The renin test measures the amount of renin in the blood.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site if there is any bleeding.

The health care provider may advise you to withhold drugs that can affect the test.

Consume a normal, balanced diet with low-sodium content (about 3 gm/day) for 3 days before the test.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Plasma renin activity (PRA) is measured as part of the diagnosis and treatment of hypertension  (high blood pressure).

Patients with primary hyperaldosteronism will have increased aldosterone (for more information see the aldosterone test) production associated with a decreased PRA. Patients with secondary hyperaldosteronism (that is, caused by kidney disease or renal vascular disease) will have increased plasma levels of renin and aldosterone.

Patients with essential hypertension may have renin and aldosterone levels checked to evaluate if they are salt-sensitive, which causes a low renin with normal aldosterone levels, and this helps to guide the physician in choosing the correct medication for these patients. Salt-sensitive patients with low renin hypertension respond well to diuretic medications.

Renin is an enzyme released by specialized cells of the kidney into the blood. It is in response to sodium depletion or low blood volume. Renin converts angiotensinogen (a protein released into the blood by the liver) to angiotensin I.

Angiotensin I is converted to angiotensin II by an enzyme in the veins of the lungs. Angiotensin II acts on the adrenal cortex to stimulate the release of aldosterone. Aldosterone acts on the distal tubules of the kidneys to decrease the loss of sodium ions and secondarily fluid. This has the effect of increasing blood pressure. In addition, angiotensin causes constriction of small blood vessels, which also increases blood pressure.