Diagnosis of primary aldosteronism (PA) for many patients depends on positive results for the saline infusion test (SIT). Plasma aldosterone is often measured by immunoassays, which can return inaccurate results. The objective of this study was to establish whether differences in aldosterone measurements by immunoassay versus mass spectrometry (MS) might impact confirmatory testing for PA. This study, involving 240 patients tested using the SIT at five tertiary-care centers, assessed discordance between immunoassay and MS-based measurements of plasma aldosterone. Plasma aldosterone measured by Liaison and iSYS immunoassays were respectively 86% and 58% higher than by MS. With an immunoassay-based SIT cut-off for aldosterone of 170 pmol/L, 78 and 162 patients had respective negative and positive results. All former patients had MS-based measurements of aldosterone <117 pmol/L, below MS-based cutoffs of 162 pmol/L. Among the 162 patients with pathogenic SIT results, MS returned non-pathologic results in 62, including 32 under 117 pmol/L. Repeat measurements by an independent MS method confirmed non-pathogenic results in 53 patients with discordant results. Patients with discordant results showed a higher (P<0.0001) prevalence of non-lateralized than lateralized adrenal aldosterone production than patients with concordant results (83%vs28%). Among patients with non-lateralized aldosterone production, 66% had discordant results. Discordance was more prevalent for the Liaison than iSYS immunoassay (32%vs16% P=0.0065) and was eliminated by plasma purification to remove interferents. These findings raise concerns about the validity of immunoassay-based diagnosis of PA in over 60% of patients with presumed bilateral disease. We provide a simple solution to minimize immunoassay inaccuracy-associated misdiagnosis of PA.
