Single Assay for Amino-Terminal Fragments of Cardiac A- and B-Type Natriuretic Peptides

BACKGROUND: High circulating concentrations of N-terminal fragments of A- and B-type natriuretic peptides (NT-proANP and NT-proBNP) identify patients with impaired cardiac function. ProANP-derived peptides are particularly sensitive to increased preload of the heart and proBNP-derived peptides to increased afterload; therefore, combining the information from the ANP and BNP systems into a single analyte could produce an assay with increased diagnostic and prognostic power. METHODS: We prepared a hybrid peptide containing peptide sequences from both NT-proBNP and NT-proANP (referred to as NT-proXNP) by recombinant techniques and used it to develop a RIA combining weighed concentrations of NT-proANP and NT-proBNP into a new virtual analyte, NT-proXNP. We used the novel method to measure the circulating concentrations in healthy persons and in patients with cardiac disorders. We also characterized the assay by HPLC analysis of the immunoreactive molecular forms in human plasma and serum. RESULTS: The results from the novel assay correlated well with independent home-made NT-proANP and NT-proBNP assays (r2 = 0.75-0.85) as well with the arithmetic sum of NT-proANP and NT-proBNP (r2 = 0.92). Patients with valvular heart disease (VHD) and coronary artery disease (CAD) had significantly increased NT-proXNP concentrations. The areas under the curve (AUC) of the NT-proXNP assay in detecting VHD and CAD (0.961 and 0.924, respectively) were significantly larger than the AUC of either NT-proANP (0.947 and 0.872) or NT-proBNP (0.913 and 0.782) assays. HPLC analysis showed that the novel NT-proXNP assay detects two major classes of circulating immunoreactivity corresponding to peptides derived from NT-proANP and NT-proBNP. CONCLUSIONS: Our novel immunoassay mimics the physiologic signaling system working in the body by converging the information obtained from the activation of ANP and BNP into a single virtual analyte, NT-proXNP. It appears to have a diagnostic efficiency equal to or slightly better than that of individual NT-proANP or NT-proBNP assays.