The 50:50 mixing study

What it is

The mixing study is done on a prolonged APTT. A 1:1 mix of the patient with a control is made, and the APTT of that mix is taken twice, at 0 hours and again at 2 hours, alongside the patient's own APTT and the control's APTT. One described set of results: an APTT of 68 seconds against a control's APTT of 36s, with the 1:1 mix 68s at 0 hours and 65s at 2 hours, that is, a mix that shows no correction. The rest of the coagulation screen is read with it, and in that same set the other values were normal: a PT of 11 seconds against a normal range of 9-12s, an APTT of 68 seconds against a normal range of 28-40s, a thrombin time of 16 seconds against a normal range of 12-18s, and a fibrinogen of 2.5 g/L against a normal range of 1.7-3.6 g/L.

Causes and risk

The study is asked for because an isolated prolonged PTT has a short list of causes: deficiency of a coagulation factor in the intrinsic pathway, contact factor deficiency, and a circulating anticoagulant, that is an inhibitor, of which lupus anticoagulant is the named example. When the PT is prolonged alongside the PTT the list is a different one: massive transfusion of PRBC, disseminated intravascular coagulation, liver disease, direct thrombin inhibitors, thrombolysis, combined factor deficiency, and common pathway factor deficiency.