Unexpected Troponin Elevation in a Patient Treated with Atorvastatin

The measurement of the cardiac troponins cardiac troponin T (cTnT) and cardiac troponin I (cTnI) represents a paradigm shift in the ability to diagnose myocardial infarction compared to the conventional cardiac enzymes creatine kinase (CK) and its MB isoenzyme (CK-MB). A particular problem with CK and CK-MB is the ability to detect myocardial injury in the presence of skeletal muscle injury. Although early problems were documented with the first-generation cTnT assay, the problem of cross-reaction with skeletal troponin T was removed in subsequent generations of the cTnT assay. It has therefore always been considered that measurement of cTnT using the current assay is not subject to interference by skeletal muscle injury. A female Asian, age 65 years, was referred for statin intolerance. She had previously been taking atorvastatin 40 mg daily for secondary prevention following triple-vessel coronary artery bypass grafting 7 years previously (2011). She had noted some generalized muscle weakness, most marked in her quadriceps complex prior to presentation. When going on holiday she was transferring from the airport to her hotel but found herself unable to get out of the car on reaching her hotel. She had no cardiac symptoms at the time she was seen and no shortness of breath. Her past medical history included type II diabetes mellitus (for the past 8 years) and asthma. Medication at first consultation was metformin 500 mg bd, zafirlukast 20 mg od, vitamin D3 (cholecalciferol) 1000 units daily, clopidogrel 75 mg od, omeprazole 20 mg od, losartan 50 mg od, ezetimibe 10 mg od, tiotropium inhaler 18 µg, and Spiro base inhaler. Both troponin assays were highly sensitivite (hs). All measurements were performed on a Roche Diagnostics Cobas system (Roche Diagnostics) except cardiac troponin I (hs cTnI, Architect, Abbott Diagnostics LOD 1.1 ng/L, 10% CV 4.7 ng/L, reference interval (RI) female 15.6 ng/L) with analytical range, coefficient of variation (CV) and RI as follows: CK, 7–2000 U/L, 0.5–1.4%, <200 U/L; alanine transaminase (ALT), 5–700 U/L, 2.9–3.6%, <40 U/L (female); hs cTnT, fifth-generation assay (3–10 000 ng/L, 15–1.3%, 10% CV13 ng/L, <14 ng/L; total 25-OH vitamin D 7.5–175 nmol/L, 6.8–2.2%, >50 nmol/L. On attendance (June 2017) she was noted to have an elevated CK (2853 U/L) with an elevated cTnT (55 ng/L). Total 25-OH vitamin D was acceptable at 60 nmol/L. Genetic testing (next-generation sequencing) excluded familial hypercholesterolemia with intermediate risk of polygenic hypercholesterolemia and the Rs4149056 genotype T/C (SLCOB1 gene, intermediate risk of myositis). A rheumatological opinion was sought and clinically confirmed the diagnosis of myositis. A screen for myositis-specific and myositis -associated antibodies was performed and found to be negative for isoleucyl tRNA (OJ), glycyl tRNA synthetase (EJ), alanyl tRNA synthetase (PL-12), threonyl tRNA synthetase (PL-7), signal recognition particle (SRP), histidyly tRNA synthetase (Jo-1), Pm-Scl75, Pm-Scl100, Ku, small ubiquitin-like modifier 1 activating enzyme (SAE), NXP-2, melanoma differentiation associated gene (MDA5), TIF1γ/α, helicase protein (Mi-2), and Ro52, but positive for an antibody to HMGCoA reductase. The conclusion was an autoimmune myositis induced by statin therapy. Her symptoms resolved with statin cessation with a fall in CK. Her serial biochemical results are summarized in Table 1. Serial results of investigations performed following the initial consultation and subsequent follow-up. Abbreviations: CRP, C-reactive protein; ALT, alanine transaminase; CK, creatine kinase. Serial results of investigations performed following the initial consultation and subsequent follow-up. Abbreviations: CRP, C-reactive protein; ALT, alanine transaminase; CK, creatine kinase. Because no cardiac symptoms were reported at any time, further cardiac investigations were not performed. In the absence of cardiac symptoms and in the presence of an elevated CK and cTnT, a false positive cTnT due to assay interference was suspected. A sample was therefore sent for simultaneous measurement of cTnT and cardiac troponin I (cTnI). The cTnT was 122 ng/L