This website describes the renin test http://my.webmd.com/hw/hypertension/hw203228.asp
I'm editing the part about drawing blood.
Renin is an enzyme produced by specific cells in the kidneys. Renin interacts with aldosterone (a hormone produced by the adrenal glands) and several other substances to help regulate sodium and potassium in the blood, fluid levels in the body, and blood pressure.
A renin test, or plasma renin activity (PRA) test, is done to determine the cause of high blood pressure (hypertension). It is usually done at the same time as an aldosterone test (see the medical test Aldosterone). In some situations, it may be normal to have high blood levels of both renin and aldosterone. However, if renin levels are low and aldosterone levels are high, an aldosterone-producing tumor of the adrenal glands may be present.
This test is usually done on a blood sample taken from a vein.
Why It Is Done
A plasma renin activity test is done to determine the cause of high blood pressure (hypertension), especially when potassium levels in the blood are low.
How To Prepare
For about 2 to 4 weeks before the test, you will have to stop taking medications that can interfere with the test, such as diuretics, birth control pills, and high blood pressure medications (especially beta-blockers and ACE inhibitors ). Talk to your doctor about which medications you need to stop taking. Your doctor may substitute your medications with others that will not interfere with the renin blood test results.
Do not eat natural black licorice for at least 2 weeks before the test, and avoid eating or drinking foods containing caffeine the day before the test. Natural licorice and caffeine can interfere with test results.
Recent diagnostic imaging procedures that required the use of a radioactive tracer (such as a thyroid scan or bone scan) can interfere with renin test results. Inform your doctor if you have undergone any test within 3 days that used a radioactive tracer.
For 3 days before a renin test, you may be required to follow a special diet that contains known amounts of sodium.
How It Is Done
You may need to sit or lie down for 1 to 2 hours before your blood is collected. In addition, a second blood sample may be collected after you stand for 2 hours.
The results depend upon the time of day and your position (standing, sitting, or lying down) before the blood sample is collected, your age, and the level of sodium in your blood.
Normal values may vary from lab to lab.
Plasma renin activity Adult (upright position, normal sodium diet):
1–6 nanograms per milliliter per hour (ng/mL/hr)
Greater-than-normal values may mean
A high renin value can indicate kidney disease, blockage of an artery leading to a kidney, Addison's disease, cirrhosis, excessive bleeding (hemorrhage), or malignant high blood pressure.
Lower-than-normal values may mean
A low renin value can indicate Conn's syndrome.
What Affects the Test
A diet high in natural black licorice can lower renin values.
Many medications used to treat high blood pressure can raise or lower renin levels.
Your position (standing, sitting, or lying) before the test is done or the time of day when the blood sample is drawn, as well as recent salt intake, can affect renin levels.
Very high doses of corticosteroids can lower renin values.
Pregnancy causes renin levels to increase.
Rough handling, contamination, or inadequate refrigeration of the blood sample can cause inaccurate test results.
What To Think About
Many factors can affect renin test results. Therefore, only a doctor who has experience with renin tests should interpret the results.
A renin stimulation test may occasionally be done if blood renin levels are low. To prepare for this test, you will follow a low-sodium diet for 3 days. A blood renin level will then be drawn and a diuretic, usually furosemide (Lasix), will be injected through an IV. Normally, blood renin levels will increase, whereas in Conn's syndrome blood renin levels will not change.
A plasma renin activity (PRA) test measures renin indirectly by measuring an enzyme called angiotensin I rather than renin itself. Many hospitals now measure renin activity directly through a test called the renin direct immunoassay, which measures the amount of renin in the blood. A renin direct immunoassay is easier to do and can generally be substituted for the PRA test. However, in some situations (such as pregnancy), PRA results may be more accurate than the renin direct immunoassay.