SAT-253 24-Hour Urinary Aldosterone Measured by Immunoassay for the Diagnostic Work-up of Primary Aldosteronism

Abstract Disclosure: E. Ng: None. H.Q. Shen: None. X. Lim: None. P. Marcus: None. S.M. Gwini: None. M. Thuzar: None. M. Stowasser: None. P.J. Fuller: None. J. Hu: None. J. Yang: None. In primary aldosteronism (PA), the variability in plasma aldosterone concentration (PAC) at a single time point is well documented. Measurement of 24-hour urinary aldosterone excretion (24h-UAE) may overcome this variability. An elevated 24h-UAE > 12 ug (33 nmol)/day, combined with high urinary sodium excretion (24h-UNa) > 200 mmol/day, is a recognised method for confirming PA. However, the rationale for this threshold and its diagnostic accuracy are not well established. This study aims to assess the diagnostic accuracy of the historical cut-off in Australian and Chinese cohorts and evaluate the optimal 24h-UAE threshold for diagnosing PA, using the saline suppression test (SST) as the reference standard. Patients referred for PA workup from 2018 to 2023 at two tertiary centres in Melbourne, Australia (n=215), and Chongqing, China (n=894) were included in the study if they had a 24h-UAE and SST performed. Blood and urine collections for aldosterone measurements were performed after washout of interfering medications. 24h urine sample collection was performed in an ambulatory setting for the Australian cohort and in an inpatient setting for the Chinese cohort. PAC, DRC and 24h-UAE were measured using an automated chemiluminescence immunoassays (DiaSorin). The diagnostic accuracy of 24h-UAE was compared to the SST result. The optimal 24h-UAE was determined using the Liu optimal cut-point and Youden index. PA was diagnosed in 138/215 (64%) of the Australian and 688/894 (77%) of the Chinese cohorts based on SST. The 24h-UAE cut-off of > 33 nmol/day (regardless of 24h-UNa) demonstrated 53% sensitivity and 79% specificity in the Australian cohort, and 36% sensitivity and 93% specificity in the Chinese cohort. The addition of 24h-UNa > 200 mmol/day improved sensitivity to 57 and 41%, respectively, but reduced specificity in the Australian cohort (75%), with no change in the Chinese cohort. In the Australian cohort, where 24h-UNa >190 mmol/day, the optimal 24h-UAE threshold was > 31 nmol/day, yielding 64% sensitivity and 77% specificity for diagnosing PA. In those with a DRC < 10 mU/L and 24h-UNa >190 mmol/day, a 24h-UAE > 22 nmol/day achieved 76% sensitivity and 83% specificity. Comparable diagnostic accuracy was seen in the Chinese cohort with the same DRC and 24h-UNa cut-off at a lower 24h-UAE cut-off of >18 nmol/day. This study highlights the suboptimal diagnostic accuracy of 24h-UAE measured by immunoassay and challenges the validity of the historical 24h-UAE cutoff of 33 nmol/day. Utility of the 24h-UAE may be improved by using liquid chromatography-mass spectrometry to measure aldosterone, but this remains to be formally evaluated. Presentation: Saturday, July 12, 2025