This test is also known as the Partial Thromboplastin Time with Kaolin (PTTK) and the Kaolin Cephalin Clotting Time (KCCT) reflecting the methods used to perform the test. The APTT is the main screening test for the Intrinsic clotting system and is the second most common coagulation test being exceeded in frequency only by the prothrombin time. The Partial Thromboplastin of the APTT is a phospholipid extract of animal tissue or from vegetable sources. The phospholipids act as a platelet substitute in the Intrinsic system. The test incorporates an activator, in the absence of added Thromboplastin, which shortens the test and increases the precision and reproducibility by eliminating the variable effects of contact from glass surfaces and by effecting maximum activation. The activator is used to stimulate the production of FXIIa by providing a surface for the function of high molecular weight Kininogen, Kallikrein and FXIIa. The contact activation occurs for a time at 37oC. Calcium is then added to trigger further reactions and the time required for clot formation measured. Standardised Phospholipids are required to form complexes, which activate FX and Prothrombin, which allows the test to be conducted in patient Platelet poor plasma (PPP). The test depends not only on the contact factors and factors VIII and IX, but also on the reactions with factors X, V, Prothrombin and Fibrinogen. It is also sensitive to the presence of circulating anticoagulants (inhibitors) and Heparin.
Preparation of patients: Patients should be relaxed pre-venepuncture. Excessive stress and exercise will increase Factor VIII, vWF antigen and fibrinolysis. Veno-occlusion should be avoided.
Precautions: APTT results may be affected by many commonly administered drugs and further studies should be made to determine the source of unexpected abnormal results. Oral contraceptive and Oestrogen therapy in males have been found to decrease APTT in vivo. Conversely, Heparin, Warfarin, Lupus anticoagulant and radio therapy have been found to increase APTT in vivo.
