BNP and NT-proBNP: Heart Failure Blood Tests

Natriuretic Peptide Biology

32 amino acids

BNP is a 32-amino-acid cardiac peptide hormone that dilates blood vessels, promotes sodium excretion, and suppresses the renin-angiotensin-aldosterone system in response to ventricular wall stress.

Nishikimi et al., J Cardiovasc Dev Dis, 2022

Nishikimi et al., J Cardiovasc Dev Dis, 2022

View as image

BNP (B-type natriuretic peptide) is the second member of the natriuretic peptide family, discovered five years after ANP (atrial natriuretic peptide), the family's founding member. Where ANP is primarily produced by atrial cardiomyocytes in response to atrial stretch, BNP is produced mainly by ventricular cardiomyocytes in response to ventricular volume and pressure overload. This anatomical difference has clinical consequences: BNP and its inactive fragment NT-proBNP are more sensitive markers of ventricular dysfunction than ANP, making them the preferred blood tests for diagnosing and monitoring heart failure. The natriuretic peptide system is the heart's built-in counter-regulatory mechanism against the renin-angiotensin-aldosterone system (RAAS). When RAAS promotes sodium retention, vasoconstriction, and volume expansion, natriuretic peptides push back with natriuresis, vasodilation, and volume reduction.^[1] Understanding how BNP works as both a hormone and a biomarker is essential for interpreting the blood tests that carry its name. For the broader natriuretic peptide family, see how natriuretic peptides protect your heart and kidneys.

The natriuretic peptide family

Three natriuretic peptides are produced in humans: ANP (atrial natriuretic peptide), BNP (B-type natriuretic peptide), and CNP (C-type natriuretic peptide). All three share a 17-amino-acid ring structure formed by a disulfide bond between two cysteine residues. This ring is essential for receptor binding and biological activity.

ANP is produced primarily in the atria and released in response to atrial stretch. BNP is produced primarily in the ventricles and released in response to ventricular wall stress. CNP is produced mainly by endothelial cells and acts locally as a paracrine regulator rather than a circulating hormone. ANP and BNP both bind natriuretic peptide receptor A (NPR-A), activating guanylyl cyclase to produce cGMP. CNP binds NPR-B. All three are cleared in part by NPR-C, a clearance receptor that internalizes and degrades natriuretic peptides.

Silver (2006) described how the natriuretic peptide system coordinates kidney and cardiovascular effects: natriuretic peptides increase glomerular filtration rate, inhibit sodium reabsorption in the collecting duct, suppress renin secretion, and reduce aldosterone production. The net effect is reduced blood volume, lower blood pressure, and decreased cardiac workload.^[5]

BNP was originally named "brain natriuretic peptide" because it was first isolated from porcine brain tissue in 1988 by Sudoh et al. Despite this historical naming, the heart is by far the dominant source in humans. Ventricular cardiomyocytes produce BNP at rates proportional to wall stress, with production increasing up to 100-fold in severe heart failure compared to healthy individuals. The brain does produce small amounts of BNP, but at concentrations too low to affect circulating levels. The misnomer persists in clinical practice, though many cardiologists now prefer "B-type natriuretic peptide" to avoid the confusion.

The natriuretic peptide system is evolutionarily ancient. Homologs have been identified in fish, amphibians, and all vertebrate lineages, suggesting the system evolved at least 400 million years ago as an essential regulator of fluid and electrolyte balance.

From gene to blood test: how BNP is produced

The BNP gene (NPPB) encodes a 134-amino-acid preproBNP protein. A 26-amino-acid signal peptide is removed during translation, yielding 108-amino-acid proBNP. This precursor is stored in secretory granules within ventricular cardiomyocytes.

