NT-proBNP

Your heart is a muscle that makes hormones of its own. When the wall of a heart chamber is stretched more than usual, because more blood is in it or because the chamber struggles to pump it away, the muscle cells make a precursor hormone that lets the kidneys shed salt and water and lets the vessels relax, so the pressure drops. That precursor hormone splits into two pieces: the active BNP, which does the work and is gone again after minutes, and an inactive leftover fragment that circulates for hours. That leftover fragment is NT-proBNP. Precisely because it stays measurable longer, it is usually that inactive piece that is measured. Both say the same about the stretch on your heart, but the numbers are not interchangeable: the cut-off for BNP is 35 pg/ml and that for NT-proBNP 125 pg/ml. The fragment leaves your body through the kidneys, and that is why it rises with a lower kidney function and with age, even when your heart does nothing different. Belly fat works the other way: fat tissue breaks the fragment down, so people with excess weight come out lower than their heart deserves. Your report prints NT-proBNP in pg/ml or ng/l, and that is the same number; if it says pmol/l, the number is about a factor of eight smaller, and 125 pg/ml belongs beside 15 pmol/l. Healthy is below 125 pg/ml, at every age. In addition the band on your report rises with age, from 86 ng/l for a man younger than 45 years to 738 for a woman older than 75 years, and describes what is usual, not what is healthy.

This value is built to rule something out. With shortness of breath, tiredness, swollen ankles or not being able to lie flat, the question is whether the heart can be the cause, and an NT-proBNP below 125 pg/ml makes that direction unlikely: then the heart chamber is not under pressure and the explanation lies elsewhere, in the lungs, the blood or fitness. That is what the test is strongest at, and a low value therefore says more than a high one. The second reason is that the value moves early. A heart that stiffens through years of high blood pressure, excess weight or poorly controlled diabetes shows that in this fragment earlier than in complaints. Anyone who gets their blood pressure, weight and fitness in order relieves the heart chamber and sees the value fall; in people who already have heart failure a falling NT-proBNP is the sign that the treatment is working. The honest limitation is that a high value is not a diagnosis. Getting older, a lower kidney function, atrial fibrillation, a pulmonary embolism, an infection and heavy exertion lift the value without there being heart failure, and above 125 pg/ml an echo of the heart is the next step, not a conclusion. Above 2000 pg/ml heart failure is virtually certain. In between lies the grey zone where age and kidney function make the most difference.

A low value is the outcome this test was made for: there is no raised pressure on your heart chamber, and heart failure as the explanation for your complaints is unlikely. Two things undermine that certainty. In atrial fibrillation the value rises through the irregular heartbeat itself and a low value says too little to rule heart failure out. And with marked excess weight fat tissue breaks the fragment down, so a heart under pressure can still come out below 125 pg/ml. A high value means stretch on the heart chamber or a slower clearance. In order of how often it occurs: getting older, because the band rises in men from 86 ng/l younger than 45 years to 486 above 75 years, and in women more strongly still; a lower kidney function, because the fragment leaves your body through the kidneys; atrial fibrillation, which stretches the atria; and a higher pressure in the heart chamber itself, through a stiff or weakened heart muscle, a leaking valve or years of high blood pressure. A pulmonary embolism, an infection and heavy exertion shortly before also lift the value temporarily. Always read NT-proBNP beside your eGFR and creatinine, because a lower kidney function lifts the value outside your heart, beside your age and the band that goes with it, beside your blood pressure and weight, which determine the stretch on the chamber, and beside your previous result from the same lab, because the trend says more than one number.