Diagnosis of Primary Aldosteronism by Seated Saline Suppression Test – Variability Between Immunoassay and HPLC-MS/MS

Background: In primary aldosteronism (PA), excessive, autonomous secretion of aldosterone is not suppressed by salt loading or fludrocortisone. For seated saline suppression testing (SSST), the recommended diagnostic cut-off 4-hour plasma aldosterone concentration (PAC) measured by HPLC-MS/MS is 162pmol/L. Most diagnostic laboratories, however, use immunoassays to measure PAC. The cut-off for SSST using immunoassay is not known. We hypothesized that the cut-off is different between the assays. Methods: We analysed 80 of the 87 SSST tests that were performed during the recent study defining the HPLC-MS/MS cut-off. PA was confirmed in 65 by positive fludrocortisone suppression testing (FST) and/or lateralisation on adrenal venous sampling and excluded in 15 by negative FST. PAC was measured by a chemiluminescence immunoassay (PACIA) in the SSST samples using the DiaSorin Liaison XL analyser, and receiver operating characteristics (ROC) analysis was performed to identify the PACIA cut-off. Results: ROC revealed good performance (AUC=0.893; P 217pmol/L by chemiluminescence immunoassay. A grey zone exists between cut-offs of 171 and 217pmol/L, likely reflecting a lower specificity of immunoassay.