Factor VIII activity has been historically measured with two types of methods: the one-stage assay, based on the activated partial thromboplastin time (APTT) (Chapter 5) and the two-stage assay, based on the thromboplastin generation test. The relative merits of the two assays have been debated1,2. In general, results obtained on the same samples are similar; however, discrepancies can be seen in plasmas which contain activated factors of the coagulation cascade. These factors make the clotting times obtained with the one-stage assay shorter than expected on the basis of the factor VIII content, resulting in a significant overestimation of the factor VIII activity. For the same reason, the one-stage assay is often unsuitable to determine factor VIII in concentrates. Even though the two-stage assay does not seem to suffer from the same drawback, it is difficult to perform because the reagents employed need a high degree of standardization difficult to achieve in less specialized clinical laboratories.