A renin test, also called a renin assay, is a screening test used to check for hypertension, or high blood pressure. It is often performed at the same time as an aldosterone test. The renin test results might vary depending on the time of day, the types of medication that the patient is using and how much salt the patient consumed before the test.
Renin is a protein or enzyme that works with a hormone called aldosterone to regulate the body's levels of potassium and sodium. The kidneys release renin when the patient's sodium and potassium levels are running low. Aldosterone tests can be performed on either blood or urine, but a renin test requires a blood sample.
The physician performs a renin test by extracting blood from the large vein on the back of the patient's hand or from the vein on his or her inner elbow. First, the doctor cleans the area with an antiseptic solution or wipe. Next, the doctor places an elastic band snugly around the top part of the patient's arm, engorging the lower veins with blood.
The doctor then inserts a needle, which is attached to a small tube or vial, into the vein. He or she fills the container, withdraws the needle and places a cotton ball on the site of the puncture to stop any extra bleeding. Some people feel only mild discomfort during the test, but others complain of a stinging or pulsating sensation.
Patients who have low levels of renin might have salt-sensitive hypertension. People whose renin levels are high might have renovascular hypertension, a condition that occurs because of narrowed arteries, or malignant hypertension, which is a disorder in which the blood pressure skyrockets within a short period of time. Several other conditions can also cause high or low renin levels.
The doctor might tell the patient to avoid certain drugs, such as oral birth control pills, diuretics or blood pressure medications, for several days or weeks before a renin test. He or she might also advise the patient to watch his or her sodium levels. The licorice herb contains a chemical that can affect the outcome of the test, so patients should not consume real licorice products before the test, either.
Renin levels can change as a person changes positions. The patient might be required to sit or lie in a certain position for several hours before the doctor performs the renin test. The physician might also take a second test several hours later, after the patient has been moving around for a while. The doctor determines a treatment method based on the test results.