Asymptomatic homozygous nt 20210 G to A prothrombin polymorphism in two blood donors belonging to two different kindreds

The nucleotide (nt) 20210 G to A prothrombin polymorphism has been associated with an increased incidence of thrombosis, particularly venous thrombosis. The increased incidence of thrombosis resulted from an increase in prothrombin levels. Two homozygous patients with this abnormality were found to be completely asymptomatic. In one of the two cases there was no thrombosis even after the additional risk factor of oral contraceptive therapy for several months. Prothrombin activity and antigen as assayed by several methods were at the upper limits of normalcy in one case and slightly elevated in the other. The two cases described in this article do not rule out the possibility that this prothrombin abnormality be associated, given special acquired circumstances, with thrombosis. However, they indicate that extreme caution Is needed to avoid an inaccurate conclusion. The fact that homozygous patients may remain asymptomatic indicates that the prothrombotic defect is very mild, if any.