Response to warfarin therapy in obese pediatric patients dosed according to institutional guidelines

Current warfarin dosing guidelines for pediatric patients do not account for obesity. Published data from adults suggest that obesity may affect warfarin dosing requirements. Obesity is prevalent in the pediatric population, and current warfarin dosing methods should be evaluated in obese pediatric patients.

Patients aged 2 to 18 years who were obese and initiated on warfarin therapy at our institution as inpatients from 2004 to 2010 were identified and matched in a 1:2 ratio by age and sex with nonobese patients who were initiated on warfarin therapy. Patients were categorized obese per Centers for Disease Control guidelines. Demographic and disease state information, warfarin dosing information, INR values, and interacting medications were collected. Warfarin was dosed according to the institutional guidelines adapted from the published literature. Time to therapeutic INR value was the primary endpoint and percent of patients with supratherapeutic INR values was the secondary endpoint.

A total of 30 patients met the study criteria (10 obese, 20 nonobese), and baseline demographic variables were similar. No significant differences were noted in the number of INR values drawn, number of warfarin doses administered, or length of stay. Initial and maximum doses of warfarin per kg were significantly lower in obese patients compared with nonobese patients (P<0.05). Median time to therapeutic INR value was twice as long in obese patients as in nonobese patients (median=6 [range, 4 to 28 d] versus median=3 [range, 1 to 10 d]; P<0.01).

Obese pediatric patients have an increased time to therapeutic INR value when traditional warfarin dosing guidelines are used.