Summary
Plasma levels of total prothrombin and fully-carboxylated (native) prothrombin were
compared with results of prothrombin time (PT) assays for patients undergoing oral
anticoagulant therapy. Mean concentrations of total and native prothrombin in non-anticoagulated
patients were 119 ± 13 µg/ml and 118 ± 22 µg/ml, respectively. In anticoagulated patients,
INR values ranged as high as 9, and levels of total prothrombin and native prothrombin
decreased with increasing INR to minimum values of 40 µg/ml and 5 µg/ml, respectively.
Des-carboxy-prothrombin increased with INR, to a maximum of 60 µg/ml. The strongest
correlation was observed between native prothrombin and the reciprocal of the INR
(1/INR) (r = 0.89, slope = 122 µg/ml, n = 200). These results indicated that native prothrombin varied over a wider range
and was more closely related to INR values than either total or des-carboxy-prothrombin.
Levels of native prothrombin were decreased 2-fold from normal levels at INR = 2,
indicating that the native prothrombin antigen assay may be a sensitive method for
monitoring low-dose oral anticoagulant therapy. The inverse relationship between concentration
of native prothrombin and INR may help in identification of appropriate therapeutic
ranges for oral anticoagulant therapy.
