To aPTT or not to aPTT: Evaluating the optimal monitoring strategy for unfractionated heparin

Application of anti-Xa assay in monitoring unfractionated heparin therapy in contemporary antithrombotic management

INTRODUCTION

Unfractionated heparin remains the most widely used agent in the prevention and acute treatment of thrombosis. Pharmacological complexities of this intriguing agent mandate frequent monitoring of its anticoagulant properties to maintain safe and effective hematological outcomes. Although activated partial thromboplastin time has been the standard test to monitor unfractionated heparin therapy for many decades, the anti-Xa assay has emerged as a substitute or adjunct in many institutions.

AREAS COVERED

This brief report outlines the key features of anti-Xa assay in monitoring unfractionated heparin in acute management of thrombosis in various contemporary settings, with emphasis on evidence for clinical outcomes. PubMed.gov database was utilized to obtain the pertinent literature.

EXPERT OPINION

The anti-Xa activity is primarily a reflection of UFH concentration and does not account for other hematological variables frequently present in contemporary anticoagulation management. The advantage of the anti-Xa assay in monitoring UFH therapy is predicated upon its limitations to account for global physiological hemostasis. There are significant disease and drug interactions that may potentially result in false in-vitro analysis of anti-Xa activity. Routine application of the anti-Xa assay is not evidence-based at this time.

Monitoring of Unfractionated Heparin Therapy in the Intensive Care Unit Using a Point-of-Care aPTT: A Comparative, Longitudinal Observational Study with Laboratory-Based aPTT and Anti-Xa Activity Measurement

Continuous intravenous unfractionated heparin (UFH) is administered routinely in the intensive care unit (ICU) for the anticoagulation of patients, and monitoring is performed by the activated partial thromboplastin time (APTT) or anti-Xa activity. However, these strategies are associated with potentially large time intervals before dose adjustments, which could be detrimental to the patient. The aim of the study was to compare a point-of-care (POCT) version of the APTT to (i) laboratory-based APTT and (ii) measurements of anti-Xa activity in terms of correlation, agreement and turnaround time (TAT). Thirty-five ICU patients requiring UFH therapy were prospectively included and followed longitudinally for a maximum duration of 15 days. UFH was administered according to a local adaptation of Raschke and Amanzadeh’s aPTT nomograms. Simultaneous measurements of POCT-APTT (CoaguCheck® aPTT Test, Roche Diagnostics) on a drop of fresh whole blood, laboratory-based APTT (C.K. Prest®, Stago) and anti-Xa activity (STA®Liquid anti-Xa, Stago) were systematically performed two to six times a day. Antithrombin, C-reactive protein, fibrinogen, factor VIII and lupus anticoagulant were measured. The time tracking of sampling and analysis was recorded. The overall correlation between POCT-APTT and laboratory APTT (n = 795 pairs) was strongly positive (rs = 0.77, p < 0.0001), and between POCT-APTT and anti-Xa activity (n = 729 pairs) was weakly positive (rs = 0.46, p < 0.0001). Inter-method agreement (Cohen’s kappa (k)) between POCT and laboratory APTT was 0.27, and between POCT and anti-Xa activity was 0.30. The median TATs from sample collection to the lab delivery of results for lab-APTT and anti-Xa were 50.9 min (interquartile range (IQR), 38.4−69.1) and 66.3 min (IQR, 49.0−91.8), respectively, while the POCT delivered results in less than 5 min (p < 0.0001). Although the use of the POCT-APTT device significantly reduced the time to results, the results obtained were poorly consistent with those obtained by lab-APTT or anti-Xa activity, and therefore it should not be used with the nomograms developed for lab-APTT.