External Quality Assessment for Low Molecular Weight Heparin Monitoring in the Australasia/Asia-Pacific Region

Low molecular weight heparin (LMWH) remains a widely utilized parenteral anticoagulant in hospital and outpatient settings for thromboprophylaxis and treatment of venous thromboembolism. While LMWH typically does not require routine monitoring due to predictable pharmacokinetics, anti-factor Xa (anti-Xa) monitoring is indicated in specific populations, including those with renal impairment, extremes of body weight, pregnancy, or unexplained treatment failure. LMWH monitoring relies exclusively on chromogenic anti-Xa assays, as LMWH's predominant anti-Xa activity with minimal anti-IIa effect renders activated partial thromboplastin time insensitive. Therapeutic targets are context-dependent: peak anti-Xa levels of 0.5 to 1.2 U/mL for twice-daily therapeutic dosing or 1.0 to 2.0 U/mL for once-daily dosing, measured 3- to 5-hour postdose. We report external quality assessment findings for anti-Xa testing of LMWH from the past 6 years (2020-2025 inclusive) using RCPAQAP (Royal College of Pathologists of Australasia Quality Assurance Program) data, an international program with approximately 150 laboratory enrolments. Four samples are assessed annually at varied LMWH concentrations (0, 0.2-0.3, 0.5-0.6, 0.8-1.0 U/mL). Good reproducibility was demonstrated for duplicate samples across surveys. Coefficient of variation data revealed moderate variability (10-25%) for detectable LMWH levels, lower than unfractionated heparin (UFH) monitoring variability. Method-related differences were observed, with Siemens Innovance anti-Xa assays yielding slightly higher values than Werfen HemosIL or Stago STA assays. Participant interpretations of LMWH levels (below/within/above therapeutic range) showed greater variability than UFH due to multiple valid therapeutic ranges for LMWH depending on dosing context. Overall, laboratories demonstrated acceptable analytical and interpretative performance for LMWH monitoring, which should provide some reassurance to both patients and their clinical carers.