Diagnosis of primary aldosteronism: Renin activity, renin concentration and aldosterone/renin ratio.

The Renin-Angiotensin-Aldosterone System (RAAS) is a cornerstone of cardiovascular and electrolyte homeostasis, and its dysregulation is central to the pathophysiology of hypertension, heart failure, and chronic kidney disease. This chapter provides a comprehensive overview of the RAAS, from its classical and alternative signaling pathways to its clinical implications. We delve into the pathological consequences of both RAAS overactivation and its suppression. A significant focus is placed on the biochemical evaluation of the RAAS, tracing the chronological progression from early bioassays to modern automated immunoassays and high-resolution mass spectrometry for measuring renin and aldosterone. We critically examine the methodologies for determining plasma renin activity (PRA) and direct renin Concentration (DRC), and the nuances of plasma and urinary aldosterone quantification. The aldosterone-to-renin ratio (ARR) is highlighted as the essential screening tool for PA, with a detailed discussion of assay-specific cut-offs and the imperative of laboratory-specific validation. Furthermore, the chapter outlines the numerous preanalytical and analytical variables that profoundly influence RAAS component measurements, including patient posture, diet, and the significant confounding effects of various medications. Adherence to standardized protocols for sample collection, handling, and analysis is emphasized as critical for accurate diagnosis and management.