Is coding for myocardial infarction more accurate now that coding descriptions have been clarified to distinguish ST-elevation myocardial infarction from non-ST elevation myocardial infarction?

Outcomes are typically graded on the basis of diagnoses coded according to the International Classification of Diseases, Ninth Revision (ICD-9). To facilitate performance measurement, the ICD-9 codes for acute myocardial infarction changed in October 2005 to completely separate non-ST elevation myocardial infarction (NSTEMI; code 410.71) and ST elevation myocardial infarction (STEMI; all other codes 410.x), yet it is unclear whether these changes have been implemented by coders. Patients in the National Registry of Myocardial Infarction (NRMI), version 5, were categorized in 2 ways: by electrocardiographic (ECG) findings and ICD-9 codes. Agreement between ECG findings and ICD-9 codes for type of myocardial infarction (STEMI or NSTEMI) was assessed before and after ICD-9 revision. Mortality rates were measured in a subgroup of patients discharged without transfer after the coding change. There were 102,679 hospitalizations before October 2005 and 63,012 hospitalizations after the coding change, among which the mean age was 66.7 years. Previously, 81% of NSTEMIs (by ECG diagnosis) were coded ICD-9 410.71; after the reclassification of code 410.71 to reflect NSTEMI, 82% of NSTEMIs were coded 410.71 (p <0.001). Overall, the correlation of ECG diagnosis with ICD-9 code improved only slightly after the coding change. In conclusion, despite more distinctly separated definitions of STEMI and NSTEMI in the new ICD-9 coding system as of October 2005, there appears to be little change in coding, which may reflect a lack of awareness of this substantial change in classification.