Comparison of Single and Repeat Centrifugation of Blood Specimens Collected in BD Evacuated Blood Collection Tubes Containing a Clot Activator for Cardiac Troponin I Assay on the ACCESS Analyzer

Cardiac troponin I (cTnI) has become a major marker in the diagnosis and monitoring of myocardial damage. On the ACCESSTM analyzer (Beckman Coulter), serum is the biological fluid compatible with both myoglobin and cTnI tests (1). However, residual fibrin (2) and microparticles (2) have been reported to interfere with one or more cTnI assays. Some of these interferences can be avoided with repeat centrifugation of the sample and/or the use of a clot activator (3). The BD VacutainerTM HemogardTM Thrombin glass evacuated blood collection tube (BD Thrombin) contains thrombin as a clot activator and may, therefore, obviate the need for repeat centrifugation of samples and the resulting delays in testing. This open comparative study (without masking of phlebotomists, technicians, or investigators) was performed in the Laboratory of Biochemistry of the University Teaching Hospital Beaujon between August and October 1999. Its objective was to assess whether single centrifugation of blood specimens is adequate for samples collected into BD Thrombin tubes for the assay of cTnI on the ACCESS analyzer. Patients with chest pain from the emergency department and patients with acute coronary syndrome from the cardiology intensive care unit were included. Samples were drawn in duplicate into BD Thrombin and BD Vacutainer Hemogard SSTTM glass tubes (BD SST), the laboratory reference tube, whose sample is routinely centrifuged twice for this assay. Samples were centrifuged at 1300–2000g for 10 min at 10 °C. The first 64 results (cTnI range, 0–1.9 μg/L) included 10 (16%) above the threshold (0.1 μg/L). Results showed a random positive bias of the “BD Thrombin + single centrifugation” combination when compared with “BD SST + double centrifugation” (P <0.0001, paired t-test). Deming regression analysis yielded the following: slope = 1.16 [95% confidence interval (95% CI), 1.03–1.29]; intercept = 0.04 μg/L (95% CI, 0.02–0.06 μg/L). Table 1 shows the assay values of the six apparently false-positive results. These results prompted a second phase, during which the BD Thrombin sample was also centrifuged twice. Nineteen additional patients were included. The difference between the results (cTnI range, 0.01–24 μg/L, 32% above threshold) was not statistically significant as analyzed by the paired t-test and the Deming regression [slope = 1.04 (95% CI, 0.93–1.14); intercept = −0.03 μg/L (95% CI, −0.08 to 0.02 μg/L)]. We conclude that a single centrifugation of collection tubes containing thrombin as a clot activator is insufficient to avoid false-positive cTnI results on the ACCESS analyzer. Repeat centrifugation eliminates false-positive results. The presence of thrombin in collection tubes does not significantly affect cTnI results by this assay. Roberts et al. (3) published similar results. Reference tube and BD Thrombin + single centrifugation discrepant false-positive cTnl results (μg/L). Reference tube and BD Thrombin + single centrifugation discrepant false-positive cTnl results (μg/L). We thank Sanofi Diagnostics Pasteur (Marnes La Coquette, France) for providing the reagent, and BD France SA for providing the blood collection tubes.