In cardiac marker testing, choosing the right controls is critical

If there is one scenario common to all emergency rooms, it is probably the patient presenting with some combination of chest pain and shortness of breath. That is why clinical laboratories have to be prepared to run cardiac biomarker tests day and night. And since cardiac troponin tests are counted on to help distinguish between life-threatening heart attacks and relatively harmless panic attacks, they also have to be completely reliable.

With widely available cardiac troponin tests, that reliability may seem like a given. It isn’t. In fact, there are a number of challenges for assuring quality in the cardiac marker testing process. Perhaps most important is there is no established reference standard for troponin quality controls, making it difficult for laboratory professionals to know that they are using the best testing components. It is also tough to compare test results across laboratories — even labs in the same healthcare system — because testing ranges can vary by manufacturer and platform. Another factor is that normal and abnormal cardiac troponin levels are different in males and females, in older and younger individuals, and in the same people at different times of day. This limits the leeway that can be permitted in test results and increases the pressure to ensure exact answers.

The upshot is that clinical lab teams must find ways to perform the best quality control steps to keep their cardiac testing results as rigorous and reliable as possible.

Troponin testing

In the aftermath of a myocardial infarction, damage to the heart will cause cardiac-specific troponin levels in the bloodstream to surge, spiking within a few hours and remaining elevated for several days. There are two isoforms of the troponin protein relevant to this situation that can be detected with clinical cardiac assays: troponin I, which is only produced in the heart, and troponin T, which is also produced in the heart but is present at low quantities in other muscles (a third form, troponin C, is not specific to the heart and is not useful for cardiac marker testing). The higher the levels of cardiac troponin, the more severe the heart attack.

Evidence has underscored the need to act quickly in the case of myocardial infarction, which requires generating rapid results from cardiac troponin tests. Fortunately, tests have gotten faster and more sensitive in recent years, giving clinical teams the opportunity to diagnose cardiac events sooner, delivering appropriate interventions earlier and leading to better outcomes for patients.

Modes of detection

The advent of high-sensitivity and ultra-high-sensitivity testing for cardiac troponin has been a significant leap forward for the clinical laboratory community. This follows years of continued improvements in sensitivity for troponin tests; the most sensitive options now have detection limits 100-fold lower than conventional assays.¹ Even tests that don’t meet this mark are about ten times more sensitive than assays that were once common in clinical labs. Still, manufacturers continue to aim for even lower limits of detection, with publicly stated goals of detecting troponin levels at less than 1 ng/L.¹ With higher levels of sensitivity, troponin testing can be performed sooner: there’s no need to wait for levels to get high enough for detection. This enables faster results and earlier intervention. Today, high-sensitivity immunoassays are available from several commercial manufacturers.

Recently, clinical researchers have begun to evaluate a new generation of cardiac troponin tests designed to be used at the point of care. In one clinical trial, researchers reported using a rapid point-of-care test to safely discharge hospital patients sooner.² They used the test to identify patients with very low risk of experiencing a myocardial infarction in the next 30 days, finding that the approach was successful and offered an opportunity to get low-risk patients back home more quickly.

Although high-sensitivity and point-of-care assays are shaping the future of cardiac troponin testing, many clinical laboratories still rely on conventional methods due to established workflows and long-standing clinical validation. High-sensitivity cardiac troponin assays use advanced immunoassay techniques to detect very low concentrations of troponin with high precision. In core laboratory settings, common detection modes include chemiluminescent microparticle immunoassays (CMIA) and electrochemiluminescence immunoassays (ECLIA), which provide automated, ultra-sensitive measurements. For point-of-care applications, lateral flow and microfluidic devices are increasingly used to deliver rapid results near the patient. In parallel, emerging technologies — such as electrochemical immunosensors, optical fluorescence-based platforms, and impedance-based sensors — are being developed to combine portability with high analytical sensitivity, supporting faster diagnosis and broader clinical access.

