Direct renin: an automated immunoassay for renin with sensitivity comparable to plasma renin activity

Detection of low plasma renin is crucial because it determines the course of treatment of hypertensive patients. Nichols Institute Diagnostics developed an automated chemiluminometric immunoassay for renin (Direct Renin) that is sensitive enough to detect low renin samples, minimizes interference by prorenin, and it can be performed in less than one hour. Two monoclonal antibodies are used: the capture antibody is bound to biotin and recognizes both renin and prorenin. The second antibody is labeled with acridinium-ester and is specific for active renin. The method is automated on the Nichols Advantage® instrument: after a 20 minute 37° C incubation of a 0.2 ml plasma sample with both antibodies, paramagnetic particles coated with streptavidin are added and incubation extended for further 10 minutes. Detection after washing is accomplished by the addition of alkaline peroxide. The ability of this immunoassay to discriminate patients with low renin was assessed by comparing it to the Plasma Renin Activity (PRA) method of Sealey and Laragh. Two separate comparison studies were performed four months apart using different immunoassay reagent lots. The first study of 240 patient specimens included 93 samples with PRA < 0.65 ng/ml/h, 38 samples with PRA 0.65 to 1.0 ng/mg/h, 30 samples from diabetic patients and 28 from pregnant women. Using a 3-h PRA incubation and PRA and Direct Renin cutoffs of 0.65 ng/ml/h and 5 mU/L respectively, classification of low vs. non-low renin agreed in 177 samples (82.3%). Three specimens were classified as low renin by PRA and non-low by Direct Renin. Thirty three specimens were classified as low-renin by Direct Renin and non-low renin by PRA. Since most of these 33 discrepant samples had PRA activities between 0.65 and 1.0 ng/ml/h, in the second 163-sample study, the 43 specimens with PRA values < 1.0 were reanalyzed using an 18-h incubation. Again with 0.65 ng/ml/hr and 5 mU/L as cut-offs, low renin classification agreed in 152 of the 163 samples (93%); 2 samples (1.2 %) had low PRA values and non-low Direct Assay results and 9 samples (5.5 %) low Direct Assay and non-low PRA results. Although prorenin levels are increased in diabetes and pregnancy, samples from diabetic patients behaved like the other specimens in the regression analysis. Direct Renin results in samples from pregnant women were actually lower than the values expected from the regression analysis, most likely because their PRA reflect increased plasma angiotensinogen levels. These results suggest that plasma prorenin does not interfere with the Direct Renin assay. In summary, we report here a simple, fast and reproducible Direct Renin immunoassay with sensitivity and accuracy comparable to those of the classical PRA kinetic assay.