key: cord-0872930-ydkvx678
authors: Laguë, Marianne; Turgeon, Pierre Yves; Thériault, Sébastien; Steinberg, Christian
title: A false-positive troponin assay leading to the misdiagnosis of myopericarditis
date: 2022-03-28
journal: CMAJ
DOI: 10.1503/cmaj.211842
sha: 66d406415d1b5df01612120c363bd5340ce51597
doc_id: 872930
cord_uid: ydkvx678

nan

antibodies, SARS-CoV-2, Epstein-Barr virus and HIV. Genetic testing for hereditary cardiomyopathies was negative.

These negative findings on an extensive workup for myocardial injury made us suspicious that the results of the cTnI assays were false positives, a suspicion supported by normal levels of creatine kinase-MB at less than 1.0 (ULN 8.0) µg/L and of cardiac troponin T (cTnT) at 12 (ULN 14) ng/L from a high-sensitivity assay (Cobas, Roche Diagnostics). We therefore undertook additional biochemical testing. Analysis of cTnI with another method (Vitros 5600, Ortho Clinical Diagnostics) was close to normal at 11 (ULN 9) ng/L. To assess the possibility of circulating macrotroponin, we conducted a precipitation test with polyethylene glycol (PEG), which showed an initial recovery of 31%, followed by 21% after dilution (the recovery threshold for a positive test has been suggested as < 40%). 1 Therefore, we concluded that the presence of circulating macrotroponin likely caused false-positive TnI results on the Siemens' Dimension Vista 1500 assays.

Reviewing the patient's entire clinical course, we considered that the most likely diagnosis was musculoskeletal pain, and anxiety after her first episode of chest pain that was likely exacerbated by the false diagnosis of myopericarditis. We reassured the patient and discharged her without medication. She had no further episodes despite progressively resuming sport training. One year later, the result of a cTnI test was still slightly positive (130 ng/L, Dimension Vista 1500). 

Clinicians should consider the possibility of a falsely elevated troponin when they observe discordance between the troponin level and the patient's clinical presentation. High-sensitivity troponin immunoassays are subject to false-positive error from either in vivo or in vitro assay interference (Figure 2A ), which can be caused by heterophilic antibodies, autoantibodies or macrotroponin. 2 A previous study found discordance rates of about 1.5% when comparing results from 2 different cTnI assays, most of which were attributed to the presence of a macrotroponin. 3 These elevated cTnI levels from high-sensitivity assays can be clinically misleading, but most false-positive results are small elevations.

The authors of a 2013 review of heterophilic antibody interference in immunometric assays suggested that heterophilic antibodies with affinity to animal antibodies are present in as many as 40% of the population, most often without a known exposure to animal antigens. 4 They are an endogenous cause of in vitro analytical interference. In rare cases, the presence of heterophilic antibodies may lead to false-positive results as they can interact with the animal immunoglobulins in certain reagents, despite optimization of most assays to avoid this interference. A serial dilution of the sample to look for nonlinearity in the cTnI concentration or another analysis of the sample in heterophilic blocking tubes to neutralize the antibodies can help make the diagnosis. We did not perform serial dilution in our patient, and heterophilic blocking tubes are not readily available in our centre.

