Improved immunoradiometric assay for plasma renin.

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 degrees C to 37 degrees 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 degrees 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 degrees C, avoids interference by prorenin while retaining the desirable analytical characteristics of the older IRMA and requiring less time.