Sensitivity of different prothrombin time assays to factor VII deficiency in canine plasma

The factor VII sensitivity of prothrombin time (PT) in dogs was tested using five different PT reagents and a commercial PT variant. The five PT reagents were used according to manufacturers' instructions (standard test, PT([ST])) and also using a modified test instruction (modified test, PT([MT])). Plasma samples with defined factor VII levels (10-100%) were prepared by adding increasing quantities of canine factor VII deficient plasma to the pooled plasma of healthy dogs. Statistical comparison based on prothrombin time ratios (PTR = PT sample: PT measured for 100% factor VII activity level) revealed significant differences between different reagents for PT([ST]) and also for PT([MT]). Factor VII activity at which PT was prolonged to the upper limit of the reference values (FVII([X(0.975)])) was 16-39% (PT([ST])) and 23-35% (PT([MT])). Factor VII sensitivity measured by PTR and also by FVII([X(0.975)]) values, was higher in four of five PT reagents using PT([MT]) when compared with PT([ST]). The results of this study indicate the importance of selecting a sensitive reagent and method for PT measurement and for careful interpretation of PT test results using canine plasma.