We found major deviations from guideline recommendations for troponin laboratory reporting in our study. Laboratory reports only rarely identified a 99th percentile or 10% CV level, and more than half of hospitals identified an indeterminate range classifying MI based on a higher threshold than the guideline-recommended approach [1–3, 20]. In this context, many microsize MIs may be difficult to recognize clinically.
Although our study was not able to examine the proportion of microsize MIs missed clinically by treating physicians as a result of hospital reporting practices, it is likely that some of these events were in fact not treated as MI. The potential misclassification of microsize MIs as “normal” has significant long-term prognostic implications. The higher long-term mortality risk we observed for those suffering microsize MIs was similar to past reports in which the in-hospital risks were low, but by one month and one year were similar to those with usual MIs [21, 22]. This finding underscores both the importance of long-term risk factor management in patients with microsize MIs and the relatively low short-term risks [1–4, 21, 23–26].
A recent report from Scotland illustrates the clinical importance of detecting microsize MIs . Investigators at the Royal Infirmary in Edinburgh took advantage of the introduction of a more sensitive troponin assay to study clinical management and outcomes among individuals analogous to our microsize MI group. In their study, the focus was on individuals whose peak troponin elevations were above the threshold for the new assay (0.05) and below the threshold for the older, less sensitive assay (0.20), both before and after changes in lab reports. They found that, after introduction of the lower decision threshold on lab reports, secondary preventive measures increased by 15-30% in the microsize MI group but remained similar for those in the usual MI or no MI groups. They also reported reductions in clinical outcomes, including the rate of MIs and deaths at 3 and 12 months in the group with microsize MIs after introduction of the lower threshold on lab reports. While this is an observational study from a single site, it does provide evidence of the potential impact of appropriate troponin reporting to optimize clinical management.
These findings together with our results suggest that improved reporting of hospital troponin results in the US is warranted; the US now has a system of public accountability for the quality of health care that provides a ready infrastructure to enact rapid change. A quality indicator assessing whether the recommended decision threshold is clearly identified on laboratory reports and assessing whether indeterminate ranges are used could be added rapidly to standards for accreditation by the Joint Commission or to the Centers for Medicare and Medicaid Services’ pay-for-performance programs, such as the Premier Hospital Quality Incentive Demonstration [27, 28].
Our study included a large proportion of black participants, which has not been the case in previous reports stemming from non-US samples . Our finding that black participants had 62% higher odds for microsize MIs raises concerns that some microsize MIs among black individuals may not be recognized clinically. Indeed, the last decade has demonstrated a concerning widening of the CHD mortality disparity between black and white people in the US, with white mortality rates declining faster than black rates . Similar to past reports, we found that those with a past history of CHD were also more likely to be in the microsize MI group . The clinical consequences of overlooking a microsize MI in individuals with a history of CHD may be minimized because many such patients may already be receiving secondary prevention, including aggressive smoking cessation counseling, lipid management, beta blockers, or anti-platelet therapy. On the other hand, as many as half of individuals on chronic disease preventive therapies discontinue their medications [30–32]; thus, an overlooked microsize MI may translate into a missed opportunity to redouble secondary preventive efforts in these individuals.
Our study’s limitations include our inability to examine how many cases classified as microsize MI by our rigorous adjudication methods were also recognized as MI by the treating physicians. We also did not have available the 99th percentile or 10% CV value at each hospital at the time of the event. Like other large epidemiology studies, we had to rely on clinical laboratories from many hospitals to classify MI and, given the variations we described, it is possible that, while state-of-the art and rigorous, the adjudication process under-detected some microsize MIs. This challenge would bias our results towards finding less difference in mortality rates than may actually exist; thus, the mortality risks we describe may underestimate the true risks. Our team of experts only classified events as MI if there was a clear rising and/or falling pattern of troponin, and perennial low level elevations seen in conditions such as heart failure or renal failure would therefore not have been classified as MI; however, it is possible that some misclassification could have occurred. Unfortunately, this study was not designed to detect differences in clinical management in microsize and usual MIs, but recent reports suggest that this may be an important issue . Future studies should examine what proportion of microsize MIs were not managed as acute MIs. Another limitation is the context of a national epidemiologic cohort study; thus, participants may not be representative of the US population. Although we included 649 hospitals, they may not be representative of all US acute care hospitals. In addition, event accrual may permit future analyses of CHD or cardiovascular disease deaths, in contrast to the all-cause mortality we used in this study.