Utility of aPTT in Monitoring Unfractionated Heparin in Children

Abstract number: OR360

Chan AKC, Black L, Ing C, Williams S, Brandao L, Walter K, Abdelhaleem M

Unfractionated heparin (UFH) is used with increasing frequency in children. Although current recommendations are to use activated partial thromboplastin time (aPTT) or a chromogenic anti-factor Xa level for monitoring UFH, data suggest that there is a lack of correlation between aPTT and anti-factor Xa in children. The current study was conducted to investigate the relationship between aPTT and anti-factor Xa levels in children receiving UFH and ascertain if other coagulation tests affect these correlations. Consecutive samples received by the Coagulation Laboratory at the Hospital for Sick Children for monitoring UFH therapy were analyzed for aPTT and anti-factor Xa. In a separate cohort, where adequate samples were left after clinical testing was done, the following tests were performed: aPTT, anti-factor Xa activity, 1 unit thrombin time, fibrinogen and factor VIII:C (FVIII:C). Of the 52 consecutive samples, there was very poor correlation between aPTT and anti-factor Xa activity (r²= 0.09). However, the correlation between the two assays was better for children > 2 years of age (r²= 0.27) compared to those of less than two years of age (r²= 0.08). In the cohort where multiple tests could be performed, thrombin time correlated better with anti-factor Xa activity (r²= 0.40) compared to aPTT (r²= 0.27). Forty-six% of 28 patients had FVIII:C > 2.0 U/mL. Interestingly, patients with FVIII:C > 2.0 U/mL had better correlation between aPTT and anti-factor Xa (r²= 0.85) than those with normal FVIII < 2 U/mL (r²= 0.19). In conclusion, aPTT was not a good predictor of anti-factor Xa activity; however, it had a better correlation in older children. Our data suggest that thrombin time is a better test for monitoring UFH. Larger studies are required to determine the age at which aPTT is a good predictor of anti-factor Xa activity. These studies will help determine the effect of FVIII:C on aPTT and if thrombin time is a better test for UFH monitoring.