Prothrombin time (PT) and activated partial thromboplastin time (APTT) are basic laboratory tests for diagnosis of bleeding diseases such as congenital hemophilia. In addition, abnormally prolonged clotting times are observed in patients who have inhibitors. Lupus anticoagulant and inhibitors against coagulation factors are two clinically important inhibitors. However, non-specific inhibitors are also observed. Differential diagnosis of these inhibitors is important. Clotting times are also prolonged by artifactitious factors, such as contamination of heparin. Mixing tests are relatively easy and useful methods which potentially differentiate these abnormalities. This article is summarized the causes of prolongation of clotting times and the usefulness of mixing tests for differentiation of the abnormalities.