Comparison of Abbott Architect high-sensitivity troponin I in Rapid Serum Tubes and plasma

A rapid clotting serum tube containing a thrombin-based clot activator, the BD Vacutainer® Rapid Serum Tube (RSTTM; BD, Franklin Lakes, NJ, USA), gives a faster clotting time of around 5 min, compared to 30 min for standard serum tubes (BD). The faster clotting time offers potential improvement to turnaround times for serum samples. RST tubes have therefore, previously been evaluated using various cardiac troponin (cTn) assays (including contemporary cTnI assays [1, 2] and cTnT assays [3–5], showing generally good agreement with serum [4, 5] and plasma specimens [1–3]. We wanted to compare cTnI results in RST and plasma (a common specimen for stat testing) using a new high-sensitivity cTnI assay. We collected three blood tubes, including an RST, BD Vacutainer PSTTM II (PST; heparin plasma, gel separator) and a BD Vacutainer Heparin Tube (non-gel, heparin plasma), from patients presenting to an emergency department (ED) and requiring a blood test, over an 8-day period. cTnI was promptly assayed on all three tubes using the STAT High Sensitive Troponin-I assay (Abbott Diagnostics, Abbott Park, IL, USA) with laboratory decision levels of ≤ 16 ng/L in females and ≤ 34 ng/L in males. In-house quality control (QC) data gave overall %CV (for our two Abbott Architect analysers) of 12.9% (9.2 ng/L), 7.0% (51.0 ng/L), 5.8% (1213.9 ng/L) and 9.6% (6375.5 ng/L) [using Liquichek Cardiac Markers Plus Control LT (BioRad Laboratories, Hercules, CA, USA)]. Differences in cTnI results between the tubes were assessed using Wilcoxon-signed rank test and differences in the proportion of elevated results, with χ2-test statistic, using software from SigmaPlot (Systat Software, San Jose, CA, USA) and SAS 9.3 (SAS, Cary, NC, USA). For the comparison statistics, results below the assay limit of detection (1.9 ng/L) were assigned that value. Details of the sample collection can be found in [6]. In total, 347 patients were included in the study (median age 50 years and 53.0% female). The cTnI median value (in PST) was 2.4 ng/L (range < 1.9 ng/L–35,021 ng/L) (Table 1, Figure 1). In total 57.3% (199/347) of PST samples had detectable cTnI and 12.4% (43/347) had elevated results above the decision levels. A small but statistically significant increase in cTnI was detected in the RST group compared to the plasma specimens (median difference ≤ 1.1 ng/L in samples with detectable cTnI), but no significant difference was found between the two plasma tubes (Table 1). The proportion of elevated results (above the decision levels) did not significantly differ between any of the tube groups (χ2 = 0.013, p = 0.91). In two patients, however, results in the different tubes were on opposing sides of the decision levels, but in both cases the values were all close to the cut-off concentration. The respective PST, RST and non-gel plasma concentrations for these two patients were 33.8 ng/L, 38.5 ng/L, 34.5 ng/L; and 32.0 ng/L, 36.6 ng/L, and 37.6 ng/L. Larger differences were observed in some specimens (Figure 1). In samples with cTnI concentrations ≤ 50 ng/L (n = 325), however, the absolute differences between tubes were all ≤ 8.9 ng/L (except for one patient with concentrations of 21.9 ng/L, 32.7 ng/L and 20.2 ng/L in the PST, RST and non-gel specimens, respectively). Additional data on samples with elevated results is available in the Supplemental Data (Table 1, Figure 1) that accompanies the online version of this letter at http:// *Corresponding author: Joshua B. Ryan, Biochemistry Department, Canterbury Health Laboratories, Christchurch, New Zealand, Phone: +64 3 3640889, Fax: +64 3 3640889, E-mail: Josh.Ryan@cdhb.health. nz Sandy J. Southby, Lesney A. Stuart, Christopher M. Florkowski and Peter M. George: Canterbury Health Laboratories, Christchurch, New Zealand