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Lab test update - BNP

Chemical pathologist Dr Paul Collinson uses a case study to discuss use of B-type natriuretic peptide in suspected cardiac failure

Chemical pathologist Dr Paul Collinson uses a case study to discuss use of B-type natriuretic peptide in suspected cardiac failure

How useful is the NTproBNP test as a predictor of cardiac failure?

41208322The measurement of B-type natriuretic peptide, either as B-type natriuretic peptide (BNP) or the prohormone N-terminal proBNP (NTproBNP) is an excellent test for the exclusion of cardiac failure in primary care. A value below the appropriate cut-off for the age of the patient excludes cardiac failure with a very high negative predictive value. Values above the cut-off require further investigation by echocardiography.

Very high values make acute cardiac failure likely but there are a range of causes of moderate elevation of NTproBNP/BNP that are not primarily due to heart failure, though they do mean cardiac dysfunction.

An elevated NTproBNP always indicates that there is something wrong with the patient and is associated with a reduced long-term survival. The value in this case suggests chronic heart failure and further investigation is required but makes acute heart failure unlikely. Examples of cut-off values for NTproBNP are shown below.

Always remember that there is no such thing as a perfect diagnostic test – of any type. All tests must always be interpreted in the context of the clinical features of the patient. Clinical judgement must be an important part of the diagnosis.

An ECG is often recommended as a tool to find out who should be sent for echocardiography – how does this tie in with BNP measurement?

41208321Current guidelines recommend a combination of an ECG together with measurement of BNP. An entirely normal ECG also has a strong negative predictive value for heart failure. However, when comparing the diagnostic power of the ECG with BNP measurements, the ECG has been found to be inferior in routine clinical practice. If both tests are normal, heart failure is unlikely. If the NTproBNP/BNP is elevated but ECG is normal, believe the NTproBNP/BNP.

In established cardiac failure, does the NTproBNP/BNP have prognostic value?

The magnitude of elevation of NTproBNP/BNP is prognostic in patients with cardiac failure and indeed across the entire range of people with cardiac disease. The degree of elevation is proportional to the degree of risk of premature death.

And does the level fall with treatment of cardiac failure?

Successful treatment of heart failure results in a fall in NTproBNP/BNP. Failure of NTproBNP/BNP to fall in response to treatment is associated with a much worse long-term outcome. Studies that have used measurement of NTproBNP/BNP to both monitor and to direct treatment have shown that active treatment to reduce the level of the hormone is associated with a better outcome.

What could be the explanation for a very high BNP in the light of a normal echocardiogram?

An elevated NTproBNP/BNP indicates atrial and ventricular dysfunction from any cause. Echocardiography is very good at demonstrating impaired systolic function by measurement of the ejection fraction. As well as systolic dysfunction, there may be diastolic dysfunction, which is difficult to diagnose on the echocardiogram but also causes a rise in the NTproBNP/BNP. Other causes of cardiac structural dysfunction such as cardiomyopathy may also cause a rise as well as valvular disease and atrial fibrillation. Renal failure often with fluid overload and infective conditions such as pneumonia may also cause any rise in NTproBNP/BNP. Any raised value requires further investigation and explanation, as it will be associated with heart disease.

Dr Paul Collinson is consultant chemical pathologist at St George's Hospital London. He has also written a Q&A article for about troponins in acute coronary syndrome.

cutoff case BNP

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