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Technical Briefs |
1
Laboratoire de Biochimie Médicale and
2
Service de Cardiologie, Hôpital Charles Nicolle, CHUR de Rouen, 76031 Rouen cedex, France
a author for
correspondence: fax 33-2-32-88-87-80, e-mail
alain.lavoinne{at}chu-rouen.fr
Cardiac troponin I (cTnI) is now regarded as one of the most specific markers for cardiac injury, and an increasing number of commercial methods are available. We report here a study of the Immulite cTnI assay manufactured by DPC.
The Immulite cTnI assay is a sandwich immunoassay that uses a monoclonal antibody immobilized on beads and a goat polyclonal antibody labeled with alkaline phosphatase as a tracer (1); both antibodies recognize epitopes localized in the N-terminal part (residues 33–110) of the protein. The chemiluminescent substrate used for the enzymatic reaction is an ester of adamantyl dioxetane phosphate. We used as calibrators purified human cTnI in horse serum. The within- and between-run imprecisions (CV) were <3.9% and 3.6%, respectively, at a mean cTnI of 1.5 µg/L (n = 20). The minimal cTnI value quantified was 0.2 µg/L, and no detectable cTnI was observed in 20 runs of assay diluent. The lowest cTnI concentration giving rise to a within-assay CV <20% was 0.33 µg/L. Linearity was good up to 150 µg/L (r = 0.996).
As observed with the Stratus cTnI assay routinely used in our
laboratory, the results obtained for plasma (y) were 
10%
lower than those obtained for serum (x): y =
0.88x - 0.7; r = 0.999; n = 24.
To simulate moderate and severe icterus, we added up to 375 µmol/L
unconjugated bilirubin to three serum samples (cTnI concentrations of
1.3, 3.7, and 21.6 µg/L). A decrease in the cTnI concentration was
observed at 100 µmol/L bilirubin (10% for each sample). No
detectable interference was observed for hemoglobin (up to 3.4 g/L) or
triglycerides (22 g/L).
In 90 healthy individuals (34 males and 56 females; ages 19–57 years), only two results were above the detection limit (0.34 and 0.41 µg/L). Specificity was determined in 10 patients with rhabdomyolysis (total creatine kinase activity >2000 U/L) and 33 patients with chronic renal failure (creatinine >110 µmol/L) without any cardiac injury. Immulite cTnI was undetectable in 4 of the 10 patients with rhabdomyolysis (with a maximum cTnI value in the 6 remaining patients of 0.51 µg/L) and in 29 of the 33 patients with chronic renal failure (with a maximum cTnI value in the 4 remaining patients of 0.48 µg/L).
The relationship between the Immulite and Stratus systems was compared
in 365 heparinized samples after exclusion of specimens with cTnI
values outside the linear reportable range of the test methods. Of the
365 samples, 37 (10%) had one of the two results below the
detection limit (15 and 22 below the limits for the Stratus and
Immulite assays, respectively) and 42 (12%) had both results
below the respective detection limits. Regression analysis was
performed on the 268 remaining samples: Immulite cTnI = 1.84
(Stratus cTnI) - 1.1 µg/L; r = 0.977;
Sy|x = 4.7 µg/L. The high slope might
result from a lack of standardization between cTnI assays
(2) and/or a difference in the reactivities of the
antibodies used to the various circulating forms of the protein
(3)(4)(5)(6). We routinely used an upper reference limit (URL) of
0.6 µg/L for the Stratus cTnI assay. To estimate the corresponding
value for the Immulite cTnI assay, we established the relationship
between the two assays in 137 samples with Stratus cTnI values <5
µg/L: Immulite cTnI = 1.51 (Stratus cTnI) + 0.27;
r = 0.924. The estimated Immulite URL (based on the
regression) corresponding to a Stratus value of 0.6 µg/L was
1.18 µg/L (Fig. 1
). Using these cutoffs, we studied 80 patients admitted to the intensive
care unit (37 with acute myocardial infarction, 22 with unstable
angina, and 21 with chest pain). Heparinized samples were collected on
admission. The Stratus (0.6 µg/L) and estimated Immulite (1.18
µg/L) URLs gave specificities of 93% [95% confidence interval
(CI), 69.6–98.8%] and 95% (95% CI, 76.2–99.9%),
respectively, for acute coronary syndrome (vs 21 chest pain patients)
and sensitivities of 95% (95% CI, 85.9–98.9%) and 92% (95% CI,
81.3–97.2%), respectively.
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In conclusion, the data presented here demonstrated acceptable analytical performance for the Immulite cTnI assay. Furthermore, there was excellent clinical concordance between the DPC Immulite and Dade Stratus cTnI assays. Additional evaluations will be necessary to define the URL.
Acknowledgments
We gratefully acknowledge Jocelyne Guignery and Sylvie Marinier for helpful technical assistance and Dr. Brigitte Candalot, DPC France (La Garenne-Colombes, France), for kindly supplying the Immulite cTnI immunoassay.
References
The following articles in journals at HighWire Press have cited this article:
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  A. Lavoinne, B. Cauliez, H. Eltchaninoff, C. Tron, and A. Cribier Release of Macromolecular Cardiac Troponin I Complex after Successful Percutaneous Transluminal Coronary Angioplasty in Acute Myocardial Infarction Clin. Chem., March 1, 2003; 49(3): 505 - 507. [Full Text] [PDF]
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V. Scharnhorst, H. L. Vader, and F. van der Graaf Characteristics of the Cardiac Troponin I Assay on the Immulite 2000 Analyzer Clin. Chem., September 1, 2002; 48(9): 1626 - 1627. [Full Text] [PDF]
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