aPTT tests the intrinsic and common pathways of coagulation

Though it is included commonly as a part of coagulation profile assessment, it’s primary uses are to detect coagulation factor deficiency and titration of heparin therapy

An isolated elevation of aPTT may indicate

deficiency of Factor VIII or IX or XI or XII

acquired clotting factor inhibitors

presence of Lupus anticoagulant

N.B.:- Factor VIII deficiency is Haemophilia A, Factor IX deficiency is Haemophilia B and Factor XI deficiency is Haemophilia C

If factor levels are >30% of normal, aPTT may remain normal, for e.g. in mild von Willebrand disease [raised aPTT + prolonged Bleeding Time (BT)], in mild hemophilia etc

Reference: Martlew V. Peri-operative management of patients with coagulation disorders. Br J Anaesth. 2000; 85(3): 446–455.