The APTT is a sensitive screening test to detect deficiencies/abnormalities of the intrinsic coagulation pathway and for monitoring heparin therapy.
If an abnormal APTT result is obtained and the form does not state that the patient is on heparin, the laboratory may perform correction studies.
The reflexive tests often performed are:
APTT 1+1: The APTT is performed using a 1:1 mix of patient plasma with normal plasma. Shortening of the APTT back to normal with the 1+1 mix indicates a coagulation factor deficiency. No correction or only partial correction, suggests heparin, lupus anticoagulant or the presence of a coagulation factor inhibitor.
Dilute Thrombin Clotting Time (DTCT): A sensitive test for inhibitors of thrombin eg. heparin and dabigatran. Addition of Protamine sulphate inhibits heparin but not dabigatran.
Occasionally performed:
APTT with Actin: Actin is an alternative APTT reagent which is insensitive to lupus anticoagulant. A normal Actin-APTT rules out significant factor deficiency if the routine APTT is prolonged and can be used to monitor heparin therapy if the patient has a lupus anticoagulant.
Rarely performed:
APTT 1+1 2 hour: This test is only performed if a specific factor inhibitor is suspected and is used to detect time dependent inhibitors (most commonly against factor VIII).
Interpretation of APTT:
Causes for a prolonged APTT include; liver disease, anticoagulant therapy, disseminated intravascular coagulation, deficiencies in factors VIII, IX, XI or XII, factor inhibitors (factor VIII inhibitor is the most common), acidosis and lupus anticoagulant.
A shortened APTT usually suggests factor activation e.g. difficult collect. If the APTT is consistently short, this likely indicates high levels of FVIII activity.