The use of anti-Xa assay to monitor intravenous unfractionated heparin therapy.

Continuous infusion unfractionated heparin (UH) has traditionally been monitored using the activated partial thromboplastin time (aPTT). The use of this test to monitor heparin therapy is not based on randomized controlled clinical trials, and the test is associated with significant intra- and inter-patient variability that is not related to circulating blood heparin activity. Due to these and other limitations, the use of aPTT alone to monitor UF has been questioned. Many laboratories are now transitioning to monitoring actual heparin activity (by anti-factor Xa analysis). In this review, we discuss the limitations of using the aPTT to monitor UH therapy and additionally the limitations of solely using heparin activity to monitor therapy. We also include a discussion of the challenges with monitoring heparin therapy in the pediatric population.