Monitoring unfractionated heparin in ECMO and cardiac surgery: the search for a valid reference standard

Unfractionated heparin remains the standard anticoagulant during cardiopulmonary bypass (CPB) and extracorporeal membrane oxygenation (ECMO). Despite decades of clinical use, however, no universally validated reference method exists for measuring biologically active unfractionated heparin in these settings. Conventional assays such as the activated clotting time and the activated partial thromboplastin time lack specificity and standardisation. Consequently, anti–factor Xa (anti-Xa) assays are increasingly recommended as alternative methods (or even reference methods). Nevertheless, their validity as a true reference standard in CPB and ECMO has never been formally established. This situation creates a methodological paradox: an assay with context-dependent limitations is used to calibrate and validate other imperfect tests. Anti-Xa assays differ substantially in design, particularly regarding the inclusion of dextran sulphate. Dextran sulphate was introduced to prevent ex vivo neutralisation of heparin by platelet factor 4. However, platelet factor 4 release also occurs in vivo during extracorporeal circulation. Depending on assay configuration and clinical context, anti-Xa measurements may therefore overestimate or underestimate biologically active heparin—especially at low concentrations or following protamine reversal. As a result, anti-Xa assays cannot be assumed to be interchangeable in CPB and ECMO. We argue that heparin monitoring in extracorporeal circulation requires conceptual re-evaluation. Distinct clinical scenarios must be differentiated, and rigorous validation studies are essential before any assay can be regarded as a true reference standard.