaging cannot be done within 4 hours, using an anticoagulant that can be continued if DVT is confirmed; apixaban and rivaroxaban fit this and need no heparin lead-in. Doses differ between the initial and maintenance phases and by renal function, so prescribe from the SPC.
NICE NG141 gives flucloxacillin as first choice for non-severe cellulitis, with clarithromycin, erythromycin (pregnancy) or doxycycline for penicillin allergy. Compression without checking the arterial supply risks ischaemic injury.
Early mobilisation with anticoagulation is safe and improves symptoms. Recurrent cellulitis is driven by oedema and broken skin, so skin care and oedema control are the main preventive measures.
The highest-risk moment is the gap between the consultation and the scan: a documented plan for who books it, when, and who acts on the result closes that gap. A negative scan with persisting unexplained swelling needs a planned re-look for an extrinsic cause.