When the ventricle is stretched by increased filling pressure, proBNP is released and cleaved by the serine protease corin (and to a lesser extent by furin) into two fragments: the C-terminal fragment becomes active BNP (amino acids 77-108, 32 residues), and the N-terminal fragment becomes NT-proBNP (amino acids 1-76, 76 residues). Both are released into the bloodstream in approximately equimolar quantities.^[1]

In practice, the cleavage is not always complete. A significant fraction of circulating "BNP" in heart failure patients is actually unprocessed proBNP or glycosylated forms of proBNP that are not fully cleaved. This matters because different BNP immunoassays detect different combinations of BNP, proBNP, and BNP fragments, contributing to variability between assay platforms. NT-proBNP assays are more standardized because Roche holds the dominant market position with a single assay platform (Elecsys), reducing inter-laboratory variability.^[1]

Two fragments, two clearance pathways

BNP and NT-proBNP follow fundamentally different clearance pathways, which explains their different pharmacokinetic profiles.

BNP clearance: BNP is degraded by neprilysin (neutral endopeptidase, NEP), a zinc metalloprotease expressed on the surface of many cell types. BNP is also cleared by NPR-C receptor-mediated internalization and by renal filtration. The combined effect of these three clearance mechanisms gives BNP a plasma half-life of approximately 20 minutes. This rapid turnover means BNP levels respond quickly to changes in cardiac status, rising within hours of an acute decompensation and falling within hours of successful diuresis.

NT-proBNP clearance: NT-proBNP is not a substrate for neprilysin. It is cleared primarily through passive renal filtration. Its half-life is approximately 120 minutes, six times longer than BNP's. This means NT-proBNP accumulates to higher steady-state concentrations and is less susceptible to short-term fluctuations, but it is more affected by renal impairment. In chronic kidney disease, NT-proBNP levels rise independently of cardiac function, complicating interpretation.^[1]

Why sacubitril changes everything

Sacubitril/valsartan (Entresto), approved for heart failure in 2015, combines a neprilysin inhibitor (sacubitril) with an angiotensin receptor blocker (valsartan). By inhibiting neprilysin, sacubitril reduces the degradation of endogenous natriuretic peptides, allowing BNP and ANP to circulate longer and at higher concentrations. This augments the beneficial effects of natriuretic peptides: more vasodilation, more natriuresis, more RAAS suppression.

The clinical problem is that neprilysin inhibition raises BNP levels measured by immunoassays. A patient on sacubitril/valsartan who shows a rising BNP level may be improving (the drug is working, augmenting BNP's effects) rather than deteriorating. NT-proBNP, which is not degraded by neprilysin, is unaffected by sacubitril. In patients on sacubitril/valsartan, only NT-proBNP reliably reflects the underlying cardiac status.^[1]

Nishikimi et al. (2025) found that the patterns of BNP, mature BNP, and proBNP responses to sacubitril/valsartan differed between treatment responders and non-responders. Responders showed a specific pattern of BNP elevation with proportional increases in mature BNP, while non-responders showed disproportionate proBNP accumulation. This suggests that the quality of the BNP response, not just the quantity, may predict therapeutic benefit.^[4]

Selezneva et al. (2025) discovered that neprilysin 2 (NEP2), a separate enzyme from the neprilysin targeted by sacubitril, also catalyzes the degradation of natriuretic peptides and is not inhibited by sacubitrilat (the active metabolite of sacubitril). This NEP2 pathway may represent an escape mechanism that limits the efficacy of sacubitril/valsartan in some patients, and could explain why BNP augmentation varies between individuals.^[2]

When the natriuretic peptide system fails

In chronic heart failure, the natriuretic peptide system is activated but its effectiveness is progressively reduced. BNP levels rise dramatically, sometimes exceeding 1,000 pg/mL, yet the expected vasodilation, natriuresis, and RAAS suppression are blunted. This apparent paradox has several explanations.

