The Measurement of Multifactorial Thrombophilia

Six years ago a group of experts summoned by the World Health Organization defined thrombophilia as a tendency to develop venous thromboembolism that may be genetically determined, acquired or both (1). Table 1 gives a list of inherited or acquired conditions that have been associated with thrombophilia and hence with an increased risk of venous thrombosis. The list is by no means comprehensive, not including other conditions that are still not well established and need confirmation (for instance, high levels of coagulation factors IX and XI) (2). An important point is that very often more than one thrombophilic condition is concomitantly present (2, 3), as it can be predicted from the fact that some of them occur frequently in the general population. Typical examples are the combination of the gainof-function factor V or prothrombin gene mutations with pregnancy or surgery. Most importantly, it has been firmly established that the effect of the presence of more than one thrombophilic condition on the relative risk of developing clinical manifestations of venous thromboembolism is not simply the sum of the weight of each relative risk, it tends to be multiplicative (4, 5). The knowledge that thrombophilia is often multifactorial has obviously led clinicians to require laboratory methods that measure the risk of thrombosis not only in the presence of single thrombophilic conditions but also when they interact. Ideally, the clinician requires a single laboratory test that correlates with the overall risk of each single patient. This test is currently not available. Examining Table 1, one realizes that the laboratory can separately identify with relative ease and accuracy such thrombophilic conditions as the deficiencies of naturally-occurring anticoagulant proteins (antithrombin, protein C, protein S), the gain-of-function mutations of coagulation factor genes, factor V Leiden and prothrombin G20210A; activated protein C (APC) resistance not attributable to the gain-of-function gene mutations; high levels of factor VIII and hyperhomocysteinemia, whether inherited or acquired; and antiphospholipid antibodies. The circumstantial conditions associated with thrombophilia (pregnancy, oral contraceptive intake, hormone replacement therapy, surgery, immobilization and cancer) are easily identified by the clinician. However, when he/she has to tackle the practical problem of the evaluation of the overall relative risk of thrombosis in the presence of interacting thrombophilic conditions there is still no way to measure it accurately. There have been attempts to develop laboratory methods that globally measure the existence and the degree of thrombophilia. Designing such global methods is complicated, because the single measurable conditions associated with thrombophilia are not always linked and interrelated. The most extensively investigated methods are those globally exploring the protein C (PC) anticoagulant pathway, based upon the activation of plasma PC by some venoms and the comparison of clotting times obtained with or without PC activators (6-8). In several studies (6-8) the diagnostic efficacy of these methods has been found acceptable for PC deficiency and APC resistance, but not for PS deficiency and obviously not for the deficiency of antithrombin, not involved in the PC pathway. In this issue of Thrombosis and Haemostasis, Curvers et al. (9) have explored the value of another method, i.e., a functional APC resistance method based upon the quantification of the effect of exogenous APC on thrombin formation initiated in plasma via the extrinsic coagulation pathway. This method is modified from the method of Hemker et al. (10, 11), that measures thrombin generation as endogenous thrombin potential (EPT) after activation of defibrinated plasma by means of intrinsic (cephalin) or extrinsic (tissue factor) activators. The method, as modified by Rosing et al. (12) by adding APC to the system and adopted by Curvers et al. (9), has immediately attracted widespread attention because it is to date the only laboratory test that consistently demonstrates a greater degree of plasma hypercoagulability in women taking third-generation oral contraceptives, who carry a higher risk of venous thromboembolism than those taking second-generation pills (12). The study of Curvers et al. (9), besides confirming that the method gives more abnormal results in women on third-generation contraceptives, shows abnormal results during pregnancy and hormone replacement therapy and in patients homozygous and heterozygous for the factor V Leiden and prothrombin mutations, but not in those with antithrombin and PC deficiency. The significant advantage of the study, however, is the demonstration that the method picks up the higher degree of APC resistance associated with the concomitant presence of more than one genetic or acquired thrombophilic condition (for instance, factor V Leiden in association with pregnancy or oral contraceptive intake); and that there is an excellent positive correlation between the degree of the abnormality and the relative risks of venous thromboembolism associated with the presence of single and combined thrombophilic conditions. Relative risks have not actually been established in the study of Curvers et al. but are derived from clinical studies published in the literature. This gives some space for bias in the choice, even though the studies chosen are among the largest and well designed available. A limit of the method is its lack of sensitivity to PC and antithrombin deficiencies. Limits of the study are the relatively small number of patients investigated, particularly those with combinations of thrombophilic conditions; and that some frequent conditions such as major surgery and cancer, alone or in combination, have not been investigated. Whether the test as modified by Rosing (12) is a truly more sensitive measurement of thrombophilia than the original method of Hemker, remains to be established (13-15). This notwithstanding, the badly needed method that measures multifactorial thrombophilia Correspondence to: P. M. Mannucci, Via Pace 9, 20122 Milano, Italy – Tel. : +39/02/55035421; Fax: +39/02/5516093; E-mail: pmmannucci@libero.it Commentary