Changes in aldosterone during saline infusion. Serum aldosterone concentration before and after fluid challenge in A patients with initially normal or high concentrations, and B patients with initial concentrations below threshold of assay. In B, aldosterone concentrations remained below threshold in 4 patients whose lines overlap and are therefore indistinguishable. Reference range for serum aldosterone in supine patients is 135–400 pmol/l. Note that even 'normal' serum aldosterone would be inappropriately low in patients with sodium depletion.