Troponin and natriuretic peptides

What it is

Cardiac troponin (I or T) is a protein of the heart muscle's contractile apparatus that leaks into the blood when myocytes are injured. High-sensitivity assays detect very small amounts, so the key reference point is the 99th percentile of a healthy population, which each assay defines and which may differ between men and women. A value above it means myocardial injury; it becomes myocardial infarction only when the values rise and/or fall and there is evidence of ischaemia (symptoms, new ECG changes, imaging or a coronary thrombus). That is why troponin is read as a series: the change between a first and a repeat sample (the delta) separates acute injury from a chronically raised level, as seen in kidney disease or stable heart disease. Many conditions other than coronary occlusion injure the myocardium: myocarditis, pulmonary embolism, sepsis, tachyarrhythmia, heart failure, takotsubo, aortic dissection and trauma. B-type natriuretic peptide (BNP) and its inactive fragment NT-proBNP are released by ventricular myocytes when the walls are stretched by volume or pressure. Their main use is to rule out heart failure in a breathless patient: a low value makes it unlikely. They rise with age, kidney failure and atrial fibrillation and are lower in obesity.

Reference values and concepts

• High-sensitivity troponin: the upper reference limit is the assay's 99th percentile, often in ng/L and sometimes sex-specific; check your laboratory.
• Troponin starts to rise within a few hours of injury, peaks around a day and can stay raised for 1–2 weeks.
• Rapid protocols compare a sample on arrival with one at 1–3 hours, depending on the assay and validated pathway.
• Acute heart failure rule-out: BNP below 100 pg/mL or NT-proBNP below 300 pg/mL.
• Age-adjusted NT-proBNP rule-in for acute heart failure: above 450 pg/mL (under 50 years), 900 (50–75), 1800 (over 75).
• Non-acute (outpatient) heart failure is unlikely with BNP below 35 pg/mL or NT-proBNP below 125 pg/mL.

Reading it step by step

1. Look at the ECG first — ST elevation needs immediate reperfusion, whatever the troponin.
2. Note the time from symptom onset to the first sample — a very early sample can still be normal after an infarct.
3. Compare the first troponin with the assay's 99th percentile — above it means myocardial injury, not yet infarction.
4. Repeat at the interval of your local pathway and calculate the delta — a significant change means acute injury.
5. Ask whether the picture is ischaemic: pain, ECG, wall-motion changes — only then is it myocardial infarction.
6. If not ischaemic, look for other causes: PE, myocarditis, sepsis, arrhythmia, dissection, kidney disease — treat the cause.
7. In breathlessness, read BNP or NT-proBNP against the rule-out cut-off — a low value makes heart failure unlikely.
8. Adjust the reading for age, kidney function, rhythm, weight and drugs — they shift natriuretic peptide levels.

Troponin and natriuretic peptide patterns

Patterns and common mistakes

• Calling every raised troponin an infarct: type 2 infarction and non-ischaemic injury (sepsis, PE, tachycardia) are common.
• Using a single troponin to rule out infarction when the sample was taken very soon after the pain began.
• Dismissing a stable, chronically high troponin in kidney disease as normal: it still carries risk, and a new delta still means acute injury.
• Reassuring because the troponin is normal in a patient whose ECG shows ST elevation or who is still in pain: treat the patient.
• Reading a high NT-proBNP in an older patient with kidney failure or atrial fibrillation as proof of heart failure; echo confirms it.
• Falsely low natriuretic peptides in obesity and in flash pulmonary oedema measured within the first hour.

Next step

• ST elevation or ongoing ischaemic pain: activate the reperfusion pathway and call the senior or cardiology at once.
• Rising troponin with ischaemia: manage as acute coronary syndrome by the local protocol and discuss with cardiology.
• Raised troponin without ischaemia: look for and treat the cause (CT pulmonary angiography, echo, cultures) and repeat the troponin.
• High natriuretic peptide in a breathless patient: request an echocardiogram to confirm heart failure and its type.
• Record sample times on every troponin request so the next doctor can calculate the delta.

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Elsewhere in DurgiPedia

• Cardiac troponin as a lab test
• Creatine kinase (CK) and CK-MB as lab tests
• Heart failure physiology
• The ECG in ischemia and infarction
• Chest pain: ischemic, pericardial, pleural and other patterns
• Heart failure syndrome: left- and right-sided signs
• Ischaemic heart disease

Connections

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Analytical and Clinical Aspects of Troponin Testing (StatPearls (NCBI Bookshelf), 2026); Natriuretic Peptide B Type Test (StatPearls (NCBI Bookshelf), 2025).

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