What is being tested?
Aldosterone is a hormone that regulates the retention of sodium (salt) and water by the kidney and also regulates the excretion of potassium. It plays an important role in the control of blood pressure.

Aldosterone is produced by the adrenal glands, located at the top of each kidney. Its production is stimulated by a complex process that includes several other "hormones", the most important of these being renin and angiotensin II. Renin, produced by the kidney, stimulates production of angiotensin II in the bloodstream. Angiotensin II then regulates production of aldosterone. Normally when renin increases, aldosterone increases; when renin is low, aldosterone decreases.

How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm for measuring a plasma aldosterone and/or renin level. Because the levels of aldosterone and renin change when a person goes from lying down to standing up, your doctor may collect one sample whilst you are lying down and another after you have been upright for a few hours. A 24-hour urine collection for aldosterone is preferred by some physicians, since plasma aldosterone levels vary by the time of day the samples are collected.