Treatment with the antibiotic dicloxacillin may cause a significant decrease in international normalized ratio (INR) levels among patients taking vitamin K antagonists (VKAs), according to research published in JAMA.
Researchers studied 7400 patients on VKA therapy and found that 61% of patients taking warfarin and dicloxacillin experienced a decrease in INR after dicloxacillin exposure.
Forty-one percent of patients taking phenprocoumon had a decrease in INR after exposure to the antibiotic.
Anton Pottegard, PhD, of the University of Southern Denmark, Odense, and his colleagues conducted this research using Thrombobase, a clinical database of all VKA-treated patients followed up by 3 outpatient clinics and 50 general practitioners in Funen, Denmark.
The researchers included all patients who filled a prescription for dicloxacillin while receiving warfarin or phenprocoumon between March 1998 and November 2012.
INR results were grouped by the week relative to dicloxacillin exposure. The last INR measurement before dicloxacillin exposure was compared with the first measurement within weeks 2 to 4 after dicloxacillin exposure.
Of the 519 patients taking warfarin and initiating treatment with dicloxacillin, 236 met inclusion criteria. The average INR level was 2.6 prior to dicloxacillin exposure and 2 at two to four weeks after dicloxacillin exposure.
In total, 61% of patients (n=144) experienced sub-therapeutic INR levels (<2.0) within 2 to 4 weeks of dicloxacillin exposure.
Among patients taking phenprocoumon (n=64), average INR levels were 2.6 before exposure to dicloxacillin and 2.3 at two to four weeks after exposure. The proportion of patients with sub-therapeutic INR levels after dicloxacillin exposure was 41% (n=26).
The researchers said these results suggest treatment with dicloxacillin can cause a significant decrease in INR levels among patients taking VKAs.
