The impact of child-specific characteristics on warfarin dosing requirements.

Background: The influence of child characteristics on warfarin dosing has been reported; however, there is no consensus on the nature and extent of this effect.

Objectives: To investigate the impacts of the demographic and clinical characteristics of children on the warfarin dose required to achieve a therapeutic international normalization ratio (INR).

Methods: This retrospective cohort study included children aged 3 months to 14 years old who were prescribed warfarin for 3 months or longer with a "stable INR." The primary outcome was the total daily dose (TDD) and total weekly dose of warfarin required to achieve a therapeutic INR target.

Results: We included 127 patients with a mean age of 7.7 ± 3.7 years and a median weight of 22 (IQR, 16-33) kg. Of the sample, 55 patients (43.3%) required a TDD of ≤0.1 mg/kg. The TDD for children younger than 5 years, 5 to 10 years, and older than 10 years were 0.14 ± 0.06 mg/kg, 0.12 ± 0.05 mg/kg, and 0.096 ± 0.04 mg/kg, respectively ( = .002). Overweight and obese children required a smaller TDD than normal-weight children: 0.09 ± 0.05 vs 0.13 ± 0.05 mg/kg ( = .004), which was similar for underweight children. A lower body surface area (<0.5 m) required a higher dose. All the other variables did not affect warfarin doses. The incidence of a subtherapeutic or supratherapeutic INR was independent of demographic or clinical variables.

Conclusion: The study confirmed that the patient demographics affect the daily warfarin dose required to achieve the INR target. However, they do not have any predictive value for the incidence of out-of-range-INR.