Influence of sample predilution on the sensitivity of prothrombin time in feline plasma

The prothrombin time (PT) is measured in feline plasma usually by means of a method that has been optimized for measurements in human plasma (standard test). In the present study the sensitivity of this test was investigated in comparison to a modified PT using a human placental thromboplastin. In the modified test, 100 microliter sample predilution (1:5, 1:10, and 1:20) were incubated with 100 microliter of a human fibrinogen solution (2 g/l) until 100 microliter Ca-thromboplastin was added. When measured in 31 samples with a reduced, and 22 samples with an increased activity of the coagulation factors II, V, VII and/or X the modified test showed an increased sensitivity as well as a correlation between PT and the changes in single factor activity which was distinctly closer compared to the standard test. The highest sensitivity was produced by a 1:10 or 1:20 sample dilution. A test procedure modified in this way, is thus, a more reliable screening test for the extrinsic system in cats and should be used for routine diagnostics.