Deep vein thrombosis (DVT) is a blood clot (thrombus) forming in a deep vein — most commonly in the calf, thigh or pelvis. It becomes life-threatening if the clot breaks off and travels to the lungs (pulmonary embolism, PE). Together, DVT and PE are known as venous thromboembolism (VTE) and cause an estimated 300,000–600,000 deaths per year in the US alone.
Symptoms — and why they're easily missed
DVT symptoms are notoriously non-specific. Up to 50% of DVTs — particularly calf DVTs — are completely asymptomatic and are only discovered incidentally during imaging. When symptoms do occur, they overlap with muscle injuries, cellulitis and other common conditions.
- Swelling in one leg (or, rarely, both) — particularly the calf or thigh
- Pain or tenderness in the leg — classically described as cramping or heaviness
- Warmth over the affected area
- Redness or discolouration of the skin
- A palpable cord in the calf (rare, but specific when present)
- Symptoms of PE if clot has migrated: sudden breathlessness, chest pain, rapid heart rate, coughing blood — these are medical emergencies
Risk factors
| Risk factor | Mechanism |
|---|---|
| Immobility (long flights, bed rest, post-surgery) | Reduced blood flow velocity in veins |
| Recent surgery or trauma | Tissue damage activates clotting cascade |
| Cancer and its treatments | Hypercoagulable state; chemotherapy endothelial damage |
| Oral contraceptives / HRT | Oestrogen increases clotting factors |
| Pregnancy and postpartum period | Increased clotting factors; venous compression |
| Previous DVT or PE | Damaged vessel wall and post-thrombotic syndrome |
| Inherited thrombophilia (e.g. Factor V Leiden) | Inherited hypercoagulability |
| Obesity | Venous stasis and inflammatory effects |
Diagnosis
The Wells Score is the standard pre-test clinical probability tool. It assigns points for risk factors and symptoms, stratifying patients into low, moderate or high probability before imaging.
D-dimer blood test is used in low-to-moderate probability cases — a negative D-dimer effectively rules out DVT in most patients. A positive D-dimer (which is non-specific and raised in many conditions) requires imaging to confirm or exclude a clot. Compression ultrasound of the leg veins is the primary imaging test — it is non-invasive, widely available and highly accurate for proximal DVT.
"A DVT that is found and treated is a near-universal recovery story. A DVT that becomes a pulmonary embolism is a different story entirely."
Dr. Lina Haddad, CMO, Looms
Treatment
Anticoagulation (blood thinning) is the mainstay of DVT treatment. Direct oral anticoagulants (DOACs) — rivaroxaban, apixaban — are now first-line and do not require routine monitoring. Treatment typically lasts 3–6 months for a provoked DVT and indefinitely for unprovoked or recurrent clots.
Below-knee compression stockings reduce the risk of post-thrombotic syndrome — persistent swelling, pain and skin changes in the affected leg. They are recommended for all patients with proximal DVT.