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Departments of
1
Clinical Chemistry, and
2
Cardiology, Liège University Hospital, B35 Sart Tilman, B-4000 Liège, Belgium
a Author for correspondence. Fax 32-4-3667666; e-mail JP.Chapelle{at}chu.ulg.ac.be
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To the Editor: |
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The Stratus CS uses closed routine sample tubes containing
anticoagulated whole blood (lithium heparin). The system takes 
14 min
to centrifuge the tube and perform the analysis. The Stratus CS can
also manage precentrifuged plasma samples. In the present study, the
Stratus CS was used to determine cTnI on whole blood and on
preprocessed plasma samples drawn from the same patients. Our purpose
was to check the similarity of the results obtained for both samples in
the entire measuring range. We also aimed at comparing results from the
Stratus CS with cTnI results yielded by the Abbott AxSYM analyzer, with
special attention to concentrations below the threshold for AMI.
The Stratus CS is a fluorometric enzyme immunoassay analyzer for quantitative determination of the cardiac markers creatine kinase-MB mass, myoglobin, and cTnI (Dade Behring). The test system uses radial partition immunoassay technology, which has been enhanced through the use of a monoclonal capture antibody coupled to Starburst® dendrimers (7). According to the insert for the cTnI method, coronary risk increases at cTnI concentrations >0.4 µg/L, and this value was considered as the cutoff for AMI in the present study. Reference values are <0.08 µg/L (6). The two monoclonal antibodies used in the method recognize both free and complexed cTnI. The results were compared with those obtained by the AxSYM cTnI microparticle enzyme immunoassay (Abbott Laboratories). This system also allows for automated quantification of cTnI in serum or plasma (lithium heparin) (8)(9). According to the manufacturer’s package insert, cTnI values >0.4 µg/L are increased above the reference values established in blood donors, whereas values >2.0 µg/L are indicative of AMI.
Blood samples were obtained from 85 patients hospitalized at the University Hospital of Liège, Belgium. Twenty-three patients were admitted for suspected AMI; diagnosis was confirmed in 16 cases (14 Q-wave AMI and 2 non-Q-wave AMI), and a myocardial lesion was excluded in 7 cases. Twelve patients had unstable angina pectoris, 9 had stable angina, and 19 had undergone cardiac surgery. In the remaining patients were 5 patients with polytrauma, 1 non-cardiac surgery patient, and 16 chronically hemodialyzed patients.
In this study, we used two different types of samples: one tube (tube I; Hemogard lithium heparin, Vacutainer; Becton Dickinson) to be analyzed without further treatment on the Stratus CS, and one (tube II; Venoject-II lithium heparin + separator; Terumo Europe) to provide the plasma to be analyzed on the two analyzers. A sample was collected into tube II from each of the 85 above-mentioned patients. In a subgroup of 53 patients, tube I was taken in addition to tube II. Both tubes were taken within periods of time not exceeding 2 min. The order in which the tubes were sampled (tube I first or tube II first) was randomized in each category of patients. Tube I was kept at room temperature and analyzed within 2 h. No manual treatment of the tube was required prior to the assay.
cTnI results obtained for whole blood were compared with the corresponding results obtained for plasma aliquots from tube II, also analyzed by the Stratus CS using the special design for plasma samples (comparison A1). For 15 patients, selected at random among the patients with cTnI results <0.15 µg/L on the Stratus CS, tube I was removed from the Stratus CS after completion of the analysis, decapped, and centrifuged manually (8000g for 10 min). The supernatant was transferred to a cup, kept at 4 °C, and reanalyzed on the Stratus CS. Each plasma sample was analyzed within 1 h after the corresponding whole blood sample, and the results were used for a second comparison of whole blood with plasma on the Stratus CS (comparison A2). Tube II was kept at room temperature and centrifuged at 3000g within 1 h. The supernatant was divided into two aliquots and kept at 4 °C. One aliquot was analyzed on the Stratus CS, the other on the AxSYM; both plasma samples were analyzed simultaneously on the two analyzers within 2 h after blood collection (comparison B1). We also compared the cTnI results obtained on the Stratus CS (whole blood) with those obtained on the AxSYM, using an aliquot of plasma from tube II (comparison B2).
