Positive Interference from Contrast Media in Cardiac Troponin I Immunoassays

Cardiac troponin I (cTnI) has been found to be a sensitive and reliable marker of myocardial damage, and elevated levels of cTnI can indicate high risk for acute coronary syndrome. To determine how to intervene in possible cases of acute coronary syndrome, cTnI levels must be measured by immunoassay. However, cTnI immunoassay results are prone to interference from many substances such as heparin and common drugs. The contrast media used in the coronary angiography might also interfere with results. To explore this possibility, we performed two in vivo and two in vitro studies. In the first in vivo study, we evaluated the effects of contrast media on cTnI immunoassays by collecting blood samples from 45 patients undergoing coronary angiography before and after the procedure. We used the Opus Magnum immunoassay system to measure cTnI levels. In the second in vivo study, we collected 25 blood samples from another group of patients also undergoing angiography at various times before and after the procedure to determine cTnI values by both the Opus Magnum and ACCESS systems. In the first in vitro study, 12 different contrast media were treated as samples to disclose the potential interference of measurement in the two assay systems. In the second in vitro study, we made sequential dilutions of iopromide (Ultravist; Schering) with serum to explore their potential for interfering with the detection of cTnI by the Opus Magnum and ACCESS assays. In the first in vivo study using the Opus Magnum assay, cTnI concentrations in samples taken after angiography were significantly higher at 5 minutes than at 30 minutes, and, at 60 minutes, all cTnI concentrations had dropped below the cutoff point. In the second in vivo study, we found a substantial difference in detection of cTnI by the Opus Magnum and ACCESS assays. All cTnI concentrations checked by ACCESS assay were below the cutoff value. In our first in vitro study, the Opus Magnum assay gave false positive results for all 12 contrast media; the ACCESS assay gave a positive result for only one contrast medium, poppy-seed oil (Lipiodol; Guebert). In our second in vitro study, we found that, in the Opus Magnum assay, the more concentrated the contrast medium, the higher the cTnI value, but not in the ACCESS assay. We conclude that contrast media may cause false-positive results in cTnI assays and that, when contrast media are being used for angiography, cTnI results, especially those based on samples taken within the first hour of the procedure, should be interpreted carefully.

Key Words:  immunoassay , interference , cardiac troponin I , contrast media

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