External Quality Assessment Testing Near the Limit of Detection for High-Sensitivity Cardiac Troponin Assays.

To the Editor: The latest laboratory recommendations on high-sensitivity cardiac troponin (hs-cTn)1 endorse imprecision ≤10% at the 99th percentile with a total analytic error of <3.5 ng/L for concentrations ≤10 ng/L (1). However, for physicians using the European Society of Cardiology guideline's alternative 0/1-h early rule-in and rule-out algorithm, which uses the limit of detection (LoD) for an immediate rule-out, laboratories may need to limit total error to <1 ng/L to prevent patient misclassification (1). This metric may be difficult for laboratories to achieve and maintain over the long term when instrument performance and reagent lot-to-lot variation can easily exceed this error limit (2). There is an increasing appreciation that proficiency testing materials are required at <10 ng/L if physicians are making clinical decisions on hs-cTn results in this low range (1, 2). In this regard, the United Kingdom–National External Quality Assessment Service (UK-NEQAS) is producing material at this low concentration range, with a lyophilized sample in the concentration range of 3 to 20 ng/L being produced and used for the hs-cTn assays (3). This material (which needs to be reconstituted with 500 μL of distilled water) for the UK-NEQAS cardiac markers survey is manufactured using pooled female human plasma or serum base matrix with different levels obtained by the addition of the troponin complex (3). The initial published report from 33 instruments on 1 such low external quality assessment (EQA) material (which is not commutable) yielded …