Natriuretic Peptides in Heart Failure
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Direct answer
Natriuretic peptides, chiefly BNP and NT-proBNP, are released by ventricular myocytes under wall stretch and are measured to support or exclude heart failure. In the acute setting a BNP below 100 pg/mL or an NT-proBNP below 300 pg/mL makes heart failure unlikely. Obesity lowers levels, while kidney failure, atrial fibrillation and older age raise them, so every value is read against the clinical picture.
What you must remember
- BNP origin: brain natriuretic peptide is secreted mainly by the ventricles, despite its name, and rises with wall stress.
- NT-proBNP: the inactive N-terminal fragment cleaved from the same precursor, with a longer half-life than BNP and no effect from neprilysin.
- Rule-out cut-offs: BNP below 100 pg/mL and NT-proBNP below 300 pg/mL in acute dyspnoea make heart failure unlikely, with a grey zone between these values and the rule-in thresholds.
- Obesity: lowers natriuretic peptide levels, so a normal value in an obese patient does not exclude heart failure.
- CKD: reduced clearance raises both peptides, and NT-proBNP is affected more than BNP.
- Neprilysin inhibition: sacubitril, given with valsartan, blocks the enzyme that degrades BNP, so BNP rises on treatment while NT-proBNP is the better marker.
- Other raisers: atrial fibrillation, left ventricular hypertrophy, pulmonary embolism with right ventricular strain, sepsis and advancing age all increase levels.
Choosing the right marker
Both peptides come from the same precursor, but their behaviour differs in ways that change how the result is read. The table compares them side by side.
| Feature | BNP | NT-proBNP |
|---|---|---|
| Biological activity | Active hormone | Inactive fragment |
| Half-life | Short, about 20 minutes | Longer, about 120 minutes |
| Effect of sacubitril | Levels rise | Levels not affected |
| Acute rule-out cut-off | Below 100 pg/mL | Below 300 pg/mL |
| Preferred in ARNI therapy | No | Yes |
A single measurement is most useful when the marker matches the clinical question, such as diagnosis at presentation or follow-up on sacubitril-valsartan. Because the two assays have different cut-offs, values must never be interchanged between them.
Serial measurements are more informative than a single value, because a falling trend during treatment suggests that filling pressures are coming down. The same assay should be used at each time point, since switching assays changes the numbers and can mislead the trend. Such trends are always read alongside weight, symptoms and examination findings.
Where the exam tries to catch you
Peptide questions test whether the student knows what alters the level, so the traps below focus on the common confounders.
- Trap: a low BNP in an obese patient is used to exclude heart failure. Correct: obesity lowers natriuretic peptide levels, so symptoms and echocardiography still decide the diagnosis.
- Trap: a rising BNP on sacubitril-valsartan is read as worsening heart failure. Correct: neprilysin degrades BNP, so BNP rises with treatment, and NT-proBNP is the marker to follow.
- Trap: a high NT-proBNP in an elderly patient with CKD is taken as proof of heart failure. Correct: kidney disease, atrial fibrillation and age all raise levels, so the value alone does not confirm the diagnosis.
Frequently asked questions
What BNP level rules out acute heart failure?
In the acute setting a BNP below 100 pg/mL makes heart failure unlikely, and an NT-proBNP below 300 pg/mL does the same. Results between the rule-out and rule-in values need clinical correlation and echocardiography.
Why does sacubitril-valsartan raise BNP levels?
Sacubitril inhibits neprilysin, the enzyme that breaks down BNP, so the BNP level in the blood rises during treatment. NT-proBNP is not degraded by neprilysin, which is why it is the preferred marker in patients on sacubitril-valsartan.
Why does obesity lower natriuretic peptide levels?
Obesity lowers BNP and NT-proBNP levels, possibly through increased peptide clearance and lower release per unit of cardiac strain. A normal value in an obese patient with heart failure symptoms therefore does not exclude the diagnosis.
Which conditions raise BNP apart from heart failure?
Chronic kidney disease, atrial fibrillation, left ventricular hypertrophy, pulmonary embolism with right ventricular strain, sepsis and advancing age all raise natriuretic peptide levels. Their presence makes a high value less specific for heart failure.
How does NT-proBNP differ from BNP as a test?
BNP is the biologically active hormone, while NT-proBNP is an inactive fragment cleaved from the same precursor. NT-proBNP has a longer half-life and is unaffected by neprilysin inhibition, but its cut-offs differ from those for BNP.