A renin assay blood test is done to find the cause of high blood pressure (hypertension). Renin is an enzyme made by special cells in the kidneys. Renin works with aldosterone (a hormone made by the adrenal glands) and several other substances to help balance sodium and potassium levels in the blood and fluid levels in the body, which affects your blood pressure.

A renin test is often done at the same time as an aldosterone test. In some people, it may be normal to have high blood levels of both renin and aldosterone. If renin levels are low and aldosterone levels are high, a tumor may be present in the adrenal glands.

Why It Is Done

A renin test is done to find the cause of high blood pressure (hypertension), especially when potassium levels in the blood are low.

How To Prepare

For 2 to 4 weeks before the test, you will be asked to stop taking medicines that can affect the test, such as diuretics, estrogens, and high blood pressure medicines (especially beta-blockers and ACE inhibitors). Your health professional may have you take other medicines for a few weeks that will not change the renin test results.

Do not eat natural black licorice for 2 weeks before the test. Do not eat or drink foods that contain caffeine the day before the test. Natural licorice and caffeine can change the test results.

For 3 days before a renin test, you may be asked to follow a special low-sodium diet.

You will be asked to not eat or drink anything for 8 hours before the test.

How It Is Done

You may need to sit or lie down to relax for 1 to 2 hours before your blood is collected. A second blood sample may be collected after you move around for 2 hours.

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.