with a cTnI of 5 ng/L. This excluded myocardial injury as a cause of the cTnT elevation. Serial dilution of the sample showed linear dilution, excluding a heterophile antibody. A subsequent sample was diluted 1 in 2 with troponin-free diluent and with polyethylene glycol (PEG) (25% w/v). Samples were incubated at room temperature for 10 min and centrifuged at 14 000 g for 5 min, and cTnT was analyzed in the supernatant. The original cTnT was 25 ng/L. Following a 1:2 dilution value with diluent, a cTnT value of 13.59 ng/L was obtained. After 1:2 dilution with PEG, a cTnT value of 15.21 ng/L was obtained. This gave a bioactive cTnT of 30.42 ng/L with recovery of 111.9% hence over-recovery. It was concluded that macrotroponin T was not present, as previous studies have shown that recovery of 5–10% occurs in samples containing macrotroponin T. The elevated cTnT with cTnI within the reference interval excluded myocardial injury. Polyethylene precipitation excluded a macrotroponin causing cTnT elevation. On follow up over the subsequent 15 months, cTnT fell in parallel with CK (Table 1). She was not rechallenged with a statin. The first generation cTnT assay used a cardiac-specific capture antibody but a noncardiac-specific detection antibody. Because of the assay format, an analytical interference occurred in the presence of high concentrations of skeletal muscle troponin T (sTnT) (1,). The second-generation assay used two cardiac-specific antibodies and completely resolved the problem. The current high-sensitivity (hs) cTnT assay uses fragment antigen binding (FAB) portions of 2 cTnT-specific mouse MAbs directed against epitopes in the central region of human cTnT. It is documented as having no assay interference from sTnT (2). Elevation of cTnT but not cTnI has been reported in conditions with muscle damage and muscle regeneration, specifically in patients with renal failure. However, there is controversy in the interpretation of the methodology used in the original studies (3,). It is now thought that the elevation of cTnT seen in patients with renal failure is not artifactual and reflects myocardial injury (4). The question of potential assay interference in patients with chronic muscle diseases remains an open question. There have been a number of reports of elevation of troponin T but not troponin I in patients with muscle disease (5–8,). The mechanism is considered to be re-expression of a cardiac isoform of troponin T in regenerating skeletal muscle (9,). A recent publication supports this with demonstration of mRNA to the TNN2 gene, a cardiac isoform of troponin T, plus detectable cTnT in inflammatory muscle disease (10). This case represents autoimmune myositis secondary to statin therapy with skeletal muscle regeneration and likely re-expression of a cross-reacting TnT isoform. True false-positive elevations of cTnT are rare but do exist. When clinical features do not match the laboratory findings, discussion with the laboratory and further investigations are required. Laboratories should have a protocol to identify potential false-positive results, including reciprocal arrangements to measure the analyte by a different methodology. Exclusion of macrotroponin is a straightforward procedure. Identification of the false-positive result prevented unnecessary invasive cardiac investigations. All authors confirmed they have contributed to the intellectual content of this paper and have met the following 4 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; (c) final approval of the published article; and (d) agreement to be accountable for all aspects of the article thus ensuring that questions related to the accuracy or integrity of any part of the article are appropriately investigated and resolved. Authors’ Disclosures or Potential Conflicts of Interest: Upon manuscript submission, all authors completed the author disclosure form. Authors' disclosures and/or potential conflicts of interest: Employment or Leadership: P.O. Collinson, The Journal of Applied Laboratory Medicine, AACC. Consultant or Advisory Role: None declared. Stock Ownership: None declared. Honoraria: None declared. Research Funding: None declared. Expert Testimony: None declared. Patents: None declared.