Theoretical approaches to estimation of plasma renin activity: a review and some original observations.

Performance of accurrate, reproducible, and interpretable assays for plasma renin activity and other components of the renin/angiotensin system in the clinical setting requires a clear understanding of the various reactions in the renin/angiotensin cascade and the nature of their interactions. Plasma renin activity, the rate of angiotensin generations from plasma incubated in vitro, is the most commonly used clinical index of function in the renin/angiotensin system. Renin activity is measured by radioimmunoassay of angiotensin I generated in vitro under carefully controlled conditions. The value obtained for plasma renin activity depends on pH and duration of incubation and on the method used to protect the angiotensin I generated. We recommend incubation at neutral pH in buffered plasma for three hours in the presence of either ethylenediaminetetraacetate + 8-hydroxyquinolone + dimercaprol or ethylenediaminetetraacetate + phenylmethylsulfonylfluoride. Addition of a standard preparation of human renin to the plasma incubation step of the renin activity assay serves the dual purpose of permitting measurement of renin activity and furnishing an internal standard for comparison of assay procedures. The many variables among renin assay methods can be cancelled by referring to a common internal renin standard.