Adopting ‘ng/L’ as the units for high-sensitivity cardiac troponin assays and commitment by the entire health-care team could be the key for adopting recommendations

McKeeman and Auld on behalf of the National Clinical Biochemistry Audit Group (NCBAG) of the Association for Clinical Biochemistry and Laboratory Medicine should be commended on performing and reporting the results of a national survey of cardiac troponin (cTn) testing. ¹ The findings of this survey of the UK and Ireland, together with those of a recent US survey on cTn testing, highlight significant gaps and variation in laboratory practice with respect to cTn.^1,2 The Annals publication makes 12 recommendations for improving the application of high-sensitivity cTn (hs-cTn) assays in clinical practice, of which recommendation number 5 we feel is essential: ‘Laboratories should report hs-troponin as ng/L’. ¹

Originally, the recommendation to adopt the International System of Units (SI units) for hs-cTn assays was made in order to avoid errors in the reporting of hs-cTn concentrations and misunderstanding about which kind of cTn assay was used (i.e. standard vs. high sensitivity). ³ Subsequently, both laboratory and clinical societies as well as the International Federation of Clinical Chemistry Task Force on Clinical Applications of Cardiac Biomarkers have likewise advocated the use of ng/L as the units for measuring and reporting hs-cTn assay results.^4–6 For educational purposes and for implementation strategies, reporting hs-cTn in ng/L may be key to achieving awareness of this new test and its associated analytical characteristics. ⁷

One area where there is evidence and agreement is on shortening the time interval between presentation and the second sample in the emergency department (ED) when using hs-cTn assays. ⁸ The universal definition of myocardial infarction (MI), published in 2007, recommended 6–9 h between the initial and subsequent sample, while the third universal definition of MI, published in 2012, shortened this time interval to 3–6 h.^9,10 Our hospital changed from the standard Roche cTnT assay to the Abbott ARCHITECT sensitive cTnI assay in November 2012 and to the Abbott ARCHITECT hs-cTnI assay in November 2014. We sought to assess if, during the six months following the transition to cTnI (1 December 2012–1 June 2013), and hs-cTnI (1 December 2014–1 June 2015), there was any decrease in the time between measurements in the ED based on the recommendations. During the transition from the standard cTnT assay to the sensitive cTnI assay, the units remained the same (μg/L). However, we adopted the 99th percentile as the cut-off for the sensitive cTnI assay and recommended the 3–6 h time interval. Clinical services were notified of these recommendations, but did not necessarily commit to adopt them. In the six months following the introduction of the sensitive cTnI assay, the median (interquartile) time interval between first and second sample for cTnI measurements (n = 4485 pairs) in the ED was 7.2 h (4.3–8.8), consistent with the previous 6–9 h time interval and not in accordance with the recommendation. During the transition from the sensitive cTnI assay to the hs-cTnI assay, we changed the units to ng/L, maintained the 99th percentile cut-off, recommended a 3 h time interval between measurements, and this time obtained prior agreement and commitment from the clinical services for this transition. ⁷ In the six months following the implementation of hs-cTnI testing, the median time interval between the first and second sample for hs-cTnI measurements (n = 5479 pairs) in the ED was significantly shorter, at 3.7 h vs. 7.2 h; P < 0.001 by Mann-Whitney U test.

In summary, it was only when we changed to the hs-cTnI assay with clear communication, clinical agreement between all relevant services and a distinction between this assay (ng/L) and previous cTn assays (μg/L), that we were we able to achieve the sampling intervals recommended by the guideline. Perhaps, there should be an additional recommendation for hs-cTn application in clinical practice: ‘There should be agreement and commitment by the entire health-care team to follow the specific guidelines and/or evidence’ if the benefits of hs-cTn testing may be realized.