The ICTH/WFH study of the partial thromboplastin time in mild haemophilia.

A study has been carried out in two stages to compare the sensitivity of different Partial Thromboplastin Time (P.T.T.) methods to partial deficiencies of factor VIII. In Stage 1, 63 laboratories throughout the world compared their own methods with a reference method, all using samples of the same three mild haemophilic plasmas. Wide variation was found between laboratories using the same method, especially for those methods which were not well standardised. There was fairly good agreement for the reference method. All methods correctly diagnosed the three mild haemophilic plasmas, but their factor VIII levels were rather low. Stage 2 was restricted to seven laboratories in Great Britain, comparing seven well-standardised commercial methods and the same reference method. A much wider range of abnormal plasmas was tested, and a variety of normal ones. P.T.T.'s for all plasmas (factor VIII levels from 5 to 58 iu/dL) fell outside the normal range by all methods, with a few exceptions. It was not possible to rank the methods in any order.