| Summary: | Background: The recommended approach to screening for primary aldosteronism (PA) in at-risk populations is to determine the ratio of aldosterone concentration (serum (SAC)/plasma (PAC)) to renin measured in plasma as activity (PRA) or concentration (DRC). However, lack of assay standardisation mandates the need for method-specific decision thresholds and clinical validation in the local population. Aim: The study objective was to establish method-specific aldosterone: renin ratio (ARR) cut-offs for PA in men and women using the IDS-iSYS® assay system (IDS plc). Methods: A prospective cohort study design was used. PAC and DRC were measured immunochemically in ethylenediamine-tetraacetic acid (EDTA) plasma on the IDS-iSYS® instrument. Results: A total of 437 subjects (218 men, 219 women) were recruited including: healthy normotensive volunteers (n=266) and women taking the oral contraceptive pill (OCP; n=15); patients with essential hypertension (EH; n=128); confirmed PA (n=16); adrenal cortical carcinoma (ACC; n=3); Addison's disease (AD; n=4) and phaeochromocytoma/paraganglioma (PPGL; n=5). In this population, an ARR cut-off at>37.4 pmol/mIU provided 100% diagnostic sensitivity, 96% specificity and positive likelihood ratio for PA of 23:1. When the ARR decision threshold was stratified according to gender, a cut-off of>26.1 pmol/mIU in men and>113.6 pmol/mIU in women resulted in diagnostic sensitivity and specificity of 100%. Conclusion: This study demonstrates that decision thresholds for PA should not only be method-specific but also gender-specific. However, given the small number of PA patients (n=16), particularly women (n=4), further validation through a prospective study with a larger PA cohort is required before the thresholds presented here could be recommended for routine clinical use. Keywords: Primary aldosteronism, Renin, Aldosterone, Aldosterone: renin ratio (ARR), Sensitivity, Specificity
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