Inherited thrombophilia is a genetically determined tendency to thrombosis.¹ In 1965 the first family with antithrombin deficiency was described, and for many years this was the only identifiable cause of thrombophilia.² More recently, pedigree and case–control studies have confirmed that the risk of venous thrombosis is increased by deficiencies of antithrombin, protein C, and protein S, and by resistance to activated protein C.^3–5 Other candidate genetic factors are included in table 1.J Clin Pathol 2000;53:167–170

The value of obtaining laboratory evidence of thrombophilia is the ability to predict the likelihood of recurrence in symptomatic patients and the risk of thrombosis in their relatives. Thus thrombophilia testing would be used to optimise the benefit/risk ratio of anticoagulant treatment. Therapeutic recommendations would have to be based on a risk–benefit analysis that considers the risk of the disease, the effectiveness of treatment, the risk of treatment, and the predictive value of the laboratory tests used to establish the diagnosis of thrombophilia. Venous thromboembolism is a common disease with a significant risk. The risk of death from recurrence in the first three months after a pulmonary embolus is 1–2%.⁶ The risk of recurrence of deep vein thrombosis is 17.5% after two years and 24.6% after five years.⁷ Treatment with oral anticoagulation is extremely effective as long as the international normalised ratio (INR) is maintained above 2.0.^8,9 However, this treatment is potentially dangerous—in any one year there is a 1% chance of a major haemorrhage, and one quarter of these are fatal.¹⁰ Therefore the disease carries a high risk, and treatment, while effective, is also associated with significant danger. The ability to distinguish patients at high and low risk of thrombosis would help to optimise therapeutic decisions. The identification of laboratory evidence of thrombophilia would seem …