In vivo interference In vitro interference Circulating autoantibodies against cTnI or cTnT can cause analytical interference and are sometimes associated with autoimmune disorders such as rheumatoid arthritis. 5 The prevalence of cTnI autoantibodies has been reported to be as high as 12.7% in normal blood donors. 5 Macrotroponin, a complex of immunglobulin and cardiac troponin (most often cTnI), is the most common cause of discrepancies between different troponin assays. 1, 6 A recent study showed a potentially higher incidence of false-positive results from macrotroponin with Siemens cTnI assays, including the one used in our case. 1 These high-molecular-weight immune complexes result from the binding of protein fragments or enzymes with circulating immunoglobulins, with or without cardiac muscle injury. This phenomenon has also been observed with several other biomarkers, notably creatine kinase, aspartate aminotransferase, amylase and prolactin. These complexes tend to persist in the blood owing to their reduced clearance and increased half-life, resulting in falsely elevated levels of the biomarker. The troponin level measured by immunoassays when macrotroponins are present is unpredictable, but it often produces an elevation over the 99th percentile. 6 The presence of these physiologically inactive immune complexes has been proposed to result from prior sensitization with infectious antigens, or with immunization to cTnI or cTnT after myocardial injury or muscle damage. 5 Many biochemical tests can be used to confirm the presence of macrotroponin; PEG precipitation tests are increasingly used for this purpose ( Figure 2B ). 1 The patient's serum is mixed with PEG and, after an incubation period that allows for heavy complexes to precipitate, the supernatant is then reanalyzed. Low recovery of cTnI indicates the presence of macrotroponin. The level of recovery after PEG precipitation that determines the presence of a macrotroponin is not well established and likely depends on the protocol and the assay that is used, as well as the troponin level. A study of 6 different assays proposed a threshold of 40% recovery and showed good concordance between PEG precipitation and incubation with protein A at this threshold. 1 Although other studies have used lower thresholds, the recovery observed in our case was lower than what has been reported in control samples. 3, 6 Precipitation with an immunoglobulin-binding protein, like protein A, protein G or another antihuman immunoglobulin G (IgG), is another method to initiate immunoglobulin depletion, which has been shown to have a good concordance with PEG precipitation but is not readily available in our centre. 1, 6 Removing the IgG fragment of the macrocomplex and subsequently measuring cTnI and IgG levels can also indicate the presence of a macrotroponin.

In our patient, we identified macrotroponin I using PEG precipitation. Notably, it was not detected using a different cTnI assay, which could be attributed to the 2 assays recognizing different epitopes. Although less common, we cannot exclude the presence of a heterophilic or other antibody contributing to assay interference in our patient. Interestingly, the patient's serial cTnI levels decreased on follow-up assays; it is possible that the anti-inflammatory treatments she was prescribed caused a decrease in antibody production.

Myopericarditis has the same pathophysiology as myocarditis. For myocarditis, the inflammation is located exclusively in the myocardium, whereas the pericardium is also affected in myopericarditis. The inflammation of the pericardium can cause the typical pleuritic pain of pericarditis, which is worsened by supine position. Other than the spurious troponin elevation, this patient had no specific symptoms or signs of myopericarditis or pericarditis after extensive work-up.

Our case stresses the importance of carefully interpreting laboratory results in the clinical context of each patient, with special attention given to cases with discordance. When discordance is observed, a different laboratory method for quantification of cardiac troponin levels can be used to identify false-positive results. The early recognition of false-positive elevations in troponin could spare patients many invasive and expensive procedures and mitigate anxiety.

Discrepancy between cardiac troponin assays due to endogenous antibodies

What to do when you question cardiac troponin values

High incidence of macrotroponin I with a high-sensitivity troponin I assay

Heterophilic antibody interference in immunometric assays

Circulating cardiac troponin-I autoantibodies in human plasma and serum

The section Cases presents brief case reports that convey clear, practical lessons. Preference is given to common presentations of important rare conditions, and important unusual presentations of common problems. Articles start with a case presentation (500 words maximum), and a discussion of the underlying condition follows (1000 words maximum). Visual elements (e.g., tables of the differential diagnosis, clinical features or diagnostic approach) are encouraged. Consent from patients for publication of their story is a necessity

This article has been peer reviewed.The authors have obtained patient consent. Contributors: All of the authors contributed to the conception and design of the work. Marianne Laguë and Pierre Yves Turgeon drafted the manuscript. All authors revised it critically for important intellectual content, gave final approval of the version to be published and agreed to be accountable for all aspects of the work.

This is an Open Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY-NC-ND 4.0) licence, which permits use, distribution and reproduction in any medium, provided that the original publication is properly cited, the use is noncommercial (i.e., research or educational use), and no modifications or adaptations are made. See: https://creativecommons.org/ licenses/by-nc-nd/4.0/ Correspondence to: Pierre Yves Turgeon, pierre-yves.turgeon@criucpq.ulaval.ca