PB0089 - Unfractionated Heparin measurement with Anti-Xa assays from the UK National External Quality assessment Blood Coagulation 2023.

Background: Monitoring or measuring unfractionated heparin (UFH) in patients can be undertaken by APTT Ratios (APTTR), Anti-Xa assays or point of care devices. NEQAS BC data have highlighted that there are differences between the UFH Xa measurement across the Hyphen, Werfen and Stago group. The samples distributed in both of these exercises were constructed by addition of UFH to normal plasma. We received APTTR and UFH Xa results from participants. The Xa assays are calibrated with unicalibrators or multicalibrators. Both the Hyphen Xa and Werfen Xa kits have dextran sulphate which is absent from the Stago Xa kit.

Aims: The therapeutic range for UFH for an anti-Xa assay is 0.3-0.7IU/ml. Results in Table 1 suggest that the median values returned for kits with and without Dextran Sulphate are within the therapeutic range. There were a number of outliers above and below the therapeutic window of 0.3-0.7IU/ml.

Methods: Local study on individual non lyophilised patient samples showed a similar pattern of results with the three kits thereby confirming that NEQAS BC samples were commutable to patient samples, data not shown.

Results: The Anti-Xa assay overall medians of 0.51 (23/22) and 0.15 (22/49) IU/ml with and without dextran sulphate in the kit, have much better interpretation consensus of 95% and 100% respectively. Interpretation of APTT ratios for the sample with 0.5 IU/ml UFH varied more according to the reagent used.
UK participants registered for Heparin Dosage Assessment by APTT ratio/UFH Xa is a split of 81%/19%.

Conclusion(s): Despite some kit related differences, presence or absence of dextran sulphate in reagents there was more consensus amongst Anti Xa users than APTTR users in interpretation of heparin dosage.