aPTT Mixing Studies

An isolated prolonged aPTT result with a normal prothrombin time implies either the presence of heparin and inhibitor (either specific factor or lupus anticoagulant) or a factor deficiency of the intrinsic system.⁷ Unless the cause for the prolonged aPTT is known, as in the case of heparin therapy, mixing studies may be necessary to determine the etiology of the prolonged result. If the result obtained with the immediate normal plasma mixture corrects to within the reference range and the saline mixture result increases dramatically, a factor deficiency or specific factor inhibitor is suspected.⁷

With factor deficiency, heparin, or a specific factor inhibitor, the 1:1 saline mix shows significant prolongation of the aPTT result. If the result obtained with the immediate normal plasma mixture shows only partial or no correction and the result obtained with the saline mixture shows near correction to the reference range or only mild prolongation, then a nonspecific inhibitor, such as lupus anticoagulant (LA), is suspected. After incubation at 37°C, an aPTT result equivalent to that of the original mixture is indicative of a factor deficiency or heparin. If the result of the aPTT on the incubated mixture demonstrates further prolongation (with the control remaining equivalent to the original result), a specific factor inhibitor is indicated. A specific factor VIII inhibitor may show initial correction of the original mixture, but will demonstrate prolongation upon incubation.

If mixing studies indicate the possibility of single or multiple factor deficiencies, a PT may be performed to differentiate between common pathway and intrinsic pathway deficiencies. Factor assays should then be performed to determine specific factors involved. If a factor inhibitor is suspected, specific factor assays and inhibitor titers may be necessary. If heparin is suspected without clinical confirmation, a thrombin time with addition of heparin neutralizer should be performed on the original specimen. Plasma suspected of containing a lupus anticoagulant (LA) should be further tested with low phospholipid tests and associated confirmatory tests. A patient with an initial positive antiphospholipid antibody or lupus anticoagulant test should be tested after 12 or more weeks to determine if the antibody is persistent.