B-type natriuretic peptide (BNP) is a hormone produced primarily by the muscle cells of the heart’s ventricles. The heart releases this peptide into the bloodstream when the ventricular walls experience stretching or stress due to high pressure or volume overload. Measuring the concentration of this hormone in the blood helps in the diagnosis and ongoing management of heart failure, a condition where the heart struggles to pump blood efficiently.
What the BNP Blood Test Measures
The BNP hormone functions as a natural defense mechanism against excessive pressure buildup within the heart chambers. It is synthesized as a larger prohormone, which is then cleaved into the biologically active BNP and an inactive fragment, NT-proBNP. Both fragments are measurable in the bloodstream. The release of BNP is directly proportional to the degree of stretch experienced by the ventricular walls, making it an excellent biomarker for cardiac strain.
Once released, BNP acts on the body in several ways to try and correct the volume overload and high pressure. One of its main actions is to signal the kidneys to increase the excretion of salt and water, a process known as natriuresis and diuresis. This action helps reduce the total blood volume and, consequently, the pressure the heart must work against.
The hormone also promotes vasodilation, causing blood vessels to widen, which further reduces blood pressure and the overall workload on the heart. BNP works to counteract the effects of other systems in the body, such such as the renin-angiotensin-aldosterone system, that would otherwise cause fluid retention and vessel constriction. The blood test, therefore, provides a direct look at how much stress the heart is currently under.
Identifying the Correct Collection Tube
The BNP blood test requires a very specific collection method to ensure the sample remains stable for accurate measurement. The correct container for collecting a BNP sample is the Lavender or Purple top tube. This tube is color-coded because it contains the anticoagulant ethylenediaminetetraacetic acid (EDTA).
EDTA is mandated because of its ability to stabilize the BNP peptide after the blood is drawn. BNP is a fragile molecule that is rapidly degraded by natural enzymes, called proteases, present in the blood sample. EDTA works by binding to metal ions required for these enzymes to function, effectively stopping the degradation process and preserving the true BNP concentration.
If the sample is collected in a tube without this specific anticoagulant, the BNP concentration would quickly decrease, leading to a falsely low and inaccurate result. Beyond the correct tube, the sample handling is equally important because of the peptide’s instability. The blood must be processed quickly, often by centrifuging to separate the plasma, and then frequently frozen for transport and testing to ensure the integrity of the BNP molecule.
Interpreting BNP Test Results
The numerical result from a BNP test is used by healthcare providers to assess the likelihood and severity of heart failure. A normal BNP level is considered under 100 picograms per milliliter (pg/mL). A value below this cutoff suggests that heart failure is unlikely to be the cause of a patient’s symptoms, such as shortness of breath.
As the heart experiences greater stress, the levels of BNP released into the blood increase. Results higher than the normal threshold correlate with a greater degree of heart failure severity. For instance, levels exceeding several hundred or a thousand pg/mL indicate moderate to severe heart failure.
The test is used for initial diagnosis and for monitoring how well a patient’s treatment plan is working. A successful treatment regimen, which may involve diuretics or other heart medications, should result in a measurable decrease in BNP levels, indicating reduced cardiac strain. Conversely, a rising BNP level in a patient with an existing diagnosis signals that their heart failure is worsening or that the current treatment requires adjustment.