In the present study, we compared data from samples collected from patients undergoing clinical testing for suspected or confirmed MI, rather than those from samples obtained from healthy volunteers in a non-emergency environment. We found that the Spearman correlation coefficient was unity, indicating a perfect correlation, when hs-cTnT and CK-MB test results on samples collected into RSTs and SSTs were compared.
Clotting should continue for 30 min to ensure complete clot formation with SST tubes
. If a tube is centrifuged too soon, gelatinous and/or fibrin-containing serum samples will be obtained; these will clog the analyzer probes. Roberts et al.
 observed that incompletely clotted specimen contributed to the false elevations in cardiac TnI concentrations with the Stratus II batch analyzer (Dade International). In emergency laboratories, this timeframe is often unrealistic. Complete clotting may occur in 5 min if blood sample tubes contain thrombin
, the RSTs used in the present study are such tubes. We have shown herein that simultaneous quantification of CK-MB and hs-cTnT in samples derived from RSTs and SSTs yielded similar results.
In our present work, Deming regression analysis yielded the appropriate y-intercept value of zero for both tests but the slope of the regression line did not approximate unity, suggesting that, in addition to the presence of a small constant bias, a small (and nonsignificant) proportional difference existed between the test results obtained upon use of the two tube types
The acceptable degree of imprecision of both the CK-MB and hs-cTnT assays is < 10% at the 99th percentile reference level
. In our hands, the imprecision levels of either assay were below this value. Notably, participation of our laboratory in internal and external quality assurance programs, and use of an automated analyzer, indicate that RST tubes may be used to reduce TAT. The quality of assay data is high. Importantly, many reports have shown that serum, rather than heparinized plasma, should be used for cardiac marker determination
[10, 11]. In a recent study, Strathmann et al.
 showed that specimens collected into RSTs yielded fewer false-positive immunoassay results than did those collected as heparinized plasma samples when the Beckman Coulter Unicel DxI instrument was used to evaluate the levels of troponin I and the creatine kinase-MB isozyme.
The US CLIA 1988 rules indicate that differences of 7.8% in CK-MB levels, and 23% in cTnT levels, around target values, are acceptable
 because of biological variation. In the present study, we observed that the hs-cTnT concentrations of serum samples collected into SSTs were slightly higher than were those of samples collected into RSTs. We also found that the serum CK-MB concentrations of samples collected into SSTs were somewhat higher than were those of samples collected into RSTs, but no detected difference was clinically significant. The bias values were smaller than are the US CLIA 1988 targets. The bias may be attributable to the fact that samples taken into SSTs have a longer clotting time; RST samples are analyzed about 30 min earlier than are SST samples. Also, the two tube types employ different clotting agents.
An increase of >2.0 ng/ml in CK-MB level compared to baseline concentrations is indicative of myocardial damage in patients without AMI. For both tube types, the CK-MB assay bias was 0.249 ng/ml, with zero included in the 95% confidence intervals. Such biases are acceptable
, although additional data are needed to support the hypothesis that results from SST and RST are clinically interchangeable.
Heparin is utilized clinically to inhibit clotting in critical care patients. Blood samples from patients administered heparin prior to blood collection can contain excess concentrations of heparin, increasing clotting time in the collection tube and thereby increasing the potential for the formation of “latent” fibrin in the preanalytical phase. Although few of the samples in this study were from patients receiving heparin therapy, the inclusion of this subgroup may have confounded our results and constitutes a limitation of this study. Additional studies are needed to assess the influence of heparin therapy on CK-MB and hs-cTnT concentrations measured in RST and SST tubes.
Preanalytical variables associated with blood collection should be further standardized to ensure the accuracy of test results. It is impractical to expect tube manufacturers to test their tubes on all possible assay platforms; this is a task for individual laboratories.