Background: Although International Classification of Diseases, 9th Revision (ICD-9) codes have been in use for decades to classify patients with acute myocardial infarction (AMI) in the United States, they did not allow characterization of patients with ST elevation MI (STEMI) versus non-ST elevation myocardial infarction (NSTEMI).
Methods: We evaluated all discharged patients with AMI participating in the National Registry of Myocardial Infarction 2, 3, and 4 databases with coded ICD-9 information. Patients who were transferred out to a different institution were excluded. A comparison of the ICD-9 codes used for STEMI and NSTEMI patients, as determined by electrocardiography (ECG), was performed. A comparison of mortality for patients that were neither transferred in or out was also performed.
Results: Among 1.45 million patients, 521,332 (36.1%) had STEMI and 923,757 (63.9%) had NSTEMI based on presenting ECG criteria. There were 10.9% of those with STEMI who were coded as 410.71 (subendocardial MI). In contrast, 38.8% of NSTEMI patients had an ICD-9 code other than 410.71. Mortality across ICD-9 codes was similar between the STEMI and NSTEMI groups overall and within each ICD-9 subsection.
Conclusion: Substantial overlap and inconsistency exists in ICD-9 codes for STEMI and NSTEMI patients, yet both ICD-9 code and STEMI predicted mortality. The new changes to ICD-9 coding, implemented in October 2005 to segregate STEMI and NSTEMI, should allow better description of quality measures and outcomes of these 2 distinct types of MI patients.