Multicenter evaluation of a chromogenic substrate method for photometric determination of prothrombin time

A multicenter study of a chromogenic substrate method for photometric determination of prothrombin time was conducted in order to evaluate its clinical application. Seven laboratories participated in the study using a total of 742 plasma samples from 417 patients on oral anticoagulant therapy, 261 healthy subjects and 64 patients with different diseases especially of the liver as well as 30 patients with hereditary deficiency of coagulation factors II, V, VII, X. The chromogenic PT method was compared to a standardized coagulometric PT assay which uses the same sensitive human placenta thromboplastin calibrated against international reference preparations. A high correlation of the prothrombin ratio values of the chromogenic and the coagulometric assay was obtained in 402 plasma samples (r = 0.940; y = 1.02x - 0.1). The study showed that the chromogenic PT reagent is sensitive to deficiency of the coagulation factors of the extrinsic pathway but not affected by heparin up to 1 IU/ml because of the heparin antagonist added. The precision (coefficient of variation) of the photometric method ranged between 0.6 and 3% (intraassay CV) and between 1.4 and 5.8 (interassay CV). The International Sensitivity Index (ISI) obtained for the used lot was 1.09. The therapeutical range in percentage activity for patients in a stable phase of an anticoagulant therapy was found to be from 15 to 27 percent of normal. The results of the clinical evaluation proved the good comparability of the new chromogenic PT test with coagulometric methods, its high factor sensitivity, good reproducibility and easy performance.