Prior to any non-pharmaceutical denitive treatment, conrmation of a unilateral source of independent and excessive aldosterone production as well as lateraliza­tion is necessary [7]. This is achieved with adrenal vein sampling (AVS). Rarely, unilateral adrenal hyperplasia and contralateral nonfunctioning adenoma can coexist. If resection of the adenoma is performed without prior AVS conrmation, the patient will continue to suffer from PA and miss the opportunity for curative resection. There exist different technical protocols on how to perform AVS.The most rigorous, sensitive, and specic one includes pre- and post-adrenocortico­tropic hormone (ACTH) stimulation sampling and the use of C-arm CT to conrm selectivity in order to render biochemical lateralization reliable [3, 8]. Briey, both adrenal veins are selected with catheters from a common femoral vein approach (Fig.16.1cand d). A C-arm CT is performed during contrast injection to conrm selectivity (Fig.16.1cand d). Venous blood sampling is performed, followed by ACTH administration. Sampling is repeated 30 min post-ACTH stimulation. Cortisol adrenal vein levels are compared to that from inferior vena cava (IVC) levels to conrm catheter selectivity. Once selectivity is conrmed, aldosterone levels are compared between adrenal vein samples to conrm lateral­ization. If both catheter selectivity and biochemical lateralization are conrmed, surgical or image-guided treatment can be undertaken. Less rigorous protocols sample the adrenal veins only post- ACTH stimulation. Though less time-consum­ing, this protocol can result in false- negative results in approximately 22% of patients tested [8].