The saline infusion test for primary aldosteronism: implications of immunoassay inaccuracy

CONTEXT 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. OBJECTIVE Establish whether differences in aldosterone measurements by immunoassay versus mass spectrometry (MS) might impact confirmatory testing for PA. METHODS 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. RESULTS 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