Comparison of Serum and Heparin-Plasma Samples in Different Generations of Dimension Troponin I Assay

An editorial in the June issue of the Journal pointed out the usefulness of plasma instead of serum for cardiac troponin I (cTnI) determinations. However, because some analytical systems yield significant differences between serum and plasma troponin concentrations, the author of the editorial advocates more peer-reviewed studies before new assays are implemented in hospital-based laboratories (1). In September 2001 and March 2002, we carried out two comparison studies of cTnI measurements in plasma and serum samples, using different generations of the same assay (Cardiac Troponin-I Flex; Dade Behring). Both serum and plasma samples have been reported as suitable for cTnI determinations in the first (2) and second (3) generations of the assay. We collected two different series of 39 blood samples from patients admitted to the intensive care unit of our hospital during each month. For each patient, a serum (Venoject II, Autosep Gel + Act Z; Terumo Europe) and a plasma specimen (Venoject II, Autosep Gel + lithium heparin, 75 IU/tube) were obtained. cTnI was measured in duplicate in both series with reagents that came with the Cardiac Troponin-I Flex reagent (batch BD1349 in September 2001 and batch CK3031 in March 2002) and cTnI calibrators (batches OMD095 and IHD045, respectively) on the Dimension RxL Analyzer (Dade Behring). Linear regression analysis of the September 2001 results produced a y-intercept of 0.389 μg/L (SE, 0.332 μg/L) and a slope of 0.978 (0.014). The correlation was good [r = 0.9977; 95% confidence interval (CI), 0.9956–0.9988; P <0.0001]. Results for serum [median cTnI value (range), 4.08 (0.30–61.99) μg/L] and plasma [5.50 (0.30–57.58) μg/L] did not show a significant difference (Wilcoxon rank-sum test for paired data).