Endocrine Abstracts

Background: An elevated plasma aldosterone to renin ratio (ARR) is widely accepted as being important for the detection of primary hyperaldosteronism. The ability to measure renin at extremely low concentrations is the most critical factor in determining the diagnostic reliability of the ARR. We carried out this study to establish a reference interval for the ARR as part of an evaluation of a new automated platform for the simultaneous measurement of aldosterone (PAC) and direct renin (PRC) in plasma.

Methods: We studied 78 non-smoking normotensive volunteers. Blood for renin and aldosterone was collected after 10 minutes sitting and overnight fasting. PAC and PRC were measured simultaneously in EDTA plasma on the Nichols Advantage chemiluminescence analyzer. PAC was also measured using a solid phase (coated tube) radioimmunoassay (RIA) kit (Diagnostics Products Inc) and plasma renin activity (PRA) was determined by RIA measurement of angiotensin 1 after generation from renin (Adaltis MIAA).

Results: PRC varied from 2.6 to 53.4 microUnits/ml with median of 22 and PRA from 0.3 to 3.5 nanograms/ml/hour with median of 1.6. There was significant correlation between the two measurements (r = 0.91, P < 0.0001). Aldosterone to PRC ratios ranged from 1.6 to 50.2 and PAC/PRA from 21.4 to 651.4 with good correlation between the two ratios (r = 0.94, P < 0.0001). The upper (97.5th percentile) cut-off value for PAC/PRA was 513 (picomol per litre / nanogram per ml per hour) and the corresponding PAC/PRC ratio was 40 (picomol per litre/ microUnit per ml).

Conclusion: The limits of normality of PRA and PRC measured simultaneously on a new automated platform in our local population have been determined and compared with more established procedures. This information is an essential first step in establishing the diagnostic reliability of the ARR in identifying patients with primary hyperaldosteronism using this new procedure.