Implications of Misclassification of Type 2 Myocardial Infarction on Clinical Outcomes

Background: Patients with type 2 myocardial infarction (MI) are often classified under the diagnosis of non-ST-segment-elevation MI (NSTEMI) despite the significant differences in clinical characteristics, management, and outcomes between type 2 MI and type 1 NSTEMI. This may have significant implications that can lead to inaccurate assessment of quality measures by MI quality review programs.

Methods: A single-center retrospective study of 1224 patients discharged with the diagnosis of type 1 NSTEMI between January 2015 and September 2017. Based on the third universal definition of MI, we stratified patients into type 2 MI or type 1 NSTEMI. Patient's characteristics, comorbidities, medications prescribed during hospitalization and at discharge, readmissions within 30 days after discharge, and diagnostic and therapeutic interventions data was collected. The primary goal of this study was to identify how often type 2 MI patients were misclassified as type 1 NSTEMI, we also assessed the differences in treatment and outcomes between type 2 MI and type 1 NSTEMI.

Results: 1224 patients assigned the ICD-9 and ICD-10 codes of type 1 NSTEMI at discharge were evaluated for study inclusion. After application of the inclusion criteria, 945 patients were included in the final analysis. Of these 945 patients, 281 (29.7%) patients were classified as type 2 MI and 664 (70.3%) patients were classified as type 1 NSTEMI. Patients with type 2 MI were older, more likely to have systolic heart failure, had lower peak troponin levels, were less likely to receive aspirin, P2Y12 inhibitors and statin at discharge, and had longer length of stay. Compared with type 1 NSTEMI patients, those with type 2 MI had higher all cause 30-day mortality (13.5% versus 2.9%, P < 0.0001) (RR: 4.65; 95% CI, 2.85-9.65). After adjusting for patient demographics, comorbidities, and medications, patients with type 2 MI were still more likely to die within 30 days after discharge (RR: 2.89; 95% CI, 1.58-7.46). In addition, patients with type 2 MI were more likely to be readmitted within 30 days after discharge than patients with type 1 NSTEMI (17.7% versus 13.9%, P < 0.01) (RR: 1.27; 95% CI, 1.08-2.5).

Conclusions: Close to one third of patients given the diagnosis of type 1 NSTEMI at discharge at our institution were type 2 MI patients. Patients with type 2 MI are managed differently from type 1 NSTEMI patients and have higher 30-day mortality and readmission rate. Misclassification of type 2 MI as type 1 NSTEMI can have a significant impact on hospitals MI clinical performance and quality measures.