The diagnostic work-up of pheochromocytoma/paraganglioma (PPGL) among patients with chronic kidney disease (CKD) is a clinical challenge. Impaired renal elimination of catecholamines and their metabolites confounds urinary tests, while increased plasma concentrations of free metanephrines in patients with CKD also complicate interpretation of plasma tests.1-3 We therefore aimed to establish optimal reference intervals for plasma free metanephrines and 3-methoxytyramine in patients with CKD stages 3 and 4 or receiving hemodialysis (HD).