Oral Presentation ESA-SRB-AOTA 2019

Diagnosis Of Primary Aldosteronism By Seated Saline Suppression Test - Analysis Using Immunoassay (#241)

Moe Thuzar 1 2 , Karen Young 3 , Ashraf H Ahmed 2 , Greg Ward 3 , Martin Wolley 2 , Zeng Guo 2 , Richard D Gordon 2 , Brett C McWhinney 4 , Jacobus P Ungerer 4 , Michael Stowasser 2
  1. Department of Endocrinology & Diabetes, Princess Alexandra Hospital, Brisbane, Queensland, Australia
  2. Endocrine Hypertension Research Centre, University of Queensland Diamantina Institute, Princess Alexandra Hospital, Brisbane, Queensland, Australia
  3. Department of Chemical Pathology, Sullivan Nicolaides Laboratories, Brisbane, Queensland, Australia
  4. Department of Chemical Pathology, Pathology Queensland, Brisbane, Queensland, Australia

Background: Primary aldosteronism (PA) is characterized by excessive, autonomous secretion of aldosterone which fails to suppress upon administration of fludrocortisone and/or salt loading. Seated saline suppression testing (SSST) was recently reported to be superior to recumbent.1 The recommended diagnostic cut-off 4-hour plasma aldosterone concentration (PAC; 162 pmol/L) for SSST was based on that measured by HPLC-MS/MS. Most diagnostic laboratories, however, use immunoassays to measure PAC.

Objective: To define the optimal cut-off PAC measured by immunoassay for the diagnosis of PA using SSST.

Methods: This study involved 80 of the 85 SSST studies whose data were used to originally define the optimal HPLC-MS/MS cut-off. PA was confirmed in 65 (23 unilateral, 34 bilateral and 8 subtype yet to be determined) by positive fludrocortisone suppression testing (FST) and/or lateralization on adrenal venous sampling and excluded in 15 (12 cured of PA post-adrenalectomy and 3 non-PA) by negative FST. PAC was measured by a chemiluminescence immunoassay (PACIA) using the DiaSorin Liaison XL analyzer.

Results: Receiver operating characteristics analysis revealed good performance (AUC = 0.893; P<0.001) of the 4-hour PACIA for the diagnosis of PA and an optimal diagnostic cut-off of 171 pmol/L, with sensitivity and specificity of 95.4% and 80.0% respectively (Youden index = 0.754). Using this cut-off, none of the three subjects with false-negative SSST had unilateral PA. The cut-off with the next highest Youden index (0.728) was 217 pmol/L and demonstrated lower sensitivity (86.2%) but higher specificity (86.7%). PACIA measurements strongly correlated with PAC measured by HPLC-MS (Pearson correlation coefficient r = 0.94, P<0.001).

Conclusions: A higher diagnostic cut-off level for SSST should be employed when PAC is analyzed by immunoassay compared to that by HPLC-MS/MS.

Reference. 1. Stowasser M, et al. Comparison of seated with recumbent saline suppression testing for the diagnosis of primary aldosteronism. J Clin Endocrinol Metab 2018;103:4113-24.