The measuring range for cTnI on the Stratus CS is 0–50 µg/L, similar to that of the AxSYM. One sample had to be diluted on the two instruments (72.8 µg/L on Stratus CS and 369 µg/L on AxSYM). Seven additional samples (10.2–33.5 µg/L on Stratus CS) were above the measuring range on the AxSYM and had to be diluted. The calculated results (56.0–369.0 µg/L) were included in the comparisons.
In the 53 patients for whom tubes I and II were available, we compared
(comparison A1) the cTnI values obtained by the Stratus CS on whole
blood and plasma; the values were related by the equation:
 | (1) |
plasma - 0.075
(-0.20 to 0.05); r2= 0.998
When only cTnI values <0.15 µg/L (n = 29) were selected
for the comparison, the regression equation was:
 | (2) |
(-0.008 to 0.003); r2= 0.923
For 15 patients of the latter subgroup, the results obtained by the Stratus CS on whole blood were compared with those obtained for the plasma in the same system after subsequent centrifugation of the tubes (comparison A2). There was no statistically significant difference between the initial values and results obtained after manual centrifugation (Student t-test for paired observations).
The comparison of measurements obtained for plasma samples (n =
85) analyzed on the Stratus CS and AxSYM (comparison B1) indicated that
the cTnI values on the two analyzers were related by the equation:
 | (3) |
0.06 (-0.37 to 0.25);
r2= 0.981
The regression equation between results obtained for whole blood
on the Stratus CS and plasma on the AxSYM (comparison B2) was close to
Eq. 3
. When the comparison was performed for results obtained for the
64 patients with cTnI values <2.0 µg/L on the AxSYM (comparison of
plasma samples), the regression equation was:
|
| (4) |
0.015 (0.013–0.017); r2= 0.789
To analyze the discrepancies between the results obtained by the
two methods in this clinically important concentration range between
the upper reference value (URL) and the cutoff for AMI, the cTnI
results were expressed in relation to the respective URL: results
obtained on the Stratus CS were divided by 0.08 and those obtained on
the AxSYM were divided by 0.4. Fig. 1
shows the distribution of the differences between each pair of
transformed results (value on AxSYM minus value on Stratus CS) in
relation to their mean value. The mean of the differences (
) was
not statistically different from zero, indicating that there was no
systematic bias toward one method. Nevertheless, based on the threshold
values of the methods (0.08 and 0.4 µg/L for Stratus CS, 0.4 µg/L
and 2.0 µg/L for AxSYM), nine patients were classified in different
diagnostic categories according to the method used. In four patients
for whom unstable angina or AMI was excluded and who did not show
increased cardiac markers in the following hours or days, cTnI results
on AxSYM were above reference values (0.5 and 0.6 µg/L), whereas the
Stratus CS yielded negative results (<0.08 µg/L). On the other hand,
in contrast to the negative results on AxSYM, values between 0.08 and
0.4 µg/L on the Stratus CS were found in five samples from patients
with myocardial damage. One of these patients was in the early phase of
AMI (the two methods indicated positive results 2 h later), two
had unstable angina, and two others were cardiac surgery patients
(early postoperative period). Consequently, for all discordant results,
the Stratus CS was in agreement with the clinical diagnosis. There was
no discrepancy between the methods for results >2.0 µg/L on the
AxSYM.
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In conclusion, when performed on samples deriving from two different
tubes (whole blood and heparinized plasma) drawn from the same patients
at the same time, cTnI values obtained by the Stratus CS in the entire
measuring range, including values 0.15 µg/L, were statistically
indistinguishable. This attests not only to the excellent precision of
the method, as already stated by others (6), particularly in
the low concentration range, but also to the quality of the
preanalytical phase automatically performed by the system. The
agreement of the results allows the system to be used on different
specimens in the same patients.
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