Warfarin monitoring in ambulatory older individuals receiving antimicrobial therapy.

STUDY

OBJECTIVE

To determine the frequency of monitoring of international normalized ratio (INR) within 14 days of coprescription of warfarin and antimicrobial therapy and to evaluate differences in INR monitoring among antimicrobials.

DESIGN

Retrospective cohort study.

SETTING

Group model health maintenance organization.

SUBJECTS: Patients aged 65 years or older who were taking warfarin and an antimicrobial agent.

MEASUREMENTS AND MAIN

RESULTS

Patients who received dispensings of both warfarin and an antimicrobial agent were identified. We found 2959 coprescribing instances in 1816 patients. The INR values were obtained for 2267 (77%) coprescribing situations within 14 days. Monitoring occurred more frequently (p<0.001) when warfarin was coprescribed with fluoroquinolones (641 [85%] of 755 situations), metronidazole (59 [81%] of 73), tetracyclines (274 [80%] of 341), or macrolides (201 [83%] of 243) than when warfarin was coprescribed with sulfonamides (35 [66%] of 53), penicillins (604 [71%] of 856), or cephalosporins (419 [71%] of 591). Among monitored patients, a higher proportion of monitoring (p<0.001) occurred within 7 days for patients prescribed antifungals (87%), fluoroquinolones (88%), tetracyclines (82%), metronidazole (86%), sulfonamides (86%), or macrolides (85%) than for patients prescribed cephalosporins (68%) or penicillins (75%).

CONCLUSION

Most older patients coprescribed warfarin and an antimicrobial in our organization had INR monitoring within 7 days. This is consistent with appropriate practice to manage a risk of clinically important drug-drug interaction between an antimicrobial agent and warfarin. Prospective identification of patients requiring INR monitoring after coprescription of interacting drugs by using merged administrative pharmacy and laboratory data should be further evaluated as a tool to improve clinical outcomes.