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False positive analytical interference of cardiac troponin I assays: an important consideration for method selection

Molecular interferences are an important challenge in biotechnologies based on antibody–antigen interactions, such as sandwich immunoassays. We report how a sandwich immunoassay with magnetic particles as label can be used to probe interference by surfactants. Surfactants are often used to improve the performance of immunoassays, however the surfactants can affect the involved proteins and the mechanism of action of surfactant molecules on the antibody–antigen system is mostly unknown. As an example, we investigated molecular interference by a nonionic surfactant (Pluronic F-127) in a cardiac troponin (cTn) sandwich immunoassay with two monoclonal antibodies. The influence of the surfactant below the critical micelle concentration (0.00–0.04%) on dissociation properties was quantified in a magnetic tweezers setup, where a force is applied to the molecules via magnetic particle labels. The force-dependent dissociation curves revealed the existence of two distinct cTn-dependent bond types, namely a weak bond attributable to non-specific binding of cTn, and a strong bond attributable to the specific binding of cTn. The dissociation rate constant of the strong bonds increased with the surfactant concentration by about a factor of two. Circular dichroism spectroscopy data showed that the nonionic surfactant influences the conformation of cTn while not noticeably affecting the two monoclonal antibodies. This suggests that the surfactant-induced increase of the dissociation rate of the specific sandwich-type cTn binding may be related to a conformational change of the antigen molecule. The described methodology is an effective tool to study the influence of surfactants and other interferences on assays based on protein interactions.

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.

To determine whether glycoprotein (GP) IIb/IIIa inhibitors interact with cardiac marker assays: total creatine kinase (CK), CK-MB, and troponin-I (TnI).

Two blood samples were collected during coronary angiography or angioplasty and analyzed for cardiac markers: one after arterial sheath insertion and the second 10 mins later. In the first phase (n = 101), heparin and GP IIb/IIIa inhibitors were given between samples, in the second phase (n = 45) only heparin was given, and in the third phase (n = 54) neither drug was given.

In each phase, there was a significant decline in total CK over the 10 min period. There were no consistent significant changes in CK-MB or TnI.

Neither GP IIb/IIIa inhibitors nor IV heparin interact with cardiac marker assays. There is a small but significant decline in total CK levels within 10 mins of arterial puncture that is unrelated to administration of these drugs.

Coronary artery disease in uremia: Etiology, diagnosis, and therapy. Cardiovascular disease is a major challenge to nephrologists, whether we deal with patients with pre–end-stage renal failure, on dialysis or after successful renal transplantation. It is the most common cause for death in patients with a functional allograft, and prevents many dialysis patients from being engrafted. Coronary artery disease is a diagnostic and therapeutic challenge, as it differs in some respects from that seen in non-uremic cohorts, and lacks much of the evidence-base on which therapeutic intervention rests. This review examines the experimental and clinical literature on cardiovascular disease in uremia, focusing on coronary artery disease. We focus on the incidence, presenting syndromes, screening tools, and interventions in the context of acute and chronic coronary syndromes. Recent evidence comparing coronary angioplasty, coronary artery stenting, and bypass surgery in subjects with renal failure is also reviewed. Coronary artery disease is more prevalent in uremia, more difficult to diagnose and less rewarding to treat compared to non-uremic subjects. Many more randomized trials are needed. In the absence of information from such trials, we advocate aggressive control of conventional and novel cardiovascular risk factors, and early intervention for symptomatic coronary disease.