Defining Heparin Resistance: What do Laboratory Professionals Need to Know? 

In laboratory medicine, patients can be harmed when we stray from evidence-based practices. Such is the case in the assessment of antithrombin (AT) levels, a critical test used to identify heparin resistance among patients receiving unfractionated heparin (UFH) or low molecular weight heparin (LMWH). Anticoagulant agents derived from heparin (UFH, LMWH, fondaparinux) contain a pentasaccharide sequence specifically designed to bind to antithrombin. This interaction significantly enhances the inhibitory activity of antithrombin against its target, the activated coagulation factors.1 Consequently, blood-thinning medications derived from heparin cannot function optimally without an adequate supply of antithrombin. Heparin resistance, a form of drug resistance often encountered in clinical settings, is defined as the need for unusually high heparin doses to achieve a targeted level of anticoagulation. One definition refers to patients requiring 35,000 units or more of heparin per day to achieve therapeutic levels. There is no consensus, however, on the anticoagulation target or the definition for resistance.1 Mechanisms of heparin resistance include AT deficiency, increased heparin binding by plasma proteins, increased heparin clearance, elevation of fibrinogen and factor VII levels, and interaction with some medications.

AT levels can be evaluated either by measuring the activity (functional assays) or by quantifying the protein (antigen immunoassays). Functional assays, recommended as the first line of testing for monitoring UFH, could be thrombin-based assays or factor Xa-based chromogenic assays. Functional assays work by incubating the patient sample with a known concentration of thrombin in the presence of UFH, and the residual thrombin quantified by use of a specific chromogenic substrate. The residual thrombin level (as measured by chromophore release), is inversely proportional to the AT level in the sample plasma. In other words, the smaller the concentration of functional active AT, the higher the absorbance signal measured by the instrument.

When utilizing activated partial thromboplastin time (aPTT) as the primary monitoring test for UFH, elevated levels of factor VIII (FVIII) can reduce the accuracy of the test, resulting in a patient appearing to be heparin resistant.1 In a slightly different scenario, suspicion of AT deficiency may arise if a patient received a full therapeutic dose of UFH and subsequently displays unexpectedly low or subtherapeutic levels when assessed using the chromogenic anti-Xa assay, since anti-Xa assays used by most laboratories do not include AT supplementation. Given the pivotal role of antithrombin as a plasma coagulation inhibitor, this test is typically performed at specialized coagulation laboratories. AT deficiency, which is associated with increased risk of venous thromboembolism (VTE), may manifest as type I AT deficiency, marked by low AT antigenic levels, or as type II AT deficiency, characterized by a reduced AT activity level relative to antigen levels.2 The relevance of accurate testing and adherence to evidence-based protocols in this context cannot be overstated, as the consequences can be dire for patients.

Laboratories that employ anti-Xa assays supplemented with AT may hinder the identification of AT deficient patients receiving UFH or LMWH, resulting in potential clotting outcomes. 1 Consequently, patients with potential heparin resistance should be assessed by a chromogenic anti-Xa assay without AT supplementation, along with an AT functional test to evaluate whether alternate anticoagulation strategies should be used. 3-4 Furthermore, pediatric coagulation testing guidelines discourage the use of AT-supplemented anti-Xa assays in pediatric patients receiving UFH or LMWH due to the lack of an accurate, physiologically relevant measure of hemostasis in this population.5

While inherited antithrombin deficiency is a rare occurrence, hospitalized patients frequently exhibit AT levels below the lower threshold of normal (ranging from 80-120%).6 This includes patients with conditions such as liver disease, sepsis, overt disseminated intravascular coagulation (DIC), leukemia patients undergoing asparaginase treatment, and those requiring mechanical life support, such as extracorporeal membrane oxygenation (ECMO).6 Taking a broader view, for hospitalized patients in need of anticoagulation therapy, heparin and related parenteral blood thinners are commonly administered. This preference is due to factors such as cost-effectiveness, the extensive knowledge and experience of clinicians in drug management and monitoring, a relatively short half-life of 60-90 minutes, and the ease of reversibility.8

Current clinical guidelines recommend use of a chromogenic AT assay as first line of testing if AT deficiency is suspected, followed by an antigen assay to differentiate type II from type I AT deficiency.2,9 AT activity assays that employ thrombin as the activator are less prone to interference from direct oral anticoagulants, such as apixaban, rivaroxaban, and edoxaban.2 In addition, AT activity assays using bovine sources for thrombin will not be affected by heparin cofactor II, another plasma thrombin inhibitor.

In summary, AT testing holds significance not only within the traditional thrombophilia panel but also in evaluating the anticoagulation response among patients receiving unfractionated heparin or low molecular weight heparin who are suspected of heparin resistance. Employing the anti-Xa assay, followed by a chromogenic AT level assessment, can prove invaluable in identifying patients who do not exhibit the expected response to UFH and LMWH. This approach equips clinicians with precise information, enabling them to make informed decisions, including the potential transition to alternative anticoagulants when necessary.

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