Improved immunoradiometric assay for plasma renin

Abstract

Background: Our renin IRMA overestimated renin in plasmas with high prorenin-to-renin ratios. We suspected that the overestimation of renin was caused less by cross-reactivity of the renin-specific antibody with prorenin than by a conformational change of prorenin into an enzymatically active form during the assay. Methods: Because the inactive form of prorenin converts slowly into an active form at low temperature, we raised the assay temperature from 22 °C to 37 °C, simultaneously shortening the incubation time from 24 to 6 h. The former IRMA was performed in <1 working day with these modifications. Results: The comeasurement of prorenin as renin was eliminated. Reagents were Stable at 37 °C, and the new and old IRMAs were comparable in terms of precision and accuracy. The functional lower limit of the assay (4 mU/L) was below the lower reference limit (9 mU/L). The modified IRMA agreed closely with the activities measured with an enzyme-kinetic assay. Results were not influenced by the plasma concentration of angiotensinogen. At normal angiotensinogen concentrations, the IRMA closely correlated with the classical enzyme-kinetic assay of plasma renin activity. Conclusion: The modified IRMA, performed at 37 °C, avoids interference by prorenin while retaining the desirable analytical characteristics of the older IRMA and requiring less time.