Comparison of lyophilized plasmas with fresh plasmas for calibration of thromboplastin reagents in oral anticoagulant control.

In a multicentre study of four thromboplastin reference materials (three from rabbit brain and one from bovine brain combined with adsorbed bovine plasma), three types of lyophilized plasmas were included. One was a pooled normal plasma, another was made by pooling plasmas of coumarin-treated patients, and the third was prepared by adsorption of vitamin K dependent clotting factors (artificially depleted). The purpose of this study was to investigate whether these lyophilized plasmas could be used for reliable calibration of the four thromboplastins. Calibration line slopes were calculated for the lyophilized plasmas and compared to the slopes based on fresh plasmas. When a rabbit thromboplastin was calibrated against a similar but not identical reagent (like-to-like comparison), the slopes for lyophilized plasmas and for fresh plasmas were the same. However, significant differences between lyophilized and fresh plasmas were observed in the calibration of unlike thromboplastins. The deviation of the slope with artificially depleted plasma was greater than that with lyophilized coumarin plasma. Indiscriminate use of these lyophilized plasmas for thromboplastin calibration should be avoided.