Monitoring of unfractionated heparin using activated partial thromboplastin time: an assessment of the current nomogram and analysis according to age

Background: We frequently encounter high levels of activated partial thromboplastin time (aPTT) during heparin anticoagulation. The purpose of this study is, first, to investigate the rate of achieving and maintaining therapeutic aPTT in patients treated with heparin anticoagulation and second, to assess the adequacy the current nomogram.

Methods: We included 197 patients who underwent anticoagulation with unfractionated heparin (UFH) according to the standard nomogram between September 2008 and May 2010. The primary endpoints were the rate of achieving a therapeutic range (TR) at the first sample, 24 hours, or 48 hours. We also compared heparin nomograms according to age (<70 years vs ≥70 years).

Results: Of the 197 patients, 131 had heparin loading. In the heparin loading group, there were 19.1% (n=25), 69.5% (n=91), and 90.1% (n=18) achieving TR at the first aPTT, 24 hours, and 48 hours, respectively. The therapeutic aPTT proportion was 39.2%, and the rate of peak level above 90 seconds was 93.1%. Peak levels of aPTT were higher in the older age group than in the younger age group (202.3 ± 124.2 versus 152.0 ± 78.9, p=0.007).

Conclusion: Our results indicate a high rate of achieving therapeutic aPTT at 24 hous and 48 hours, but a low success rate for maintenance within the TR. Most patients had supratherapeutic aPTT of more than 90 seconds. Therefore, the TR of aPTT that matches heparin levels of 0.3 to 0.7 IU/mL measured by antifactor Xa assay should be determined. If not, we should consider adopting a new heparin dosing nomogram.