A test for B-type natriuretic peptide (BNP) or N-terminal pro b-type natriuretic peptide (NT-proBNP) is primarily used to help detect, diagnose, and evaluate the severity of heart failure. It can be used, along with other cardiac biomarker tests, to detect heart stress and damage and/or along with lung function tests to distinguish between causes of shortness of breath. 
BNP and NT-proBNP are substances that are produced in the heart and released when the heart is stretched and working hard to pump blood. 
Heart failure can be confused with other conditions, and it may co-exist with them. BNP and NT-proBNP levels can help doctors differentiate between heart failure and other problems, such as lung disease. An accurate diagnosis is important because the treatments are often different and must be started as soon as possible.
Although BNP and NT-proBNP are usually used to recognize heart failure, an increased level in people with acute coronary syndrome (ACS) indicates an increased risk of recurrent events. Thus, a health practitioner may use either BNP or NT-proBNP to evaluate risk of a future cardiac event in someone with ACS.
Higher-than-normal results suggest that a person has some degree of heart failure, and the level of BNP or NT-proBNP in the blood is related to its severity. Higher levels of BNP or NT-proBNP are often associated with a worse outlook (prognosis) for the person.
Normal results indicate that the person's symptoms are likely due to something other than heart failure. 
BNP and NT-proBNP levels decrease in most people who are taking drug therapies for heart failure, such as angiotensin-converting enzyme (ACE) inhibitors, beta blockers, and diuretics.
Levels of both BNP and NT-proBNP tend to increase with age.
Levels of NT-proBNP and BNP may be increased in persons with kidney disease due to reduced clearance.
While both BNP and NT-proBNP will rise with left ventricle dysfunction and either can be measured for diagnosis or monitoring therapy, they are not interchangeable and the results cannot be directly compared.