To assess clinical outcomes in children treated with unfractionated heparin and monitored using an anti-factor Xa (Anti-FXa)-based nomogram. We also sought to assess the correlation between activated partial thromboplastin time (APTT) and Anti-FXa.

This was a single-center, observational cohort study conducted over a 20-month period that included all pediatric patients (<21 years) who received therapeutic unfractionated heparin and were monitored using an anti–FXa-based nomogram.

In total, 95 patients met prespecified inclusion criteria, and 1098 pairs of APTT and Anti-FXa measurements were performed. The median unfractionated heparin dose required to reach therapeutic Anti-FXa goal was significantly greater in infants compared with older children (P <.0001). The median time to achieve therapeutic Anti-FXa was 10 hours (range 2-96 hours) and was significantly shorter in patients who received a bolus compared with those who did not (P = .03). Five (5.3%) major bleeding events were noted. Age, peak Anti-FXa, peak APTT, lowest platelet count, and fibrinogen were not predictive of major and clinically relevant nonmajor bleeds. Moderate correlation between the APTT and Anti-FXa (r = 0.75; 95% CI 0.72-0.77) assays was appreciated.

Using an anti–FXa-based nomogram to monitor unfractionated heparin in children is feasible. Although moderate correlation was observed between the APTT and Anti-FXa assays, the APTT frequently overestimated heparin activity. Safety and efficacy of an Anti-FXa nomogram needs further validation.