🔬 BNP

B-type Natriuretic Peptide (BNP) and NT-proBNP are substances produced by the heart in response to excessive stretching of the myocytes (wall stress). They serve as gold-standard biomarkers for the exclusion and risk-stratification of heart failure. In UK practice, NT-proBNP is the preferred peptide for triaging patients for echocardiography and specialist cardiology review.

Testing is indicated in patients presenting with new-onset breathlessness, peripheral oedema, or fatigue where heart failure is a differential diagnosis. It is also used in the acute setting to differentiate cardiac causes of dyspnoea from primary respiratory causes (e.g., COPD or pneumonia). It is not indicated as a general screening tool for asymptomatic individuals.

Measured via a venous blood sample (typically a purple EDTA or yellow serum tube, depending on the lab). The two common markers are B-type Natriuretic Peptide (BNP) and the more stable N-terminal pro-B-type Natriuretic Peptide (NT-proBNP). NT-proBNP is more commonly used in the UK due to its longer half-life and better stability in the blood sample.

A normal result (NT-proBNP <400 pg/mL in the community or <300 pg/mL in the acute setting) indicates that heart failure is highly unlikely as the cause of the patient's symptoms. Normal values vary slightly by assay, but the emphasis is on the high negative predictive value of the test.

Interpretation is based on 'rule-out' thresholds. For suspected chronic heart failure (NT-proBNP), a level <400 pg/mL makes heart failure unlikely. Levels between 400-2000 pg/mL warrant an echocardiogram within 6 weeks, while >2000 pg/mL requires assessment within 2 weeks. In acute settings, higher thresholds are often used to account for age and acute stress. Values must be interpreted in light of the patient's BMI and medication.

Elevated levels (exceeding the rule-out thresholds) indicate high likelihood of heart failure. Significantly high levels (e.g., NT-proBNP >2000 pg/mL) suggest a high risk of adverse outcomes and require urgent specialist assessment. Very high levels can also be seen in atrial fibrillation, pulmonary embolism, valvular heart disease, or significant chronic kidney disease where clearance is impaired.

BNP/NT-proBNP is the primary screening tool for heart failure in the community and the emergency department. Its high negative predictive value makes it excellent for excluding heart failure in patients presenting with breathlessness. In the UK, NICE NG106 dictates management pathways (urgent vs routine referral) based on the specific NT-proBNP level detected in primary care.

A common pitfall is treating an elevated BNP as definitive proof of heart failure without further investigation (e.g., echocardiography), as other conditions like PE or AF can raise levels. Another pitfall is ignoring a 'normal' result in a patient with classic symptoms who has a very high BMI, as their BNP may be 'falsely' suppressed into the normal range.

BNP is not a stand-alone diagnostic test; it must be followed by echocardiography. Levels can be 'falsely' low in obese patients (high BMI reduces BNP) or in those already established on certain heart failure medications like ACE inhibitors or ARBs. Conversely, levels are naturally higher in the elderly, females, and those with renal impairment, necessitating careful clinical correlation.

Essential for MLA: Know the NT-proBNP thresholds for urgent (<2 weeks) and routine (<6 weeks) referral for echocardiography. Understand that a normal BNP virtually excludes heart failure (high NPV). Be aware that obesity lowers BNP levels, which can lead to under-diagnosis if not considered.