Better QC needed

While highly sensitive assays offer real benefit for patient care, they also create even more need for robust quality control in troponin testing. High-sensitivity assays can detect cardiac troponins even in healthy individuals, underscoring the importance of establishing clear threshold levels to identify situations where intervention is required. For example, it is essential to understand how results from high-sensitivity assays compare to those from conventional tests so they can be interpreted in the context of historical data.

Identifying the values that count as elevated is not a new challenge. Joint guidelines from the American College of Cardiology and the European Society of Cardiology determined that anything higher than the 99^th percentile of troponin results in a healthy reference population should be considered elevated.³ But due to the lack of a reference standard, it is impossible to pinpoint a specific troponin value that reflects the 99^th percentile across all tests.¹ With so many cardiac troponin assays available and no reference standard to align their values, studies have found that troponin values at the 99^th percentile within the same population can vary up to five-fold.¹ (Some recent progress has been made, with Standard Reference Material 2921 now available from the National Institute of Standards and Technology; however, this reference material alone does not guarantee full harmonization across commercial assays.)

This inconsistency highlights the need for reliable quality controls to help lab teams ensure that the results they produce are accurate and comparable. But it can be a challenge to identify the best control: Recombinant or native? Dried-down or liquid format? Manufacturer-provided or third-party? The ideal control is one that can be run across multiple test platforms and assays from any vendor. It should be highly stable for ease of use, streamlined workflows, and minimal storage constraints.

Manufacturers often include calibrators and controls with troponin assay kits, along with recommended schedules for their use, to verify basic assay performance under defined conditions. However, regulatory and accreditation bodies such as CLIA and CAP require clinical laboratories to demonstrate ongoing quality control that reflects real-world use, including multiple operators, reagent lots, and instrument conditions. As a result, most laboratories establish their own QC schedules using third-party controls to provide an independent assessment of assay performance and to meet regulatory expectations for objectivity and robustness. These controls may be run daily, when a new operator begins using an instrument, following maintenance or calibration events, or at other critical workflow points. Controls are also commonly used for end-to-end workflow validation.

As a general rule, the best third-party controls should be as close as possible to patient samples being tested. Native controls tend to out perform recombinant ones because the latter may have different epitopes that don't match patient materials. Controls that are value-assigned across multiple instrument manufacturers create commutability and a consistent benchmark for enabling reliable comparison of results and streamlined quality control across the laboratory network.

The need for shelf-stable quality control is essential for reliable assay monitoring, but ease of use is also a key consideration in routine clinical workflows. 2–8°C storage of controls reduces the need for frozen storage and transport. Lyophilized controls are widely available and valued for their long shelf life, ready-to-use liquid controls are often favored in routine laboratory workflows because they eliminate reconstitution, reduce preparation errors, and save technician time.

Looking ahead

Recent developments in cardiac troponin testing have led to new opportunities to detect dangerous events earlier than ever, opening the doors for improved patient care and long-term outcomes. But as sensitivity increases, there is more need than ever for reliable controls to ensure that testing workflows are delivering accurate results. With value-assigned native controls from third-party vendors, clinical lab teams can streamline their QC processes across testing platforms from a variety of manufacturers to deliver the most consistent and comparable results for their patients.

References

1. Michos ED, Berger Z, Yeh HC, et al. Cardiac troponins used as diagnostic and prognostic tests in patients with kidney disease. Rockville, MD: Agency for Healthcare Research and Quality (US); 2014 Aug. Available from: https://www.ncbi.nlm.nih.gov/books/NBK241529/.

2. Apple FS, Smith SW, Greenslade JH, et al. Single high-sensitivity point-of-care whole-blood cardiac troponin I measurement to rule out acute myocardial infarction at low risk. Circulation. 2022;146(25):1918-1929. doi:10.1161/CIRCULATIONAHA.122.061148.

3. Morrow DA, Cannon CP, Jesse RL, et al. National Academy of Clinical Biochemistry Laboratory Medicine Practice Guidelines: Clinical characteristics and utilization of biochemical markers in acute coronary syndromes. Clin Chem. 2007;53(4):552-74. doi:10.1373/clinchem.2006.084194.