RESUMEN
Five APTT reagents, a heparin clotting assay and a chromogenic assay have been tested at 13 centres using lyophilised plasma containing different levels of added heparin (in vitro samples), and fresh normal plasma samples and samples from patients receiving heparin therapy (ex vivo samples). Marked differences in sensitivity between the reagents to in vitro and ex vivo samples were found. When calibration of the reagents was attempted using orthogonal regression analysis based on the P.T. ISI/INR system, the results were disappointing. Whichever reagent was accepted as standard, the between laboratory differences in ISI's were very large, and the within laboratory and between laboratory CV's were unacceptably high. Regression lines through normal and patient clotting times were commonly non parallel or skewed. From the results of only 5 out of more than 30 commercial reagents, it is concluded that the application of such a system would give no confidence in any individual result and could be misleading.