Antithrombin-III assay without influence of heparin cofactor II

A new biological method for the assay of plasma antithrombin-III (AT-III) activity was developed without influence from heparin cofactor II (HC-II). AT-III deficient plasma is used as a substrate and diluted prothrombin time reagent as a reaction trigger for the specific assay of AT-III. The AT-III deficient plasma is prepared by passage of plasma through a heparin-agarose column. In the presence of heparin, AT-III in the sample shows concentration dependent anticoagulant activity and calibration curve is linear on semi-logarithmic graph paper. The results of reproducibility, recoveries and correlation studies with a chromogenic assay indicate that this biological method is reliable and suitable for routine use in clinical laboratories. Influence of HC-II is minimal. The method provides several advantages over those of chromogenic substrate and fibrinogen.