Quantification of Pro-B-Type Natriuretic Peptide and Its Products in Human Plasma by Use of an Analysis Independent of Precursor Processing

BACKGROUND: Measurement of cardiac natriuretic peptides or their precursors in plasma appears promising in the diagnosis of heart failure. However, the currently available assays to measure pro-B-type natriuretic peptide (proBNP)-derived peptides have produced grossly discrepant results. METHODS: We treated plasma with trypsin before assay and used in the assay an antibody specific for a processing-independent epitope of human proBNP. We then determined the total concentration of proBNP and its products in healthy volunteers and heart-failure patients. RESULTS: The antiserum produced (no. 98192) required an intact proBNP NH2 terminus for binding and displayed a high titer, index of heterogeneity, and binding affinity, implying that the RIA was monospecific and highly sensitive. Preanalytical tryptic treatment of plasma cleaved proBNP forms to release the N-terminal 1-21 fragment. Furthermore, enzymatic treatment of plasma also was efficient in avoiding nonspecific interference from plasma proteins, making it an expedient alternative to extraction. In healthy individuals, the total proBNP concentrations increased with age from 2.0 pmol/L (range, 0-15 pmol/L; ages 51-65 years) to 22 pmol/L (range, 3-40 pmol/L; ages 66-88 years; P <0.0001). The increase in plasma proBNP in the elderly, however, also seems to reflect the prevalence of cardiac disease. Plasma concentrations in patients with heart failure were all markedly increased median, 89 pmol/L (range, 29-659) vs 1.0 pmol/L (range, 0-16) in age-matched controls; P <0.0001. CONCLUSIONS: The processing-independent analysis measures the total proBNP product irrespective of the degree of proBNP processing. The results show that proBNP and its products circulate in low picomolar concentrations in healthy individuals.