From the Guidelines
A discrepancy between a positive quantitative troponin I and a negative qualitative troponin I result is most likely due to the differences in assay sensitivity thresholds, with quantitative assays detecting troponin at lower concentrations. This discrepancy can be attributed to various clinical scenarios, including early myocardial injury, minor cardiac damage, or chronic conditions such as heart failure, renal failure, or pulmonary embolism, as outlined in the European Society of Cardiology guidelines for the management of acute coronary syndromes 1. Some key points to consider in this context include:
- The sensitivity of quantitative assays, which can detect troponin at concentrations as low as 0.01-0.04 ng/mL, compared to qualitative point-of-care tests with higher cutoff values around 0.1-0.5 ng/mL.
- Clinical conditions that may cause low-grade troponin elevation, such as severe congestive heart failure, hypertensive crisis, tachy- or bradyarrhythmias, pulmonary embolism, and inflammatory diseases like myocarditis, as noted in the guidelines 1.
- The importance of considering the timing of testing, evaluating pre-test probability of acute coronary syndrome, and assessing for non-ACS causes of troponin elevation when encountering this discrepancy.
- The potential for analytical interference affecting one assay but not the other, highlighting the need for careful interpretation of results in the context of the patient's overall clinical presentation. Given the information from the European Heart Journal study 1, it is essential to approach such discrepancies with a thorough evaluation of the patient's condition, considering both cardiac and non-cardiac causes of troponin elevation, to ensure appropriate management and minimize adverse outcomes.
From the Research
Causes of Troponin I Quantitative Positive Result but Troponin I Qualitative Negative Result
- The difference in results between quantitative and qualitative troponin I tests can be attributed to the sensitivity and specificity of the tests, as well as the cutoff values used 2.
- A study found that a qualitative troponin I kit showed 98% sensitivity and 100% specificity compared to a quantitative test with a cutoff of 0.30 ng/dl, but the qualitative test showed negative results for values of 0.39,0.40, and 0.42 ng/dl 2.
- The discrepancy in results may be due to the fact that qualitative tests are designed to detect higher levels of troponin I, typically above a certain cutoff value, whereas quantitative tests can detect lower levels of troponin I 3.
- Another study found that the sensitivity of quantitative markers was higher than qualitative methods, but the specificity and likelihood ratio were lower 3.
- The concordance analysis between qualitative and quantitative markers was adequate, with a kappa index of 0.65 (p < 0.05), suggesting that the rapid bedside qualitative test by cardiac Troponin T is a good diagnostic marker compared with conventional quantitative markers to evaluate chest pain in acute ischemic syndromes 3.
Possible Explanations
- The qualitative test may not be sensitive enough to detect low levels of troponin I, resulting in a negative result even if the quantitative test is positive 2.
- The cutoff value used for the qualitative test may be too high, resulting in false negative results for patients with troponin I levels below the cutoff value 2.
- The quantitative test may be more sensitive to minor elevations in troponin I levels, resulting in a positive result even if the qualitative test is negative 3.
Clinical Implications
- The difference in results between quantitative and qualitative troponin I tests highlights the importance of using both tests in conjunction with each other to ensure accurate diagnosis and treatment of patients with suspected acute myocardial infarction 2, 3.
- Clinicians should be aware of the limitations and potential discrepancies between quantitative and qualitative troponin I tests when interpreting results and making clinical decisions 2, 3.