Cellulitis, DVT, or Dependent Oedema?
How to distinguish cellulitis from deep vein thrombosis and dependent oedema, when to call 999 for necrotising fasciitis, and why compression stockings must not be advised before DVT is excluded.
Why this matters
Patients with a red, swollen leg often present at the pharmacy counter asking for advice or a compression stocking. Three conditions can present in a similar way, and confusing them carries significant risk. Cellulitis is a bacterial skin infection that requires prescription antibiotics and is not a Pharmacy First pathway in England. Deep vein thrombosis (DVT) causes unilateral leg swelling and calf tenderness. Compression stockings must not be recommended until DVT has been clinically assessed, because they are only started after a confirmed diagnosis and once anticoagulation has begun. Dependent oedema is usually bilateral and may be benign in isolation, but still warrants GP review to investigate the underlying cause.
At the severe end of the spectrum, necrotising fasciitis is a rapidly spreading soft tissue infection with a mortality rate of 20 to 35 percent. Its key feature is pain that is grossly out of proportion to the visible skin changes. This is a 999 emergency. Orbital cellulitis, with proptosis and restricted eye movement, is also a 999 emergency.
Red flags vs more likely benign
| Feature | More likely benign | Red flag ⚠ |
|---|---|---|
| Pain | Proportionate to visible redness and swelling | Pain grossly out of proportion to visible changes, possible necrotising fasciitis |
| Skin changes | Localised erythema, warmth, tenderness | Blistering, crepitus, or visible skin necrosis. Call 999 |
| Spread | Demarcated erythema that changes slowly | Rapidly expanding erythema over hours with systemic collapse |
| Systemic features | No fever or mild malaise, consistent with Eron Class I cellulitis | Fever, rigors, confusion, tachycardia, or hypotension, consistent with Eron Class II to IV |
| Eye involvement | Periorbital skin redness only. Eye moves freely | Proptosis, restricted eye movement, reduced vision, or pain on movement. Call 999 |
| DVT features | Bilateral, painless, pitting swelling with no calf tenderness | Unilateral leg swelling with calf tenderness. Treat as DVT until excluded |
| Compression safe? | DVT formally excluded by clinical assessment or Doppler | DVT not yet excluded. Compression stockings must not be advised |
What to do in pharmacy
Compression stockings must not be recommended from the pharmacy counter until DVT has been clinically excluded. Safety-net advice: if erythema appears, pain worsens, or fever develops, the patient should seek same-day medical assessment. Patients with bilateral oedema of unknown cause should have a GP appointment arranged for investigation of possible cardiac, renal, or hepatic causes.
Key takeaways
- Pain out of proportion to visible skin changes suggests necrotising fasciitis and is a 999 emergency. Do not wait.
- Compression stockings must not be used until DVT has been excluded. A unilateral swollen, tender leg needs urgent clinical assessment.
- Cellulitis requires prescription antibiotics and is not a Pharmacy First pathway in England. All suspected cases should be referred to a GP.