Troponin I – Its Performance Doesn’t Justify Actions

October 13, 2004
  1. An article about evaluation of a troponin I assay – A recent article in Clinical Chemistry reported on the evaluation of a troponin I assay (1). The article’s conclusion was: “Conclusion: AccuTnI is a sensitive and precise assay for the measurement of cTnI”. Yet, in the article it could be seen that the assay failed the ESC- ACC (European Society of Cardiology – American College of Cardiology) guidelines for imprecision (2). Moreover, 6% to 9% of the samples studied using other troponin I assays gave values on different sides of the medical decision cutoff. As is often the case, the study was funded by the manufacturer. Note – It was still possible that this assay was the best troponin I on the market at that time (or today), but the conclusions presented by the authors don’t match their facts.
  2. A questionable POCT strategy – In a recent article (3), the benefit of POCT (Point Of Care Testing) troponin I was reported. The variables studied were clinician satisfaction (using a survey), Emergency Department Length Of Stay, (ED LOS) and Turn-Around-Time (TAT). These results were also presented in a seminar – also published (4), in which the speaker noted the strategy for dealing with POCT Troponin I results was:
  • if the POCT troponin I result was elevated, it was followed up with a lab troponin I assay
  • if the troponin I result was not elevated, no follow up assay was conducted and the POCT result was used as part of the clinician’s rationale to rule out myocardial infarction.

Problems with the clinician satisfaction survey – The clinicians were surveyed about the “confidence in the results obtained from POCT (ie, satisfaction with test accuracy of POCT)”. There are two concepts here – the performance of the assay and the perception of the assay. The survey provides information about the perception (by the clinicians) about the assay. It is unwise to draw conclusions about performance from a survey which the authors admit was subjective. Moreover, since the TAT was better for the POCT assay, one could argue that the clinicians were biased with respect to answering other questions about the assay (e.g., assay quality)

Problems with ED LOS and the assay strategy – The strategy for POCT Troponin I reported above is a biased approach which could lead to a reduced ED LOS. That is, some elevated POCT troponin I results might not be confirmed, so these patients would be released sooner from the ED. Yet, there is no opportunity to rerun POCT troponin I results that are below the cutoff range (e.g., all of these patients do not stay long in the ED). One would like to know how many POCT troponin I results that are below the cutoff, would be above the cutoff, if repeated with a lab troponin I assay.

  1. A lab unwilling to consider alternatives – In another seminar, a speaker reviewed troponin I performance and noted that no assays (as of 2002) met the ESC – ACC guidelines for troponin I performance, with imprecision being the major point of failure. This had the implication that incorrect clinical decisions would be made, partly based on an incorrect troponin I assay. I asked the question, why don’t you run replicates for the assay. This was immediately rejected  as being too expensive.

Whereas many people would agree that running replicates is impractical financially, one may run replicates only for values close to a medical decision limit to minimize increased cost. Moreover, for any proposed cost increase, one should model the financial tradeoffs as suggested in the following table:

 

Case Benefit Cost Comment
No change No increased test cost Cost of incorrect clinical decisions Depends on likelihood of incorrect clinical decisions
Increase replicates Reduced cost of fewer incorrect clinical decisions Increased test cost

 

  1. Uettwiller-Geiger D, Wu AHB, Apple FS, Jevans AW, Venge P, Olson MD, Darte C, Woodrum DL, Roberts S and Chan S. A Multicenter Evaluation of an Automated Assay for Troponin I Clinical Chemistry. 2002;48:869-876
  2. The Joint European Society of Cardiology/American College of Cardiology Committee. Myocardial infarction redefined—a consensus document of the joint European Society of Cardiology/American College of Cardiology Committee for the redefinition of myocardial infarction. J Am Coll Cardiol 2000;36:959–69
  3. Lee-Lewandrowski E, Corboy D, Lewandrowski, K, Sinclair J, McDermot S,  Benzer, TI. Implementation of a Point-of-Care Satellite Laboratory in the Emergency Department of an Academic Medical Center. Archives of Pathology and Laboratory Medicine: Vol. 127, No. 4, pp. 456–460 (2003).
  4. Kratz A, Januzzi JL,  Lewandrowski K, and Lee-Lewandrowski E. Positive Predictive Value of a Point-of-Care Testing Strategy on First-Draw Specimens for the Emergency Department–Based Detection of Acute Coronary Syndromes. Archives of Pathology and Laboratory Medicine: Vol. 126, No. 12, pp. 1487–1493 (2002).