Caution When Using High-Sensitivity Cardiac Troponin I Assay to Rule Out Acute Ischemia: When the Delta to Rule In Is Within Analytical Variation

Concentration changes of 1-2 ng/L with Abbott’s high-sensitivity cardiac troponin I (hs-cTnI) assay are evident near 5 ng/L owing to acceptable variation of the assay at this low concentration. Accordingly, larger deltas or changes in concentrations are required to demonstrate a rise or fall in hs-cTnI, which is especially important in the acute setting where physicians are ruling in patients for possible acute myocardial infarction. In this regard, the latest approved hs-cTnI assay in Canada (Ortho Clinical Diagnostics; limit of quantification = 1.2 ng/L; overall 99th percentile = 11ng/L) has a published delta or change in hs-cTnI concentrations of ≥ 4 ng/L over 1 hour to rule in patients in the emergency setting. However, a delta of 4 ng/L from > 11 ng/L (> 99th-percentile) to < 40 ng/L falls within the acceptable total error for hs-cTnI testing. At concentrations ≥ 40 ng/L this delta represents ≤ 10% variation with acceptable imprecision (coefficient of variation [CV]) for hs-cTnI being ≤ 10%. Furthermore, some commercial quality control (QC) material may yield greater imprecision estimates compared with patient material when tested daily, thereby further limiting the ability of the laboratory to truly detect suboptimal analytical performance. Accordingly, we set out to evaluate the imprecision of QC material as well as patient material within the first 100 minutes from the first result with Ortho Clinical Diagnostics’ hs-cTnI assay to assess the robustness of a 4 ng/L delta.