Activated Partial Thromboplastin Time (APTT)
The Activated Partial Thromboplastin Time (APTT) measures the combined activities of the clotting factors in the so-called 'intrinsic' coagulation pathway. This is shown in yellow in the diagram on the right and involves Factors II, V, VII, IX, X, XI and XII. The APTT is used for three distinct purposes:
- To screen for clotting factor deficiencies in a patient with a history of bleeding or bruising. Most APTT reagents will not be prolonged by a single clotting factor deficiency until the level falls below 40% of normal.
- To monitor the effects of anticoagulant drugs such as heparin, as well as the direct thrombin inhibitors (lepirudin, argatroban and bivalirudin). For this purpose the target APTT is typically 1.5 - 2.5 times the patient's pre-treatment (baseline) value.
- To test for the presence of a Lupus Anticoagulant (LA). However, it is important that a LA-sensitive reagent be used. We recommend the PTT-LA test if a LA is suspected.
The normal range for the APTT will vary with the choice of reagent and instrument. Each laboratory should therefore determine its own normal range. In the Hemostasis and Thrombosis Laboratory at Florida Hospital, the normal range is 22.4 - 38.6 seconds.
The most common reasons for a prolonged APTT are the presence of heparin and a Lupus Anticoagulant.
Prolonged APTT results are followed up with a mixing study in which normal plasma is mixed in equal volumes with the patient's plasma and the test repeated. Failure of the clot time to correct to within 5 seconds of that given by the normal plasma alone suggests the presence of an inhibitor. It should be noted that Factor VIII inhibitors, which can be associated with significant bleeding, may show correction in a so-called 'immediate' mixing study. An unexplained prolonged APTT, especially in a patient that is bleeding, should be evaluated with a complete Inhibitor Screening Test in which the patient and normal plasma are incubated together at 37oC for 2 hours. This procedure will detect FVIII inhibitors which are typically time- and temperature-dependent.