Kuwahara et al. (2021) reviewed the natriuretic peptide system in heart failure and identified multiple points of failure: increased proBNP glycosylation reduces corin-mediated cleavage, meaning more proBNP and less active BNP in the circulation; upregulated neprilysin activity increases BNP degradation; downregulation of NPR-A receptors on target tissues reduces the biological response to BNP; and competing RAAS activation overwhelms the natriuretic peptide counter-regulatory response.^[3]

Diez (2017) characterized chronic heart failure as a state of "reduced effectiveness of the natriuretic peptide system" and argued that therapeutic strategies should aim not just to increase natriuretic peptide levels (which are already elevated) but to restore the biological effectiveness of the natriuretic peptide response. Sacubitril/valsartan addresses one part of this by preventing BNP degradation, but it does not correct impaired proBNP processing or receptor downregulation.^[6]

This explains why very high BNP levels are prognostically ominous rather than reassuring. A BNP of 2,000 pg/mL does not mean the natriuretic peptide system is powerfully active. It means the heart is under extreme stress and the counter-regulatory response is failing to compensate. The clinical implication is that simply boosting natriuretic peptide levels is not sufficient therapy. Nesiritide, a recombinant BNP given intravenously for acute decompensated heart failure, was approved in 2001 based on short-term hemodynamic improvement. But the ASCEND-HF trial (2011), enrolling over 7,000 patients, found that nesiritide did not reduce 30-day mortality or rehospitalization compared to placebo. The drug added exogenous BNP to a system that was already producing BNP at maximum capacity but could not transduce the signal effectively. This failure underscored Diez's concept of "reduced effectiveness": the bottleneck in heart failure is not natriuretic peptide production but natriuretic peptide signal transduction.

Natriuretic peptide effects beyond the heart

The natriuretic peptide system affects multiple organs beyond the cardiovascular system.

Knany et al. (2025) described the effects of natriuretic peptides on alveolar epithelium in heart failure. BNP and ANP regulate fluid transport across the air-blood barrier in the lungs, and impaired natriuretic peptide signaling may contribute to pulmonary edema independently of hemodynamic congestion.^[7]

In the kidneys, natriuretic peptides increase glomerular filtration through afferent arteriolar dilation and efferent arteriolar constriction, directly opposing the renal effects of angiotensin II. They inhibit sodium reabsorption in the inner medullary collecting duct, producing natriuresis. In the adrenal glands, BNP directly inhibits aldosterone secretion, reducing sodium retention. In the sympathetic nervous system, natriuretic peptides reduce sympathetic outflow, lowering heart rate and vascular resistance. This multi-organ coordination makes the natriuretic peptide system a systemic counter-regulatory network rather than a single-organ response. The breadth of these effects explains why natriuretic peptide deficiency, whether from genetic variants that reduce NPR-A expression or from the acquired "resistance" seen in chronic heart failure, has consequences that extend far beyond the cardiovascular system. Salt-sensitive hypertension, chronic kidney disease progression, and metabolic syndrome have all been linked to impaired natriuretic peptide signaling, though the causation is complex and bidirectional.^[5]

How the blood test works

A BNP or NT-proBNP test requires a standard venous blood draw. No fasting is required. Results are typically available within 15-60 minutes from a central laboratory, or under 15 minutes from a point-of-care assay.

The measurement uses sandwich immunoassay technology: two antibodies bind different regions of the BNP or NT-proBNP molecule, forming a "sandwich" that is quantified by chemiluminescence or electrochemiluminescence. Each manufacturer's assay detects slightly different epitopes, which contributes to assay-to-assay variability for BNP tests. NT-proBNP testing has less inter-assay variability because the Roche Elecsys platform dominates the market.

Results are reported in pg/mL (picograms per milliliter). Because NT-proBNP has a longer half-life and accumulates to higher concentrations, its reference ranges are numerically higher than BNP's. A BNP of 100 pg/mL and an NT-proBNP of 300 pg/mL represent roughly equivalent degrees of cardiac wall stress in the acute setting. The two tests are not interchangeable: a patient should be monitored with the same assay over time, and clinicians should not compare a BNP result from one visit with an NT-proBNP result from another.

