Use of Plasma Testing to Decrease Discrepant Results With the i-STAT Method for Cardiac Troponin I

Abstract

We have previously reported that the point-of-care i-STAT method for cardiac troponin I, performed on whole blood, generally agreed well with a central laboratory method performed on plasma (Siemens TnI-Ultra), except that a few percent of positive i-STAT results could not be confirmed on plasma testing and were apparently false positives (Am J Clin Pathol. 2008;130:132Y5). Because of that finding, we modified our troponin testing protocol for the emergency department to confirm initial positive results on whole blood by retesting on the i-STAT using plasma. We report here on results of that protocol with 12,366 specimens serially received during a 1-year period. Eight hundred seventy-one (7.0%) of those specimens initially tested positive. Of those, 863 could be retested on plasma, and 803 of those (93.0%) were confirmed positive. Of the 60 (7.0%) that were not confirmed positive, in 28 cases the quantitative difference exceeded 0.10 ng/mL, and these were arbitrarily considered ''discrepant'' whole-blood results (3.4% of positive results, 0.2% of all results). Seven hundred seventy-one specimens that were confirmed positive on the i-STAT were retested again using the central laboratory method, and results were positive in all but 2 cases (0.26%). In comparing discrepant whole-blood results with nondiscrepant controls matched for demographic characteristics, the discrepant cases were associated with significantly higher white blood cell counts (14.0 vs 8.5 Â 10 9 /L) and absolute neutrophil counts (7.97 vs 5.7 Â 10 9 /L), but not with any other differences in routine test results. It is concluded that the i-STAT troponin method is vulnerable to an interference that occurs with whole blood but not plasma and that this interference may be mediated in some manner by white blood cells. RESULTSDuring the study period, 12,366 heparinized whole-blood specimens were collected in the ED for rapid cTn testing. The ORIGINAL ARTICLE