Quality Control Trials in the National Reference Thromboplastin Scheme

S ummary . In British hospitals routine measurements of prothrombin times are made either with a standardized thromboplastin reagent (the Manchester Comparative Reagent–MCR) or with the hospitals' own reagents, which have first been calibrated in terms of the national reference thromboplastin (the British Comparative Thromboplastin‐BCT). The fundamental assumption in the National Reference Scheme is that hospitals will obtain standardized results with the uniform technique and the BCT. Four collaborative studies have been conducted to evaluate this assumption. Instead of assessing their own local reagents hospitals were asked to calibrate given suspensions of unknown thromboplastin in terms of the BCT. In the last trial they were also asked to test three lyophilized plasma preparations. Since aliquots of given thromboplastin suspensions and dried plasma samples were distributed, all hospitals should have obtained similar results. There was, however, a wide variation, both in the ratios with the unknown thromboplastins that were equated with particular ratios with the BCT, and in the readings for the plasma samples. This variation was partly due to a tendency for some hospitals to obtain consistently higher readings than others. The situation might be improved by extending technical training in the prothrombin time test. If the differences between hospitals could be shown to be stable over long periods, a new system of quality control, involving the routine use of lyophilized plasma in addition to the BCT, might also be justified.