Figure 1.

The ‘intrinsic’ and ‘extrinsic’ parts of the clotting cascade. Activated partial thromboplastin time (APTT): contact activator, for example, kaolin or micronised silica is added to the test sample which produces factor XIIa, this activates the other clotting factors in the cascade. The APTT provides information on the ‘intrinsic’ pathway. Prothrombin time (PT): thromboplastin which contains tissue factor is added to the patient's plasma which leads to the activation of factor VII. The PT provides information on the ‘extrinsic’ pathway.