Activated partial thrombocplastin time (aPTT) measures the efficiency of the contact activation of clotting. It depends on factor XII, XI, IX, VIII, V and X.
Plasma is incubated with a substance like kaolin or elagic acid to bring about contact activation of factor XII to XIIa. XIIa cleaves XI to XIa. The subsequent steps in coagulation are calcium dependent and take place only when calcium is added. Phospholipid is added to provide a platelet sunstitute.
The test involves mixing platelet poor plasma and a factor XII activator e.g. silica, celite, kaolin, micronized silica or ellagic
acid with phospholipid containing reagent. Calcium chloride is then added. The time take for a clot to form after adding calcium chloride is the activated partial thromboplastin time. The test is performed with paired samples one test and one control.