It is important to confirm that the aPTT is reproducibly prolonged. Falsely prolonged values can be caused by pre-analytic variables such as improper specimen collection (eg, an underfilled tube). For patients with vascular access lines, heparin contamination is common even when the line is flushed with saline.1
The strategies outlined below may help identify the etiology of the prolonged aPTT:
a. Mixing study: this test may help guide the evaluation in the direction of either an inhibitor or a factor(s) deficiency.
b. Specific factors and/or inhibitor panels (eg, a lupus anticoagulant evaluation): the selection of these tests is based on the results of other screening tests (eg, thrombin time and prothrombin time [PT]/international normalized ratio) and the patient’s symptoms (bleeding/bruising, thrombosis, or asymptomatic).