Immunoradiometric assay of atrial natriuretic peptide in unextracted plasma.

We have developed and validated a two-site liquid-phase immunoradiometric assay (IRMA) of atrial natriuretic peptide (ANP) in unextracted human plasma. Both radiolabeled rabbit anti-ANP IgG and polyclonal mouse anti-ANP must bind to ANP for detection, and the assay is specific for peptides with both an intact C-terminus and a disulfide bridge. The assay sensitivity (detection limit) is 0.96 pmol/L, and the working range is 2.3-300 pmol/L, with the hook effect occurring above 500 pmol/L. Results for diluted plasma from normal subjects and from patients with renal failure paralleled the standard curve; analytical recovery of ANP added to such samples averaged 94%. The between- and within-assay CVs at 8 pmol/L were 10% and 5%, respectively. The assay is sufficiently sensitive and precise to detect the postural change in ANP concentrations in normal subjects.