Interpreting Tricky Biomarkers: When Are They Misleading?

1. The Misleading Troponin: Injury vs. Infarction

The single most important concept to master is this: Troponin indicates myocardial injury, not necessarily a Type 1 myocardial infarction (heart attack from a plaque rupture).

As you know from your rotations, any time you see a positive troponin, the first thought is often ACS. However, with the advent of high-sensitivity troponin assays (hs-cTn), we detect even minor degrees of myocardial injury from a wide variety of causes. Your job is to determine the cause of the injury.

This diagnostic flowchart provides a clear framework for thinking through an elevated troponin level.

As the chart illustrates, the key questions are:

1. Is the troponin level changing? A dynamic rise and/or fall suggests an acute process. A stable, chronically elevated level suggests chronic myocardial injury.
2. Are there signs of acute myocardial ischemia? (e.g., ischemic chest pain, new ECG changes). If yes, this points towards an acute myocardial infarction (MI).
   • Type 1 MI: Ischemia due to a primary coronary event (plaque rupture).
   • Type 2 MI: Ischemia due to a supply-demand mismatch (e.g., sepsis causing tachycardia and hypotension in a patient with stable coronary artery disease).
3. If there's no evidence of ischemia? This is classified as acute or chronic myocardial injury, not infarction.

Common Non-ACS Causes of Troponin Elevation

So, what are these other conditions that can cause myocardial injury? The list is long, but we can group them into logical categories.

Based on that reading, let's highlight the most common clinical scenarios you'll encounter:

• Heart Failure: Both acute and chronic heart failure can cause troponin elevation due to increased ventricular wall stress, subendocardial ischemia, and myocyte apoptosis.
• Pulmonary Embolism (PE): A large PE causes acute right ventricular strain and pressure overload, leading to RV injury and troponin release.
• Sepsis & Critical Illness: Severe systemic inflammation, hypotension, and tachycardia create a supply-demand mismatch (Type 2 MI) or direct inflammatory injury to the heart.
• Chronic Kidney Disease (CKD): Troponin is cleared by the kidneys. In CKD, baseline troponin levels are often elevated due to reduced clearance and a higher prevalence of underlying structural heart disease. In these patients, you must look for a significant change from their baseline, not just a single elevated value.
• Tachyarrhythmias: Very fast heart rates (e.g., atrial fibrillation with RVR, SVT) increase myocardial oxygen demand and can cause a Type 2 MI.
• Myocarditis/Pericarditis: Direct inflammation of the heart muscle or surrounding sac causes myocyte injury.

2. The Misleading BNP: When Stretch Isn't Heart Failure

In our last lesson, you learned that BNP and NT-proBNP are excellent for ruling out heart failure. However, interpreting an elevated level requires caution. Just as troponin reflects injury from many causes, BNP reflects stretch from many causes.

This infographic provides a great visual summary of factors that can push BNP levels up or down.

Common Causes of Misleading BNP/NT-proBNP Levels

Let's consolidate what you've learned into a practical list of pitfalls.

Key conditions causing falsely HIGH levels:

• Renal Failure: This is the most common confounder. Both peptides are cleared by the kidney (especially NT-proBNP), so levels will be higher at baseline. You may need to use higher diagnostic cutoffs in this population.
• Older Age & Female Sex: Levels naturally rise with age, and women tend to have higher levels than men.
• Atrial Fibrillation: Loss of coordinated atrial contraction and rapid ventricular rates lead to increased wall stress.
• Pulmonary Disease: Right heart strain from PE, pulmonary hypertension, or severe COPD will raise BNP.

Key conditions causing falsely LOW levels:

• Obesity: This is a major diagnostic trap. Adipose tissue expresses receptors that clear BNP from circulation, leading to paradoxically lower levels. An obese patient can be in florid pulmonary edema with a BNP in the "gray zone" (e.g., 100-400 pg/mL). You must maintain a high clinical suspicion and rely more on your exam and other data in this population.
• Flash Pulmonary Edema: In the very early hours of a sudden-onset event, it takes time for the ventricles to synthesize and release BNP, so the initial level may be deceptively low.
• Constrictive Pericarditis/Restrictive Cardiomyopathy: In these "stiff heart" conditions, the ventricles can't stretch as much, leading to significant symptoms of heart failure with only modestly elevated BNP levels.

3. The Misleading Inflammatory Markers: CRP and ESR

The final part of our learning outcome concerns inflammatory markers. The two most common are C-reactive protein (CRP) and the erythrocyte sedimentation rate (ESR). Their primary "pitfall" is their profound non-specificity.

• C-Reactive Protein (CRP): An acute-phase reactant produced by the liver in response to inflammation (primarily driven by IL-6). It rises and falls relatively quickly (half-life ~19 hours).
• Erythrocyte Sedimentation Rate (ESR): An indirect measure of inflammation. Inflammatory proteins (like fibrinogen) coat red blood cells, causing them to clump together and settle faster in a test tube. It rises and falls slowly over days to weeks.

When are they misleading?

The main error is to attribute an elevated CRP or ESR to a single suspected cause without considering the vast differential.

Common causes of chronically mild-to-moderately elevated CRP/ESR:

• Older age
• Obesity (adipose tissue is pro-inflammatory)
• Smoking
• Renal disease
• Depression

Situations causing falsely low or misleading readings:

• Liver Failure: Since CRP is synthesized in the liver, a patient with severe liver disease may fail to mount a CRP response to infection.
• Certain Red Blood Cell Abnormalities: Conditions like sickle cell disease or spherocytosis can prevent RBCs from clumping, leading to a falsely low ESR even with significant inflammation.
• Polycythemia: Too many red blood cells "crowd" the tube and slow down settling, causing a falsely low ESR.

Clinical Pearl: The utility of CRP and ESR is often in their negative predictive value. For example, in a patient with suspected giant cell arteritis or polymyalgia rheumatica, a normal ESR and CRP make the diagnosis much less likely. An elevated level simply tells you "inflammation is present," and the real work of finding the cause begins.

Conclusion

You've now completed your deep dive into cardiac and inflammatory markers. The overarching lesson is that no lab test provides a diagnosis in a vacuum. Your role as a clinician is to be the final integrator of all data.

Key Takeaways:

• An elevated troponin means myocardial injury. You must use the clinical context and the trend of the values to determine if the cause is ACS (Type 1 MI), supply-demand mismatch (Type 2 MI), or another non-ischemic injury (e.g., myocarditis, PE).
• An elevated BNP means myocardial stretch. While excellent for ruling out heart failure when low, an elevated level can be caused by renal failure, AFib, PE, older age, and more. Be especially wary of falsely low levels in patients with obesity.
• CRP and ESR are highly non-specific markers of inflammation. Their value is often in ruling out disease when they are normal, but an elevated result demands a broad clinical investigation.

Preview of the Next Lesson:
We will now shift gears from the heart to metabolism. Our next module begins with "Glucose Metabolism and Diabetes Monitoring." In the first lesson, you will learn to interpret fasting glucose, random glucose, and oral glucose tolerance test (OGTT) results for diabetes screening and diagnosis.