Heparin Anti-Xa

Plasma, frozen

1 mL

Blue-top (sodium citrate) tube

Determine the plasma level of unfractionated heparin (UFH) or low molecular weight heparin (LMWH)

The heparin anti-Xa assay is based on the ability of heparin to inhibit the activity of activated factor X (Xa) in the reagent. The reagent includes excess antithrombin, making the heparin in the sample the rate-limiting reagent for Xa inhibition. Heparin in the patient sample inhibits the enzymatic conversion of a Xa-specific chromogenic substrate to colored product by factor Xa. Standard curves are created using multiple concentrations of UFH and LMWH and are used to calculate concentration in the patient plasma.

UFH and LMWH are anticoagulants commonly used to decrease the risk of venous or arterial thrombosis.^6-9 Overdosing with these medications can increase the risk of hemorrhage and inadequate dosing decreases the efficacy of anticoagulation. These drugs work as anticoagulants by enhancing the ability of plasma antithrombin to bind and inactivate the serine proteases XIIa, XIa, IXa, Xa, and thrombin. Therapeutic monitoring is commonly performed because of the wide interindividual variation in response to this therapy.

Historically, many laboratories have monitored heparin levels using the activated partial thromboplastin time (aPTT) test. A large number of conditions can complicate the use of the aPTT in monitoring UFH therapy.⁶ The College of American Pathologists (CAP) divided these factors into three groups.⁶ The first group of complicating factors encompasses those that affect the bioavailability of heparin. Aging, obesity, changes in heparin binding proteins, hepatic disease, renal disease, and heparin resistance fall under this heading. The second group of complicating factors includes those that alter the aPTT dose response to heparin. Increased factor VIII or fibrinogen levels, decreased antithrombin levels, or a mild reduction in multiple factors (as might be seen in the early stages of a consumptive coagulopathy or in oral anticoagulant therapy) are examples of this type of complicating factor. Lastly, those factors that tend to cause a prolonged aPTT in the absence of heparin therapy should be considered. Lupus anticoagulants or deficiency in contact factors can produce an extended baseline aPTT. In all of these cases, the heparin anti-Xa assay may be more appropriate for monitoring heparin therapy.

The aPTT should not be used for therapeutic monitoring of low molecular weight heparin because this drug typically does not affect the aPTT significantly.^6,7