External Quality Assessment for Unfractionated Heparin Monitoring: An Update from Australasia/Asia-Pacific

Unfractionated heparin (UFH) remains a major anticoagulant therapy applied within the acute hospital system. Due to intra- and interpatient variability, UFH monitoring needs to be applied to ensure patients remain free of thrombotic and bleeding complications at too low and too high an UFH level, respectively. Monitoring of UFH therapy is usually achieved using either an activated partial thromboplastin time (aPTT) or an anti-factor Xa (anti-Xa) method. The former is a clotting assay that evaluates both the anti-factor IIa (anti-IIa or anti-thrombin) and anti-Xa anticoagulant activity of UFH and the latter is a chromogenic assay that evaluates just the anti-Xa anticoagulant activity of UFH. The aPTT method is perhaps more widely utilized since aPTT testing is performed by all hemostasis laboratories performing routine coagulation tests. However, the aPTT method requires establishment of an aPTT UFH therapeutic range. The anti-Xa method is favored in larger hospital sites and by most experts and uses a standard UFH therapeutic range. We report findings for aPTT and anti-Xa testing for UFH monitoring in our geographic region using recent data (testing for the past 5 years; 2020-2024 inclusive) from the Royal College of Pathologists of Australasia Quality Assurance Program, an international external quality assessment (EQA) program, with over 110 enrolments for this EQA module. Four samples are assessed each year, with these comprising various levels of UFH. Good reproducibility was observed for duplicate samples sent in different surveys. Coefficient of variation (%) data revealed moderate variation for samples containing UFH (10-40% for anti-Xa; 10-25% for aPTT). Anti-Xa reagents containing dextran sulphate tended to yield higher anti-Xa values than those without. Interpretations regarding UFH levels being below, within, or above therapeutic levels were generally reported as expected, according to the level of UFH present in the sample, especially for anti-Xa testing.