The validity of self-reported primary adherence among Medicaid patients discharged from the emergency department with a prescription medication

Study objective: We determine the validity of self-reported prescription filling among emergency department (ED) patients. Methods: We analyzed a subgroup of 1,026 patients enrolled in a randomized controlled trial who were prescribed at least 1 medication at ED discharge, were covered by Medicaid insurance, and completed a telephone interview 1 week after the index ED visit. We extracted all pharmacy and health care use claims information from a state Medicaid database for all subjects within 30 days of their index ED visit. We used the pharmacy claims as the criterion standard and evaluated the accuracy of self-reported prescription filling obtained during the follow-up interview by estimating its sensitivity, specificity, positive likelihood ratio and negative likelihood ratio tests. We also examined whether the accuracy of self-reported prescription filling varied significantly by patient and clinical characteristics. Results: Of the 1,635 medications prescribed, 74% were filled according to the pharmacy claims. Subjects reported filling 90% of prescriptions for a difference of 16% (95% confidence interval [CI] 14% to 18%). The self-reported data had high sensitivity (0.96; 95% CI 0.95 to 0.97) but low specificity (0.30; 95% CI 0.26 to 0.34). The positive likelihood ratio (1.37; 95% CI 1.29 to 2.46) and negative likelihood ratio (0.13; 95% CI 0.09 to 0.17) tests indicate that self-reported data are not a good indicator of prescription filling but are a moderately good indicator of nonfulfillment. Several factors were significantly associated with lower sensitivity (drug class and over-the-counter medications) and specificity (drug class, as needed, site and previous ED use). Conclusion: Self-reported prescription filling is overestimated and associated with few factors.