The Test Sample
What is being tested?
Aldosterone is a that stimulates the retention of sodium (salt) and the excretion of potassium by the kidneys. It plays an important role in maintaining normal sodium and potassium concentrations in blood and in controlling blood volume and blood pressure. Aldosterone is produced by the adrenal cortex, the outer portion of the adrenal glands located at the top of each kidney. Its production is normally regulated by two other proteins, renin and angiotensin. Renin is released from the kidneys when there is a drop in blood pressure, a decrease in sodium concentration, or an increase in potassium concentration. Renin cleaves the blood protein angiotensinogen to form angiotensin I, which is then converted by a second enzyme to angiotensin II. Angiotensin II causes blood vessels to constrict, and it stimulates the production of aldosterone. The overall effect is to raise blood pressure and keep sodium and potassium at normal levels.
A variety of conditions can lead to overproduction (hyperaldosteronism) or underproduction (hypoaldosteronism) of aldosterone. Because renin and aldosterone are so closely related, both substances are often tested together to identify the cause of an abnormal aldosterone.
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm to measure plasma aldosterone and/or renin. A
24-hour urine collection for aldosterone is preferred by some physicians since plasma aldosterone levels vary throughout the day and are affected by position. In some cases, blood is collected from the renal or adrenal veins by insertion of a catheter; this is done in the hospital by a radiologist.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
For a plasma aldosterone and renin measurement, your doctor may ask you to be upright or lying down for a period of time (e.g., 15-30 minutes) prior to the blood draw.
