Brain natriuretic peptide (BNP) is a 32 amino acid peptide, it contains a 17 amino acid ring structure that is common to all natriuretic peptides. BNP was first identified in porcine brain; however, the main source of this peptide is not the brain but the cardiac ventricle. This cardiac neurohormone is secreted from the ventricles in response to volume expansion and pressure overload. It has natriuretic and vasodilatory effects and suppresses the renin-angiotensin-aldosterone system. High levels of circulating BNP have been linked to increased risk of heart failure. BNP has been utilised in clinical trials to assess heart failure diagnosis however the assay is not yet considered robust due to high variability. The BNP (18-27) fragment provided here has been tested in an immunoaffinity LC/MS assay and was shown to provide a more specific and accurate diagnosis of heart failure than previous methods. This tryptic fragment is conserved in many degradation products and therefore makes it a useful peptide for estimation of the level of total BNP. Further work with this BNP derived fragment could improve diagnostics and management of heart failure.