Clinical features

VTE is increased 2- to 6-fold in heterozygotes and 50- to 100-fold in homozygotes. Most individuals with FVL will not develop thrombosis; other risk factors (e.g. trauma, surgery, OCP, pregnancy) are present in >50% of patients who develop a thrombotic event, and increasing age is a major risk factor. Recent studies indicate little if any value of case-finding in relatives of affected symptomatic patients.

Pregnancy—risk of VTE estimated from personal and family history will determine whether antenatal or postnatal prophylaxis is required..

OCP users have 4 x VTE risk compared to non-users generally. The FVL mutation increase the risk a further 7-fold. Thus the absolute annual VTE risk in women not taking COCs is 0.5/10,000. In women taking COCs the risk is 2/10,000. In women COC users with the FVL mutation the risk is 15/10,000.

The FVL mutation is a minimal risk factor for arterial thrombosis and of no consequence compared to typical risk factors such as smoking, hypertension and hyperlipidaemia.

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