Welcome to the fascinating world of instrumental variable analysis!
This is a retrospective cohort analysis of a large insurance claims database attempting to glean insight into the value of non-invasive testing for patients presenting to the Emergency Department with chest pain. Previous version of the American Heart Association guidelines for the evaluation of so-called “low risk” chest pain have encouraged patients to undergo some sort of objective testing with 72 hours of initial evaluation. These recommendations have waned in more recent iterations of the guideline, but many settings still routinely recommend admission and observation following an episode of chest pain.
These authors used a cohort of 926,633 unique admissions for chest pain and analyzed them to evaluate any downstream effects on subsequent morbidity and resource utilization. As part of this analysis, they also split the cohort into two groups for comparison based on the day of the week of presentation – hence the “instrumental variable” for the instrumental variable analysis performed alongside their multivariate analysis. The authors made assumptions that individual patient characteristics would be unrelated to the day of presentation, but that downstream test frequency would. The authors then use this difference in test frequency to thread the eye of the needle as a pseudo-randomization component to aid in comparison.
There were 571,988 patients presenting on a weekday, 18.1% and 26.1% of which underwent some non-invasive testing within 2 and 30 days of an ED visit, respectively. Then, there were 354,645 patients presenting on a weekend, with rates of testing 12.3% and 21.3%. There were obvious baseline differences between those undergoing testing and those who did not, and those were controlled for using multivariate techniques as well as the aforementioned instrument variable analysis.
Looking at clinical outcomes – coronary revascularization and acute MI at one year – there were mixed results: definitely more revascularization procedures associated with exposure to non-invasive testing, no increase in downstream diagnosis of AMI. The trend, if any, is actually towards increased diagnoses of AMI. The absolute numbers are quite small, on the order of a handful of extra AMIs per 1,000 patients per year, and may reflect either the complications resulting from stenting or a propensity to receive different clinical diagnoses for similar presentations after receiving a coronary stent. Or, owing to the nature of the analysis, the trend may simply be noise.
The level of evidence here is not high, considering its retrospective nature and dependence on statistical adjustments. It also cannot determine whether there are longer-term consequences or benefits beyond its one-year follow-up time-frame. Its primary value is in the context of the larger body of evidence. At the least, it suggests we have equipoise to examine which, if any, patients ought to be referred for routine follow-up – or whether the role of the ED should be limited to ruling out an acute coronary syndrome, and the downstream medical ecosystem is the most appropriate venue for determining further testing when indicated.
“Cardiovascular Testing and Clinical Outcomes in Emergency Department Patients With Chest Pain”