Haemophilia A severity is closely correlated to the factor VIII (FVIII) activity, which can be measured in different ways. The original one‐stage clotting assay is still the most widely used. The two‐stage coagulation assay eliminated many of the drawbacks of the one‐stage assay and was further developed into the chromogenic assay, a two‐staged test with purified coagulation factors in the first stage, and a FXa‐specific chromogenic substrate in the second stage. In many patients with mild or moderate haemophilia A, there is a discrepancy between the one‐stage and the two‐stage assays. If only the one‐stage assay is used, some patients will have normal FVIII levels and not be diagnosed as having haemophilia or be considered to have a milder bleeding risk than is the case. Other patients who have normal FVIII activity will be diagnosed as haemophilia A. All haemophilia treatment centre laboratories should have access to both one‐stage and chromogenic FVIII:C assays. Appropriate standards should be employed to enable accurate FVIII:C measurement. Different assays to measure inhibitor activity to infused FVIII have been developed since 1959. Inhibitor results based on the one‐stage or chromogenic FVIII:C assays are well correlated, but the one‐stage assay may be influenced by nonspecific inhibition.