OBJECTIVE: To determine the minimal clinically important difference (MCID) of warfarin therapy for the treatment of nonvalvular atrial fibrillation from the perspective of patients using 2 different elicitation methods. DESIGN: All patients completed 2 face-to-face interviews, which were 2 weeks apart. For each interview, they were randomized to receive 1 of 2 elicitation methods: ping-ponging or starting at the known efficacy. SETTING: The practices of 2 university-affiliated family medicine centers (8 physicians each), 14 community-based family physicians, and 2 cardiologists. PATIENTS: Sixty-four patients with nonvalvular atrial fibrillation who were initiated with warfarin therapy at least 3 months before the study. INTERVENTION: During each interview, the patients' MCIDs were determined by using (1) a pictorial flip chart to describe atrial fibrillation; the consequences of a minor stroke, a major stroke, and a major bleeding episode; the chance of stroke if not taking warfarin; the chance of a major bleeding episode if taking warfarin; examples of the inconvenience, minor side effects, and costs of warfarin therapy; and then (2) 1 of the 2 elicitation methods to determine their MCIDs (the smallest reduction in stroke risk at which the patients were willing to take warfarin). Patients' knowledge of their stroke risk, acceptability of the interview process, and factors determining their preferences were also assessed. MAIN RESULTS: Given a baseline risk of having a stroke in the next 2 years, if not taking warfarin, of 10 of 100, the mean MCID was 2.01 of 100 (95% confidence interval, 1.60-2.42). Fifty-two percent of the patients would take warfarin for an absolute decrease in stroke risk of 1% over 2 years. Before eliciting their MCIDs, patients showed poor knowledge of their stroke risk, which improved afterward. The interview process was well accepted by the patients. The MCID using the ping-ponging elicitation method was 1.015 of 100 smaller compared with use of the starting at the known efficacy method (P = .01). CONCLUSIONS: We were able to determine the MCID of warfarin therapy for the prevention of stroke from the perspective of patients with nonvalvular atrial fibrillation. Their MCIDs were much smaller than those that have been implied by some experts and clinicians. The interview process, using the flip chart approach, appeared to improve the patients' knowledge of their disease and its consequences and treatment. The method used to elicit the patients' MCIDs can have a clinically important effect on patient responses. The method used in our study can be generalized to other conditions and, thus, could be helpful in 3 ways: (1) from a clinical decision-making perspective, it could facilitate patient-physician communication; (2) it could clarify the patient perspective when interpreting the results of previously completed trials; and (3) it could be used to derive more clinically relevant sample sizes for randomized treatment trials.
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