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Blood. 2005 Oct 1;106(7):2363-5. Epub 2005 Jun 16.

An underestimated combination of opposites resulting in enhanced thrombotic tendency.

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Department of Medical and Surgical Sciences, Second Chair of Internal Medicine, University of Padua Medical School, Padua, Italy.


Heterozygous carriers of factor V (FV) Leiden who also carry FV deficiency often develop venous thromboembolism, but the thrombosis risk associated with this rare condition (pseudohomozygous activated protein C resistance) is still unclear. The thrombosis risk of genetically characterized pseudohomozygotes (n = 6) was compared with that of FV Leiden heterozygotes (n = 683) and homozygotes (n = 50) recruited within a large cohort study on familial thrombophilia. Both thrombin generation and Kaplan-Meier thrombosis-free survival analyses were performed in different FV genotype groups. FV Leiden pseudohomozygotes showed significantly higher thrombosis risk than heterozygotes. The thrombin generation test in pseudohomozygotes showed a pattern similar to homozygotes. Accordingly, early thrombotic manifestations occurred in pseudohomozygotes at a similar rate as in homozygotes. Thus, failure to recognize FV deficiency in FV Leiden heterozygotes may result in an underestimate of the thrombosis risk and inadequate management of affected patients.

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