Rapid blood test on arrival in hospital may help rule out heart attack

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29 October 2015

Patients who complain of severe chest pain are usually admitted to hospital, but between 60 and 80 percent of them have not suffered a heart attack. Although an electrocardiogram (ECG) can help make the diagnosis, the absence of specific ECG changes cannot exclude it. For a firm diagnosis doctors supplement clinical findings with blood tests for cardiac troponins, proteins that are released from damaged heart muscle. Diagnosis requires an increase in values during the hours after the onset of symptoms.

Cardiac troponin blood tests of high sensitivity have been developed that can detect cardiac troponin in over 50% of normal people. In October 2014 the National Institute for Health and Care Excellence (NICE) recommended two precise (repeatable) analytical methods that could be used to help doctors see a change in troponin levels sooner. Recent research has suggested that if the first high-sensitivity measurement taken more than two hours after the onset of symptoms is below a specific value, the probability of a heart attack is very low and might be used to warrant early discharge.

The idea of ruling out a heart attack has now been taken a step further. Investigators have used one of the high-sensitivity troponin tests recommended by NICE (hs-cTnI) to quantitate detectable levels in the first blood sample from patients arriving at the emergency departments of four Scottish hospitals with a suspected heart attack. They hoped to be able to define a threshold value within the normal reference range that would identify patients at very low risk. Their findings were reported in an online open-access paper in The Lancet on 8 October 2015. The work was funded by the British Heart Foundation and the Chief Scientist Office (NHS Scotland).

The study included 4,870 patients admitted to four Scottish hospitals between June 2013 and January 2014 with a suspected heart attack who had had a standard troponin requested by their doctor and an ECG done. Blood was then re-analysed by the hs-cTnI method but the results were not reported to the doctors. Two investigators independently reviewed all clinical information, investigations and patient outcomes up to 30 days after admission. After excluding 1,071 patients who on admission had ECG evidence of a heart attack or hs-cTnI values above the upper limit of normal, statistical analysis was carried out on the results from the remaining 3,799 patients. The proportion who had a heart attack diagnosed or had died was calculated across a range of hs-cTnI concentrations on admission. The researchers identified a specific threshold level within the normal reference range below which 99.6% of the patients had not subsequently been diagnosed as having had a heart attack. Those below the threshold comprised 61% of all patients who had not had an attack. It is interesting that the cut-off value was similar in men and women although, as we reported in a news item on 3 February 2015, the same researchers had found earlier that the upper limit of normal for hs-cTnI was lower in women than men.

Two follow up studies found that, of patients arriving at Edinburgh Royal Infirmary and Hennepin County Medical Center, Minnesota who had troponin concentrations below the threshold, 99.3% and 99.8% respectively had not had a heart attack.

The authors stressed that the threshold should not be applied in isolation but that all available information should be used for clinical decision making. They pointed out that studies are needed of the clinical and cost-effectiveness of their approach in routine clinical practice; they hope it will substantially reduce hospital admissions and have major benefits for both patients and health-care providers.