The determination of renin is useful for the investigation of secondary aldosteronism (e.g. renovascular disease, salt depletion, potassium loading, cardiac failure with ascites) or for the investigation of primary aldosteronism (adrenal adenoma/carcinoma and adrenal cortical hyperplasia). Renin can be measured as the plasma renin activity (PRA) or the direct renin concentration (DRC). Both are approved for clinical use in a recent Endocrine Society Practice Guideline, either by themselves or combined with aldosterone to obtain an aldosterone to renin ratio (1).

However, the sole manufacturer of the FDA approved assay kits for plasma renin activity discontinued their manufacture in December 2016. As a result, Warde Medical Laboratory validated an FDA approved kit for direct renin concentration and has converted to the methodology effective January 24, 2017.

The new assay measures the actual concentration of renin. The units are pg/mL. The old assay (Plasma Renin Activity) measured the activity of renin by measuring the production of Angiotensin 1 under controlled conditions. As an enzyme assay, it was dependent upon pH, incubation time, substrate concentration, and the presence of inhibitors or catalysts (including drugs). The units were ng/mL/hr.

There is no precise conversion factor between the two different methods. That being said, and although it varies from patient to patient, 1 ng/mL/hr of plasma renin activity approximately equals 7.6 pg/mL of direct renin concentration (range 5.5 to 9.7). So one will see higher “renin” values with the new assay.

A number of clinical studies have validated the use of the direct renin assay in screening for primary aldosteronism (2,3,4,5). In one study, at an aldosterone/ direct renin ratio of greater than 3.7 (37.0 in conventional units), the sensitivity was 90% and the specificity was 100% (2).