Standardization of Anti-Xa Assay and its Comparison with Activated Partial Thromboplastin Time for Monitoring Unfractionated Heparin Therapy.

Monitoring of anticoagulant activity of unfractionated heparin (UFH) is primarily done with activated partial thromboplastin time (aPTT), which is affected by many factors. Anti-Xa assays are considered to overcome these factors and may provide a better method for monitoring patients on UFH with a narrow therapeutic range. This study aimed to compare the effectiveness of aPTT and anti-Xa assays in UFH monitoring. A prospective non-randomized study was carried out in two stages: first, the anti-Xa assay was standardized using kit instructions; each sample was then analyzed by both tests. The outcomes of the two assays were compared and assessed for agreement of maintaining therapeutic anticoagulant levels. These levels for anti-Xa assay were between 0.3 and 0.7 IU/ml, while it was 1.5-2.5 times the control for aPTT assay. Below this range was regarded as subtherapeutic, and above this as supratherapeutic. A total of 90 samples were tested and analyzed using both assays. Most of them (> 70%) were noted to be in subtherapeutic levels with both tests. The overall concordance was 73.3%, and the estimated kappa value was 0.483 (0.396-0.57). The correlation between aPTT and anti-Xa assay was 0.74 ( p  < 0.001). With anti-Xa levels in the therapeutic range, aPTT levels were in subtherapeutic in 60% and supratherapeutic in 13.3% cases. Although both the testing strategies had a good agreement and correlation, discordance was observed in interpretative values with anti-Xa levels in therapeutic range and aPTT levels in non-therapeutic range. Its clinical implications need to be evaluated further in future studies.