Heparin anti-Xa assays can be utilized to monitor unfractionated heparin therapy, but primarily are used to monitor low molecular weight heparin (LMWH) therapy in the clinical laboratory. Though the aPTT test may be the primary assay used in certain facilities, many laboratories have added this test to their coagulation menu. Physicians have increasingly opted to order this assay for patients who may not be responding as expected to their heparin therapy according to the aPTT results. In this situation and in other conditions including: pulmonary embolism, deep venous thrombosis, presence of lupus anticoagulant, and more, the anti-Xa assay can more accurately reflect a patient's therapeutic heparin level compared to the aPTT test.
The principle of the test involves the initial binding of heparin to antithrombin. This binding forces a conformational change in antithrombin, which increases its role in the inhibition of clotting. The formation of an inactive antithrombin-Xa complex is achieved in patients undergoing heparin therapy with residual Xa left in the sample. The residual Xa is then measured using either a clotting-based or chromogenic assay. Standard curves are created by assaying the factor Xa with different known dilutions of heparin. Patient results are then extrapolated by evaluating the residual Xa activity, which is inversely proportional to the concentration of heparin in the sample.