A Comparative Study on Inter and Intra-Laboratory Reproducibility of Renin Measurement with a Conventional Enzymatic Method and a New Chemiluminescent Assay of Immunoreactive Renin

Background: The activity of the renin-angiotensin system is usually evaluated as plasma renin activity (PRA, ng AI/ml/h) but the interlaboratory reproducibility of this enzymatic assay is notoriously scarce. We compared the inter and intralaboratory reproducibility of a commercial PRA assay (DiaSorin RENCTK) with that of a new automated chemiluminescent assay (DiaSorin Liaison) which allows the direct quantification of immunoreactive renin (Ir-R, mU/ml). Methods: Aliquots from 6 pool plasmas of patients known for having from very low to very high PRA levels were measured twice in 12 specialized European centers with both the enzymatic and the direct assays. The same methods were applied to three vials (Lyphochek, Biorad) with known content of renin, the expected values being, according to the manufacturer, 1.3, 3.1 and 10.4 ng AI/ml/h and 33.7, 73.6 and 316 mU/ml. Results: In pool plasmas mean PRA values ranged from 0.13 ± 0.08 (SD) to 18.8 ± 4.2 ng AI/ml/h and those of Ir-R from 4.2 ± 1.7 to 436 ± 47 mU/ml. In control vials the mean PRA values were 1.5 ± 0.2, 3.7 ± 1.1 and 11.4 ± 4.0 ng AI/ml/h while mean Ir-R were 35.3 ± 2.1, 75.4 ± 5.5 and 306 ± 23 mU/ml. Overall there was a highly significant correlation between the mean values obtained with the two methods (r = 0.97, p<0.001). However the between laboratory coefficients of variation (CV %) observed with PRA were always higher than those of Ir-R (respectively from 63.6% to 13.2% and from 41% to 10.7%, p<0.01). Also the intralaboratory reproducibilities were higher in all but one lab for PRA than per Ir-R ranging respectively from 16% to 3.6% and from 12.7% to 2.0% (p<0.01). Conclusions: The measurement of Ir-R with DiaSorin Liaison method appears to be a precise and reliable alternative to PRA measurement with DiaSorin RENCTK, having the advantage, with respect to the enzymatic assay, of a superior inter and intralaboratory reproducibility.