B-type Natriuretic Peptide (BNP) is a hormone measured by a blood test that provides insight into the heart’s health and function. The test is frequently used in a hospital setting to help determine the cause of symptoms like shortness of breath and to assess cardiac stress. Measuring the concentration of this hormone helps evaluate how hard the heart is working to manage blood volume and pressure. The resulting numerical value serves as a biological marker, guiding further diagnostic steps and treatment decisions.
The Role and Source of BNP
BNP is a peptide hormone produced primarily by specialized muscle cells within the heart’s lower chambers, the ventricles. Its release is a direct response to increased wall tension or stretching of the ventricular muscle, which occurs when the heart is subjected to volume or pressure overload. Once released, BNP acts to restore balance by promoting vasodilation, which lowers blood pressure. The hormone also signals the kidneys to increase the excretion of sodium and water (natriuresis and diuresis). These combined effects decrease the total volume of blood returning to the heart, reducing the ventricular workload.
BNP is synthesized as proBNP, which is cleaved into the active BNP hormone and an inactive fragment called N-terminal pro-B-type Natriuretic Peptide (NT-proBNP). Both BNP and NT-proBNP are measured to assess ventricular function.
Understanding BNP Reference Ranges
For most healthy adults, a normal BNP level is considered to be below 100 picograms per milliliter (pg/mL). A result below this threshold strongly suggests that heart dysfunction is not the cause of a patient’s symptoms, such as acute shortness of breath. This low value is highly effective at ruling out the possibility of heart failure.
The exact reference range is not a single universal number and can vary slightly between different laboratories and testing methods. Furthermore, the appropriate cutoff value changes significantly with age, as BNP concentration naturally increases as people get older, even without heart disease. For example, a value considered mildly elevated in a younger adult might be expected for someone over 75 years old. Interpreting a result requires considering the patient’s demographic data alongside the laboratory’s established reference intervals, but the full context of the patient’s health must be considered.
Interpreting Elevated BNP Levels
When a BNP result is above the normal range, it indicates that the heart muscle is under stress and is struggling to handle its workload. The degree of elevation correlates with the severity of the underlying cardiac condition. For instance, in patients presenting with acute symptoms, levels greater than 400 pg/mL suggest acute heart failure.
A result falling between 100 pg/mL and 400 pg/mL is referred to as a “gray zone” and requires detailed clinical evaluation. Within this intermediate range, the diagnosis is less definitive, and a healthcare provider must integrate the BNP value with other findings, such as imaging results and patient symptoms. Higher BNP levels are also recognized as strong predictors of future adverse cardiovascular events and mortality, providing prognostic information.
BNP testing is also useful for monitoring the effectiveness of treatment in patients with a heart failure diagnosis. A successful medical regimen should lead to a reduction in BNP levels over time, reflecting decreased ventricular wall stress. Conversely, a rising BNP level may signal that the condition is worsening or that the current treatment plan needs adjustment.
Factors That Influence BNP Results
Several non-cardiac factors can affect BNP test results, leading to values that are either falsely high or falsely low. Advancing age is a significant variable that naturally elevates BNP levels, necessitating age-specific ranges for accurate interpretation. Gender also plays a role, with females generally exhibiting slightly higher BNP concentrations than males.
Kidney dysfunction causes a substantial increase in BNP levels because the kidneys are responsible for clearing the hormone from the bloodstream. Impaired renal function slows clearance, causing the concentration to build up regardless of the degree of heart failure. Conversely, patients with high body mass index or obesity often show lower than expected BNP levels.
Certain medications can also interfere with the results. For example, some heart failure drugs may lower BNP concentration, while Angiotensin Receptor Neprilysin Inhibitors (ARNIs) inhibit the enzyme that degrades BNP, causing the measured level to appear higher. The result must always be interpreted in the context of a patient’s complete medical history, existing conditions, and current medications.