There are two types of natriuretic peptide that can be measured in serum: B-type natriuretic peptide (BNP) and N-terminal pro-BNP (NT-proBNP). NT-proBNP is the inactive prohormone of BNP and is released from the left ventricle in response to ventricular strain. It acts to increase renal excretion of water and sodium, and relax vascular smooth muscle causing vasodilation.
BNP should be measured in patients with suspected heart failure who have not had a previous myocardial infarction. The serum level helps to determine the likelihood of heart failure and thus the need for specialist assessment and echocardiography. The value helps dictate the level of urgency of the referral given high BNF levels (>400) carry a poor prognosis. In these cases, a 2-week wait referral for urgent echocardiography and specialist assessment is advised.
Although elevated levels of BNP do not confirm the diagnosis of heart failure, normal levels rule the diagnosis out (highly sensitivity but varying specificity).
NICE suggest that BNP measurements are not necessary for people with suspected heart failure who have had a previous myocardial infarction. These patients require an urgent referral, echocardiography and specialist assessment because if heart failure is present this carries a poor prognosis.
Increased BNP levels (>400) are not on their own diagnostic of heart failure, and may be elevated as a result of left ventricular hypertrophy, myocardial ischaemia, atrial fibrillation, pulmonary hypertension, hypoxia, pulmonary embolism, right ventricular strain, chronic obstructive pulmonary disease, liver failure, sepsis, diabetes, and renal impairment. In addition, levels tend to be higher in women, and in people older than 70.
Heart failure is unlikely if BNP levels are low (<100). However, aldosterone antagonists, ACE inhibitors, angiotensin-II receptor antagonists, beta-blockers and diuretics can all falsely lower BNP levels, as can obesity.
Previously the first-line investigation was determined by whether the patient has previously had a myocardial infarction or not this is no longer the case - all patients should have an N-terminal pro-B-type natriuretic peptide (NT-proBNP) blood test first-line.
Interpreting the test
B-type natriuretic peptide (BNP) is a hormone produced mainly by the left ventricular myocardium in response to strain. Very high levels are associated with a poor prognosis.
| BNP | NTproBNP | |
|---|---|---|
| High levels | > 400 pg/ml (116 pmol/litre) | > 2000 pg/ml (236 pmol/litre) |
| Raised levels | 100-400 pg/ml (29-116 pmol/litre) | 400-2000 pg/ml (47-236 pmol/litre) |
| Normal levels | < 100 pg/ml (29 pmol/litre) | < 400 pg/ml (47 pmol/litre) |
Factors which alter the BNP level:
| Increase BNP levels | Decrease BNP levels |
|---|---|
| Left ventricular hypertrophy | |
| Ischaemia | |
| Tachycardia | |
| Right ventricular overload | |
| Hypoxaemia (including pulmonary embolism) | |
| GFR < 60 ml/min | |
| Sepsis | |
| COPD | |
| Diabetes | |
| Age > 70 | |
| Liver cirrhosis | Obesity |
| ACE inhibitors/ Angiotensin 2 receptor blockers | |
| Aldosterone antagonists | |
| Beta-blockers | |
| Diuretics |
🧬 Why NT-proBNP Levels Are Lower in Obesity?
4 Reasons: