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Cutoff Values for Blood Test to Determine Heart Attack May Be Imperfect

NEW YORK (360Dx) – Cutoff values for a blood test commonly used to diagnose if someone is having a heart attack may be imperfect, according to a new study.

Clinicians often, as guidelines suggest, test cardiac troponin levels to gauge whether a patient may be having a heart attack, as the protein is released into the bloodstream during a myocardial infarction.

But a study from University Hospital Southampton NHS Trust researchers has found that 1 in 20 hospital patients may have troponin levels that exceed the cutoff for being abnormal, even among patients without cardiac complaints. Troponin levels further varied by age, sex, and clinical location, the researchers reported yesterday in The BMJ. This, they added, underscores the need to interpret high-sensitivity cardiac troponin I (hs-cTnI) levels with care to avoid misdiagnoses.

"The results of this study should highlight that although hs-cTnI can contribute to the diagnosis of acute myocardial infarction, frontline clinical staff should use this test in conjunction with other key factors, such as clinical history and other investigations," Southampton's Nick Curzen and his colleagues wrote in their paper. In their observational CHARIOT study, the researchers assayed hs-cTnI levels of 20,000 consecutive patients at their hospital who were undergoing blood testing for any reason.

Currently, assay manufacturers set a recommended 99th centile to use as a cutoff to rule in or rule out an acute myocardial infarction. A task force has recommended that this 99th centile be determined using 300 healthy men and 300 healthy women.

For the test the researchers used in their study, the Beckman Coulter Access AccTnI+3 assay, that level was set at 40ng/L, but they suspected that the 99th centile of inpatients and outpatients at their hospital might differ.

Within these 20,000 patients — who were a median 61 years old and 52.9 percent female — the 99th centile was 296 ng/L, more than seven times the manufacturer-recommended cutoff. About 5 percent of these 20,000 patients had a troponin level above the manufacturer cutoff.

When the researchers excluded patients who were subsequently diagnosed with an acute myocardial infarction or who had had the troponin blood test performed for clinical reasons like chest pain or arrhythmia, the 99th centile fell to 189ng/L. Of these individuals, 4.6 percent had troponin levels past the 40ng/L threshold.

Other factors likewise affected troponin levels. For instance, they varied by whether patients were outpatients or inpatients and what ward they were on. The 99th centile among outpatients was 65 ng/L, while it was 563 ng/L among inpatients and 215 ng/L among emergency department patients whose doctors did not request testing. Of those ER patients, 6 percent had troponin levels above the recommended cutoff.

A multivariate analysis additionally identified advancing age, being male, and kidney function to be independent predictors of troponin levels.

The researchers argued that their findings suggest that using the 99th centile as a cutoff value to diagnose a patient with an acute myocardial infarction might not always be suitable. In particular, they said it might not be appropriate to use that value in populations that are older and have more comorbidities to avoid misdiagnoses and unneeded treatment.

Instead, they recommended using the troponin test in combination with patients' clinical history and evaluating the results in light of the patients' baseline health and comorbidities. "The study observations highlight the need for clinical staff to interpret hs-cTnI concentrations carefully and systematically when making a diagnosis of acute myocardial infarction," they added.