Renin is a protein (enzyme) released by special kidney cells when you have decreased salt (sodium levels) or low blood volume.

Renin also plays a role in the release of aldosterone, a hormone that helps control the body's salt and water balance.

A test can be done to measure the amount of renin in your blood.

This test is done as part of the diagnosis and treatment of high blood pressure.

If you have essential hypertension, your doctor may order a renin and aldosterone test to see if you sensitive to salt (which causes low renin with normal aldosterone levels). The test results help to guide your doctor in choosing the correct medication. Salt-sensitive patients with high blood pressure associated with low renin levels respond well to diuretic medications.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

Your health care provider may tell you to temporarily stop taking certain drugs that can affect test results.

Drugs that can affect renin measurements include:

• Birth control pills
• Blood pressure medications
• Diuretics
• Vasodilators (drugs that enlarge blood vessels; they are usually used to treat high blood pressure or congestive heart failure)

You should eat 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.