Automated Chemiluminescence-Immunoassay for Aldosterone during Dynamic Testing: Comparison to Radioimmunoassays with and without Extraction Steps

BACKGROUND: Measurements of aldosterone have become more common since the recognition that primary aldosteronism is a more frequent cause of hypertension than previously believed. Our aim was to compare concentrations reported by 4 assays for samples obtained after saline infusion during dynamic testing. METHODS: We tested 104 participants (27 with primary aldosteronism, 30 with essential hypertension, and 47 healthy controls) with the intravenous saline infusion test (2.0 L isotonic saline over 4 h), with repetitive sampling. In all blood samples, aldosterone concentration was measured by an in-house RIA after extraction and chromatography, by 2 commercially available RIAs without extraction (Aldosterone Maia, Adaltis; Active Aldosterone, Diagnostics Systems Laboratories) and by an automated CLIA (Advantage, Nichols Institute Diagnostics). RESULTS: Correlation coefficients for results of pairs of assays ranged from 0.74 to 0.98. Agreement between commercial assays and in-house RIA was best at the low to intermediate concentrations after saline infusion. Mean (SD) Adaltis and DSL RIA results were 2- to 3-times higher [healthy participants: 78 (25) ng/L and 56 (18) ng/L, respectively] than those obtained by Nichols CLIA [17 (8) ng/L] and in-house RIA [23 (18) ng/L]. Aldosterone concentrations measured by the Nichols CLIA were below the limit of detection (limit of the blank) in 27 of 47 healthy participants. CONCLUSIONS: Aldosterone concentrations reported by the Adaltis and DSL nonextraction RIAs were consistently higher than those produced by the Nichols CLIA and the in-house RIA. The convenient Nichols CLIA showed better agreement with the in-house RIA, but the concentrations in healthy participants were frequently undetectable by this method. Uncritical application of cutoff values from the literature must be avoided.