MP-4 - TROPONIN MACROCOMPLEX REPRESENTS AN UNDERAPPRECIATED CAUSE FOR PERSISTENTLY ELEVATED CARDIAC TROPONIN CONCENTRATIONS

Case background: Clinical Presentation:
A 68-year-old female with a history of dilated cardiomyopathy presented to a tertiary care hospital with worsening heart failure (HF) symptoms.
The patient described worsening dyspnea, orthopnea, and paroxysmal nocturnal dyspnea. She was hypotensive (86/63 mmHg) and had a paced rhythm of 60 BPM. Her physical examination demonstrated peripheral edema, elevated jugular venous pressure, and bibasilar lung crackles. In the emergency department, the patient was noted to have multiple runs of non-sustained ventricular tachycardia (VT) below the detection rate of her defibrillator. The patient’s cardiac resynchronization therapy defibrillator (CRT-D) was interrogated, revealing no arrhythmias detected and no therapies delivered.
The patient was admitted to the Cardiac Care Unit for management of decompensated HF. She was noted to have persistently elevated high-sensitivity cardiac troponin I (hs-cTnI) tests (post-admission day 0, PAD0: 4834, PAD1: 4189, PAD2: 3823, PAD3: 4265; upper limit of normal < 17 ng/L). On review of the electronic health record, the patient had elevated hsTnI levels recorded since 2019 (Oct 2019: 5066).
Past Medical History:

The patient had a history of non-ischemic cardiomyopathy (diagnosed in 2014); coronary angiography previously revealed no obstructive coronary artery disease. Genetic testing was felt to be non-explanatory for her cardiomyopathy.

A transthoracic echocardiogram (performed on current admission) revealed a severely dilated left ventricle (LV), global hypokinesis and LV ejection fraction 15-20%. The right ventricle was of normal size with mildly reduced systolic function. There was moderate tricuspid regurgitation and moderate mitral regurgitation.

She had a history of heart block and atrial flutter and was anticoagulated with dabigatran. She had previous episodes of VT, treated with amiodarone and an implantable cardioverter-defibrillator, later upgraded to CRT-D.

Management Challenges: Differential Diagnosis:
The patient’s decompensated HF was felt to be triggered by VT below the detection rate of her CRT-D.
The etiology for her significantly elevated hs-cTnl level was not immediately evident. She had baseline normal renal function and only grade 1 acute kidney injury. Furthermore, she had no evidence of cardiac ischemia (including no chest pain and no repolarization abnormalities on ECG when compared to previous ECGs). Supply-demand mismatch (Type 2 Myocardial Infarction) was felt to be non-explanatory as patient’s hsTnI remained elevated after appropriate diuresis. A diagnosis of myocarditis was entertained, though normal inflammatory biomarkers and the longstanding nature of the hsTnI elevation argued against this.
Investigations:
As part of the work-up of the DCM and possible myocarditis, the treating team ordered a myositis antibody panel (which returned normal).
The possibility of an erroneous lab value (or assay malfunction) was also considered.
The initial blood sample (PAD0), originally run on the Abbott hs-cTnI assay (4,265 ng/L or ~250 xULN) was re-tested on the Ortho hs-cTnI assay yielding a result of 64 ng/L (ULN < 10 ng/L or ~6 xULN). Total creatine kinase (CK) was 143 U/L (ULN < 150 U/L) and CK-myocardial band (CK-MB) was 2.6 ug/L (ULN < 2.9 ug/L); CK-MB/total ratio was 2% (normal ratio). The degree of discrepancy in elevation of hs-cTnI assays along with normal CK raised concern for a possible assay interference.
Two blood samples from POD0 were subsequently subjected to Polyethylene Glycol (PEG) precipitation. Both samples yielding a recovery rate of less than 1%, which is highly specific for the presence of a macrocomplex (i.e., an immunocomplex involving cTn and immunoglobulins which can cause falsely elevated assay levels).

Discussion:
We describe a patient with persistently elevated hs-cTnI leading to diagnostic uncertainty in the setting of DCM of unknown etiology with worsening heart failure and VT. The elevated hs-cTnI was due to troponin macrocomplexes which yielded persistently elevated concentrations. Macrocomplex interference was subsequently proven based on lack of evidence of overt muscle and myocardial injury (i.e., normal CK, CK-MB levels), highly discordant hs-cTnI concentrations using different assays, and PEG precipitation. The identification of troponin macrocomplexes provided diagnostic clarity and helped avoid potential invasive testing.
cTn represents an important diagnostic tool in the work-up of suspected ischemic heart disease and other cardiac pathologies (including myocarditis and HF). Modern hs-cTn assays utilize immunoassays with monoclonal antibodies specific to different epitopes on cTn to quantify cTn concentration. Previous reports have described several potential confounders which may contribute to erroneous cTn levels. These include blood collection tubes, sample matrices, presence of hemolysis, ingestion of biotin, presence of human anti-animal antibodies, and non-specific binding.
Troponin macrocomplexes are another etiology for discrepant hs-cTn assay results with an estimated prevalence of 1-5%.The most common form (type 1 macrocomplex) are immunocomplexes involving troponin and immunoglobulins. Macrocomplexes have been described for other laboratory tests (e.g., prolactin, CK and AST). The presence of troponin macrocomplexes may be suspected in the setting of unexplained persistently extreme elevated hs-cTn levels, with normal levels of other biomarkers with documented cardiospecificity (e.g., CK -MB, heart fatty acid binding protein).
Other cardiac biomarkers and cTn assays, such as hs-cTnT, can be utilized in suspected cases of assay interference. Although, CK-MB is no longer recommended for routine use as a cardiac biomarker, in the setting of a suspected interference of hs-cTnI assays, it can be used to rule out the presence of a large myocardial injury.
PEG precipitation is a method to separate molecules by solubility, causing complexes of high molecular weight to be removed. This includes immunoglobulins, allowing for identification of macrocomplexes. For the Abbott hsTnI assay, a decrease in cTn concentration of more than 80% (recovery < 20%) after PEG precipitation has been shown to identify the presence of macrotroponin. Anti-immunoglobulin anti-sera and Protein A or G resins can also be used for removal of macrocomplexes.
Outcomes:
The identification of troponin macrocomplexes provided diagnostic clarity. Invasive investigations for cardiac ischemia (i.e. coronary angiography) and myocarditis (i.e. endocardial biopsy) were avoided.
The patient was treated for decompensated HF with intravenous furosemide. Her CRT-D was reprogrammed with a lower monitoring zone and treatment zones. Her amiodarone dose was also increased to help prevent recurrent VT.
Unfortunately, the patient later passed away. Her end-stage heart failure and other comorbidities led to recurrent admissions to CCU. Ultimately, the patient and her family requested a comfort care approach.