Rapid, simplified radioimmunoassay of arginine-vasopressin and atrial natriuretic peptide in plasma.

We have improved a radioimmunoassay for arginine-vasopressin (AVP) and atrial natriuretic peptide (ANP) by using Sep-Pak C18 cartridges to extract AVP and ANP from acidified plasma. The analytes are co-eluted by use of a mobile phase consisting of 1,2-dimethoxyethane and 40 g/L aqueous trifluoroacetic acid (95/5, by vol). After rapid evaporation of the solvents, AVP and ANP are assayed by a nonequilibrium radioimmunoassay method in which commercially available antibodies and radiolabeled antigens are used. The bound fractions are separated from the free by use of polyethylene glycol with human gamma globulin and rabbit anti-human IgG as the second antibody. This results in very low nonspecific binding: 0.44% for the ANP assay, 0.70% for AVP. The minimum detectable amount of ANP is 0.39 pg per tube; for AVP, it is 0.13 pg per tube. Compared with other published methods, this method is substantially more reliable, economical, and easily established in a clinical chemistry laboratory.