Serum and plasma samples for ACS:systems cardiac markers.

To The Editor: The ACS:180 immunoassay system (Bayer Diagnostics) is an automated random analyzer that uses acridinium ester-based chemiluminescence detection. The architecture of the cardiac marker assays is a heterologous sandwich format, using monoclonal anti-marker antibody immobilized onto paramagnetic particles (capture) and polyclonal goat anti-human myoglobin, creatine kinase MB isoenzyme (CK-MB), or cardiac troponin I (cTnI) antibody labeled with acridinium ester (detector). Its processing time (;20 min if the analyzer is in standby mode) and precision (overall CV ,5%) make the system suitable for emergency use (1 ). However, optimal performance of an assay system for cardiac markers depends not only on the instrument’s analysis time, but also on factors that affect the overall turnaround time, including the preanalytical steps necessary to prepare the sample (2 ). The use of plasma samples eliminates the extra time needed for clotting, thereby reducing the overall preanalytical time, but there can be significant differences between serum and plasma concentrations of cardiac markers (3 ). The aim of this study was to validate the possible use of two different anticoagulants (lithium heparinate and EDTA, tripotassium salt) for cardiac marker determinations on the ACS: 180 immunoassay system. Twenty-five patients with acute myocardial infarction were studied. All gave informed consent. We collected three separate tubes (Sarstedt) to prepare serum, heparin plasma, and EDTA plasma in random order during one phlebotomy. ACS myoglobin, CK-MB, and cTnI assays were performed in duplicate on each of the 25 samples. The mean results on plasma samples were compared with the paired serum values, and the significance of the differences was evaluated (Wilcoxon rank-sum test). The results obtained are shown in Fig. 1. ACS myoglobin concentrations in serum [median myoglobin value (range), 79 mg/L (24–607 mg/ L)] and heparinized plasma [76 mg/L (24–588 mg/L)] showed no statistically significant difference (P 5 0.0799). For plasma EDTA, the assay showed a mean change in myoglobin concentration [73 mg/L (25–601 mg/ L)] of 23.5% compared with paired serum samples (P 5 0.0006). Results for serum [median cTnI value (range), 11.3 mg/L (0.4–43.6 mg/L)] and heparinized plasma [11.7 mg/L (0.4–47.7 mg/L)] also were not statistically different with the ACS cTnI test (P 5 0.704). On the other hand, for plasma EDTA, this test showed a mean change in cTnI concentration [14.7 mg/L (0.5–49.3 mg/L)] of 30% compared with paired serum samples (P ,0.0001). Finally, CK-MB values for heparin [35 mg/L (5–252 mg/ L)] and EDTA samples [34 mg/L (5– 243 mg/L)] averaged 18.6% and 14.6%, respectively, higher than those in serum samples [29 mg/L (4–199 mg/L); P ,0.0001]. Although in our study serum and heparinized plasma results for myoglobin and cTnI showed no statistical differences, we cannot definitively conclude that there is no difference in results between these sample types because the statistical power of the performed tests was too low (0.38 for myoglobin and 0.05 for cTnI) to provide an acceptable degree of certainty. Furthermore, although there may not be a substantial overall mean difference in results obtained using heparinized plasma vs serum Fig. 1. Relationship of fecal fat (n 5 111) determined with the old and new extraction procedures.