These tests measure the concentration of B-type natriuretic peptide (BNP) or N-terminal pro b-type natriuretic peptide (NT-proBNP) in the blood in order to detect and evaluate heart failure. BNP was initially called brain natriuretic peptide because it was first found in brain tissue (and to distinguish it from a similar protein made in the atria, or upper chambers, of the heart, termed ANP). BNP is actually produced primarily by the left ventricle of the heart (the heart’s main pumping chamber). It is associated with blood volume and pressure and with the work that the heart must do in pumping blood throughout the body. Small amounts of a precursor protein, pro-BNP, are continuously produced by the heart. Pro-BNP is then cleaved to release the active hormone BNP and an inactive fragment, NT-proBNP, into the blood.  Sample   Blood (serum)  Reference   Less than 125 pg/ml   Frequency   Twice a week