Abstract
BACKGROUND: The factor V (FV) Leiden and prothrombin (PT) G20210A polymorphisms in heterozygous state are present in 5% and 1–2% of Caucasians, respectively, and confer approximately 5-fold and 2-fold increases in the risk of incident venous thromboembolism (VTE). While some families who carry these genetic thrombophilia traits exhibit a prothrombotic phenotype, others have no (or only a limited) history of VTE. The ability to discern which individuals with personal and familial carriage of these genetic thrombophilias possess a clinically meaningful increase in VTE risk remains elusive, and (particularly among children) is perhaps best informed presently by family history of VTE.
OBJECTIVE AND HYPOTHESES: We sought to evaluate overall plasma coagulative capacity in FV Leiden and PT G20210A heterozygotes using the Clot Formation and Lysis (CloFAL) assay, a global turbidimetric plasma assay of tissue-factor induced fibrin clot formation and tissue-type plasminogen activator enhanced fibrinolysis. We hypothesized that children heterozygous for either thrombophilia would not uniformly demonstrate hypercoagulability, but that coagulative capacity would be increased among heterozygotes who have a family history of VTE.
PATIENTS AND METHODS: Children aged birth to 18 years (inclusive) enrolled in prospective inceptional cohort study of thrombosis/thrombophilia/stroke were included in the analysis if they were found to be heterozygous for FV Leiden or PT G20210A upon comprehensive thrombophilia testing and had undergone CloFAL assay testing on a research basis. Data on personal and family history of thrombotic events, thrombophilia testing, and CloFAL assay findings were analyzed. Intergroup comparisons of continuous data were performed by Mann-Whitney U test and proportions were compared between groups using chi-square or Fisher’s exact test, as appropriate.
RESULTS: Characteristics of the study population are shown in Table 1. Approximately 70% of patients were evaluated for a family history of VTE (with/without known thrombophilia) and nearly 50% had personal histories of VTE or arterial ischemic stroke (AIS)/recurrent transient ischemic attack (TIA); those evaluated for events were significantly older than those without events, and this difference was statistically significant among those with a positive family history fo VTE. Hypercoagulability was shown in 50% of patients and hypofibrinolysis in 13% using the CloFAL assay. Plasma coagulative capacity and maximal amplitude (MA) of the CloFAL waveform were significantly increased in patients with, versus without, family history of VTE (coagulation index, CI: 102% vs. 72% of the adult normal pooled plasma standard, respectively, p=0.04; MA: 0.415 vs. 0.322, p=0.02), and were not explained by age differences between groups. However, in this relatively small study population, the proportion of CloFAL CI results that exceeded the upper limit of normal values did not significantly differ between those with, versus without, family history of VTE. Pediatric FV Leiden or PT G20210A heterozygotes with positive family history of VTE were more likely to have multi-trait (>1) thrombophilia, in which case a trend toward increased plasma coagulability was demonstrated (CI: 139% [multi-trait] vs. 86% [isolated trait]; p=0.07); superimposed thrombophilias in this group most often consisted of elevated factor VIII activity and Lp(a) concentration.
CONCLUSIONS: The present findings using the CloFAL global assay indicate that, while pediatric FV Leiden or PT G20210A heterozygotes do not uniformly exhibit hypercoagulability, plasma coagulative capacity is nevertheless significantly increased among heterozgyotes who have a family history of VTE, which may relate to the presence of superimposed thrombophilias.
Table 1. Summary characteristics of the study population.
*VTE, AIS, or recurrent TIA **Two patients were dual heterozygotes. N 32 Median age at evaluation (range) 9.5 y (1–18 y) Personal history of events* 14 y (1–18 y) No personal history of events* 8 y (2–18 y) FV Leiden heterozygote (n) 26** PT G20210A heterozygote (n) 8** Personal history of VTE (n) 11 Personal history of AIS/recurrent TIA 4 Family history (1st/2nd degree) of VTE 71% Multi-trait (>1) thrombophilia 45% Acquired thrombophilia 24% Hypercoagulability by CloFAL assay 50% Hypofibrinolysis by CloFAL assay 13%
Clinical Utilization and Cost of Thrombophilia Testing in Patients with Venous Thromboembolism