Warfarin monitoring in ambulatory older individuals receiving antimicrobial therapy.
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.


METHODS
Retrospective cohort study.


METHODS
Group model health maintenance organization.


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


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%).


CONCLUSIONS
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.
Raebel
Marsha A
MA
Clinical Research Unit, Kaiser Permanente of Colorado, and the School of Pharmacy, University of Colorado, Denver, Colorado 80237-8066, USA. Marsha.A.Raebel@kp.org
Witt
Daniel M
DM
Carroll
Nikki M
NM
Magid
David J
DJ
D000368:Aged; D000553:Ambulatory Care / Q000706:statistics & numerical data*; D000890:Anti-Infective Agents / Q000627:therapeutic use*; D000925:Anticoagulants / Q000097:blood / Q000627:therapeutic use*; D015331:Cohort Studies; D004347:Drug Interactions; D011307:Drug Prescriptions; D005260:Female; D006801:Humans; D019934:International Normalized Ratio; D008297:Male; D008991:Monitoring, Physiologic; D011787:Quality of Health Care; D012189:Retrospective Studies; D014859:Warfarin / Q000097:blood / Q000627:therapeutic use*
D000890:Anti-Infective Agents; D000925:Anticoagulants; D014859:Warfarin