SUN-222 The Utility of 24-hour Urinary Aldosterone Concentration Analysed by LC-MS/MS for the Diagnosis of Primary Aldosteronism

Abstract Disclosure: A. Matthews: None. D. Je: None. M. Thuzar: None. J. He: None. E. Ng: None. D. Cowley: None. J. Yang: None. M. Stowasser: None. Background & Aim: Diagnosis of primary aldosteronism (PA) in current practice involves initial screening by plasma aldosterone-renin ratio (ARR) followed by confirmatory testing such as seated saline suppression test (SSST). Measurement of 24-hour urinary aldosterone concentration (24hr-UAC) collected after adequate sodium intake is a potential diagnostic test for PA, but its diagnostic performance/utility has not been well established. Furthermore, previous studies have investigated the utility of 24hr-UAC for diagnosis of PA when aldosterone concentration is measured by immunoassay, which is considered less accurate than liquid chromatography-tandem mass spectrometry (LC-MS/MS). The aim of this study was to determine the diagnostic performance of 24hr-UAC analysed using LC-MS/MS for the diagnosis of PA. Methods: This is a retrospective analysis of 24hr-UAC measured by LC-MS/MS and 24-hour urinary sodium concentration (24hr-UNa) (total n=182) collected prospectively from 167 patients who had SSST performed at the Princess Alexandra Hospital between August 2018 and July 2023. The SSST was performed due to elevated ARR (n=158) or to assess for biochemical cure post adrenalectomy for unilateral PA (n=24). All tests were performed after withdrawing interfering antihypertensive medications. Patients were encouraged to consume a liberal salt diet for at least a week prior to 24-hour urine collection and SSST. Sodium chloride tablets were not administered. Receiver operating characteristic curve (ROC) analyses were performed to assess the performance of 24hr-UAC at various 24hr-UNa for diagnosis of PA using the SSST as the reference test. Results: One-hundred and twenty-seven (127) out of 182 were diagnosed as PA by positive SSST and the remaining 55 had negative SSST. Mean 24hr-UAC was more than 2-fold higher in cases with positive SSST than that in those with negative SSST (50.6±3.2 nmol/24h vs 20.8±1.9 nmol/24h; P<0.05), with no significant difference in 24hr-UNa between the two groups (163±75.8 for positive SSST vs 147±64; P=0.07). ROC analysis revealed that 24hr-UAC ≥23.5 nmol/24h had 92.1% sensitivity and 82.6% specificity (area under the curve = 0.946) to identify PA in individuals with 24hr-UNa ≥190 mmol/24h (n=52), the 24hr-UNa level at which performance characteristics of the UAC were optimal. A higher cut-off 24hr-UAC ≥27.5 nmol/24h provided a higher specificity (92.3%) with 78.9% sensitivity, for individuals with 24hr-UNa ≥190 mmol/24h. Conclusion: 24hr-UAC analysed by LC-MS/MS in the setting of 24hr-UNa ≥190 mmol/24h can identify PA cases with high sensitivity and specificity. However, the target urinary sodium of at least 190 mmol/day was only observed in a minority of patients, suggesting the need for sodium chloride supplementation to optimize test performance of the 24hr-UAC, at least in Australian patients. Presentation: Sunday, July 13, 2025