Several factors affect reference ranges beyond cardiac function. Age increases baseline natriuretic peptide levels, with healthy 75-year-olds having two to three times higher NT-proBNP than healthy 40-year-olds. Women have higher levels than men at any age. Obesity paradoxically lowers natriuretic peptide levels because adipose tissue expresses NPR-C clearance receptors that remove BNP from the circulation. Renal impairment elevates NT-proBNP more than BNP because NT-proBNP depends on renal clearance. Atrial fibrillation raises both markers through atrial stretch independent of ventricular dysfunction. These confounders make interpretation context-dependent; no single cutoff applies universally.

For how these tests are used to guide heart failure treatment decisions, see the sibling article on natriuretic peptide-guided therapy. For the recombinant BNP that was used as a treatment, see nesiritide: the recombinant BNP.

The Bottom Line

BNP and NT-proBNP are the two measurable fragments of proBNP, a precursor released by ventricular cardiomyocytes under wall stress. BNP is biologically active, promoting vasodilation, natriuresis, and RAAS suppression with a 20-minute half-life. NT-proBNP is biologically inactive with a 120-minute half-life, cleared by the kidneys. Sacubitril/valsartan complicates BNP testing by inhibiting the enzyme that degrades it, making NT-proBNP the only reliable monitoring biomarker on this therapy. In chronic heart failure, the natriuretic peptide system is maximally activated but its effectiveness is impaired by multiple mechanisms including reduced proBNP processing and receptor downregulation.

Frequently Asked Questions

What is a BNP blood test used for?

A BNP blood test measures the level of B-type natriuretic peptide, a 32-amino-acid hormone released by the heart when it is under stress. It is primarily used to diagnose heart failure in patients with shortness of breath, to assess the severity of known heart failure, and to monitor treatment response. A BNP below 100 pg/mL in the emergency department makes heart failure unlikely; levels above 400 pg/mL strongly suggest it.

Why is NT-proBNP preferred over BNP for some patients?

NT-proBNP is preferred for patients taking sacubitril/valsartan (Entresto), a heart failure drug that inhibits neprilysin, the enzyme that degrades BNP. Neprilysin inhibition artificially raises BNP levels, making the BNP test unreliable for monitoring. NT-proBNP is not degraded by neprilysin and accurately reflects the underlying cardiac status on this therapy.^[1]

What does a high BNP level mean?

A high BNP level means the heart's ventricles are under increased wall stress, typically from volume overload or pressure overload. In heart failure, BNP levels rise as the heart works harder to pump blood. Very high levels (above 1,000 pg/mL) indicate severe heart failure and carry a worse prognosis. However, BNP can also be elevated by renal impairment, pulmonary embolism, atrial fibrillation, and sepsis.

How is BNP related to ANP?

BNP and ANP are both members of the natriuretic peptide family. ANP is produced mainly by the heart's atria in response to atrial stretch, while BNP is produced mainly by the ventricles in response to ventricular wall stress. Both bind the same receptor (NPR-A) and produce similar effects: vasodilation, sodium excretion, and suppression of RAAS. BNP is more specific for ventricular dysfunction, making it the preferred heart failure biomarker.^[5]

Can kidney disease affect BNP or NT-proBNP results?

Yes. NT-proBNP is cleared primarily by renal filtration, so kidney disease raises NT-proBNP levels independently of cardiac function. BNP is less affected by renal function because it has additional clearance pathways through neprilysin and NPR-C receptors. In patients with chronic kidney disease, clinicians must use higher diagnostic thresholds or rely more on BNP than NT-proBNP to avoid false-positive heart failure diagnoses.

Why are BNP levels high in heart failure but the hormone doesn't work?

In chronic heart failure, BNP levels are dramatically elevated because the heart is under continuous stress, but the expected biological effects (vasodilation, natriuresis) are blunted. This happens because increased proBNP glycosylation impairs cleavage into active BNP, neprilysin activity is upregulated, NPR-A receptors on target tissues are downregulated, and the competing RAAS activation overwhelms the natriuretic peptide response.^[3]