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.

Viscoelastic testing (VET) is widely used to guide hemostatic therapy in patients with coagulopathy. One important application is the detection of fibrinolysis, defined as a reduction in clot amplitude after maximum clot firmness (MCF), quantified as maximum lysis (ML). Low ML values have recently been associated with adverse outcomes in trauma and sepsis. However, the biological basis of low ML remains unclear. Objective: To determine whether low ML values reflect reduced plasmin-mediated fibrinolysis in tissue factor (TF) activated viscoelastic assays (EX-assay). Methods: A total of 120 healthy adults (52.5% female; mean age 38.2 ± 14.1 years) were studied. EX-assay without fibrinolysis inhibition were compared with assays containing the antifibrinolytic agent tranexamic acid (AP-assay). VET parameters obtained with and without fibrinolysis inhibition were compared using paired analyses, Pearson correlation, and Bland–Altman methods. Results: Clot firmness at 10 min (CA10) was similar with or without fibrinolysis inhibition; although MCF differed statistically, the difference was clinically negligible. ML ranged from 1% to 13% in both assays, with nearly identical mean values (5.9 ± 2.6% vs. 6.0 ± 2.6%). Correlation analysis demonstrated strong agreement for CA10, MCF, and ML between assays, and Bland–Altman analysis confirmed minimal bias for ML. Conclusions: Low ML values in TF-triggered viscoelastic assays were unaffected by tranexamic acid, suggesting that they are unlikely to reflect plasmin-mediated fibrinolysis. These findings support the contribution of alternative mechanisms, such as platelet-mediated clot retraction.

Introduction: A 2004 study reported high sensitivities for laboratory assays used in the preoperative detection of primary hemostasis disorders and has since been widely cited. A re-evaluation of its methodological validity is performed. Methods: The study design and calculations were reviewed focusing on the definition of “impaired hemostasis” and the derivation of diagnostic performance. A synthetic dataset of random values was generated, and assay-specific cut-offs were defined to reproduce the proportions of positive results reported in the original study. The detection of “impaired hemostasis” was then calculated using a self-referential approach. Results: Three sets of performance figures are provided in the 2004 study. In one set, the results of the evaluated tests were used to define the reference standard against which their own sensitivities and specificities were then calculated (incorporation bias). For one assay a second set of performance figures was reported. The index test had been performed first in all patients; the further diagnostic work-up and the classification of clinical findings depended on its result (verification bias). The cut-off used for the index test was determined post hoc by ROC analysis, which often inflates performance metrics. The sensitivity and specificity values reported for the individual questionnaire items are likewise affected by incorporation bias. The synthetic dataset reproduced the reported detection rates despite consisting of random numbers. Conclusion: The widely cited results of the 2004 study cannot be used as evidence of diagnostic accuracy, because the reference standard was established using the very tests whose performance was then evaluated against it.