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Assess the validity of Medicare claims for identifying myocardial infarction (MI).We used data from 9951 Medicare beneficiaries 65 years and above in the Reasons for Geographic And Racial Differences in Stroke study. Between 2003 and 2012, 669 participants had an MI identified and adjudicated through study procedures (ie, the gold standard), and 552 had an overnight inpatient claim with a code for MI (ICD-9 code 410.x0 or 410.x1) in any discharge diagnosis position.Using Medicare claims with a discharge diagnosis code for MI in any position, the positive predictive value (PPV) was 84.3% [95% confidence interval (CI), 80.9%–87.3%] and the sensitivity was 49.0% (95% CI, 44.9%–53.1%). Sensitivity was lower for men (45.8%) versus women (55.1%), microsize MIs (13.7%) versus other MIs (64.7%), type 2 (30.9%), and 4–5 MIs (11.1%) versus type 1 MIs (76.6%), and MIs occurring in-hospital (28.8%) versus out-of-hospital (66.7%). Using Medicare claims with a code for MI in the primary discharge diagnosis position, the PPV was 89.7% (95% CI, 86.3%–92.5%) and sensitivity was 40.1% (95% CI, 36.1%–44.2%). The sensitivity of claims with a code for MI in the primary discharge diagnosis position was lower for microsize versus other MIs, type 2 and 4–5 MIs versus type 1 MIs and MIs occurring in-hospital versus out-of-hospital. Hazard ratios for MI associated with participant characteristics were similar using adjudicated MIs identified through study procedures or claims for MI without further adjudication.Medicare claims have a high PPV but low sensitivity for identifying MI and can be used to investigate individual-level characteristics associated with MI.