High Sensitivity Troponin I
Work published in the BMJ this month showed that new high sensitivity Troponin I (hs cTnI) tests can detect myocardial infarction in more women than older tests (BMJ, 21/1/15). They are also able to provide diagnostic information at 3 h rather than 12 h after admission enabling targeted treatment sooner.
Unlike older assays, these hs cTnI methods have < 10 % imprecision at the 99th centile (the upper reference limit) and are able to quantify cTnI in 96 % of values below that cut off in a healthy population. Because of this, gender specific reference ranges have been developed which show that the 99th centile in women is lower than that in men (15 ng/L c.f. 34 ng/L).
At Imperial, the Blood Sciences team have been working with Acute Medicine and Cardiology. Our business case for the more expensive hs cTnI assay was successful and from Wednesday 18th February we will offer this service to all Imperial hospitals.
A comprehensive diagnostic algorithm has been developed incorporating the hs cTnI test result into the full clinical work-up of patients presenting to AE departments with chest pain. A markedly elevated hs cTnI at time 0 in an appropriate patient will be referred to cardiology. Other patients will receive a second hs cTnI at 3 h to determine whether there has been a diagnostic rise or fall. We have chosen a 50 % delta based on data from Keller et al. The PPV of hs cTnI >99th centile on admission together with a ≥50% delta at 3 hours has a 96% PPV and a stable troponin with no change and within the reference range 99.4% NPV. In collaboration with Abbott Diagnostics, we will continue to measure troponin in a 12 h sample to do a direct comparison of the old and new protocol timings in our patients over the next two months. This will be published as an audit report.
For further information contact Helen Johnstone, Clinical Scientist, Clinical Biochemistry Department