Background Increasing numbers of children are being administered warfarin therapy as thromboprophylaxis. Warfarin has a narrow therapeutic window with a target international normalised ratio (INR) of 2–3.5, called the therapeutic range. The length of time a patient's INR remains within the therapeutic range is calculated as ‘time in the therapeutic range’. Risk for haemorrhage in children receiving warfarin is 0.5%/patient-year and minor bleeding 2.3%/patient-year, which increases exponentially for INRs >5.0. Practice among non-bleeding adults with INRs ≥5 and ≤9 is to withhold warfarin and allow the INR to return to the therapeutic range. Faster warfarin clearance is correlated with younger age.

Methods and results The study objective was to determine the safety and effectiveness of a conservative approach for management of INRs >5 in children receiving warfarin. Children receiving warfarin with INRs ≥5 had warfarin withheld followed by a next day INR without vitamin K administration. Eighty-nine children (1–16 years) participated in the study with 2353 INRs performed. Twenty-six children had INRs ≥5, 5% of the total performed, with a mean INR of 5.9. The next day repeat mean INR after withholding one dose of warfarin was 3.3 (range 1.2–6.8) with 89% of INRs falling below 5. There were no overt bleeds or symptomatic thrombotic events in the month following the INR >5. Time in the therapeutic range for children with INRs ≥5 was 68%.

Conclusions Withholding warfarin alone for management of non-bleeding INRs ≥5 and ≤8 appears to be safe and effective.