A normal APTT does not exclude mild, but clinically significant, coagulation factor deficiency (eg, as in mild haemophilia, von Willebrand disease) as many reagents give a prolonged APTT only at coagulation factor levels of 30% or below, presumably.
An isolated prolongation of the APTT (PT normal) suggests deficiency of factor VIII, IX, XI or XII.
Prolongation of both the APTT and PT suggests factor X, V, II or I (fibrinogen) deficiency, all of which are rare. The APTT is normal in factor VII deficiency (PT prolonged) and factor XIII deficiency. See Figure 1.
A prolonged APTT which is not corrected by the in vitro addition of normal plasma suggests the presence of a coagulation factor (eg, VIII or IX) inhibitor or a lupus inhibitor.
Artefactual prolongation of the APTT may be due to:
the presence of heparin in the sample;
difficult or slow collection;
addition of an incorrect volume of blood to the tube;
delay in mixing blood with the citrate anticoagulant; or
suboptimal specimen storage or a prolonged interval between collection and testing.
See also Bleeding disorders and Anticoagulant monitoring.