The folic acid test measures the amount of folate 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 blood flow. 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, 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.

Fast (no eating or drinking) for 6 hours before the test. The health care provider should be notified if you are using any drugs that can affect test results, including folic acid supplements.

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

This test is used to evaluate for folic acid deficiency. It can be performed on either whole blood or red blood cells, but measuring folate in red blood cells is more accurate.

Folic acid -- one of the B vitamins -- is necessary for making the building blocks of DNA. A deficiency in making DNA can most easily be seen in quickly-dividing cells, such as immature red blood cells. Red blood cells that are formed when there is not enough folic acid have shortener life spans, are larger than normal, and have reduced ability to carry oxygen.

The normal range is 2.7 - 17.0 ng/mL (nanograms per milliliter)

Lower-than-normal folate levels may indicate:

• Inadequate diet
• Malabsorption syndrome (for example, sprue)
• Malnutrition

• Anemia due to folate deficiency
• Megaloblastic anemia

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Pregnancy increases the need for folic acid. Not getting enough folic acid during pregnancy can cause abnormalities in the fetus' nervous system formation.

Drugs that can decrease folate measurements include alcohol, aminosalicylic acid, birth control pills, estrogens, tetracyclines, ampicillin, chloramphenicol, erythromycin, methotrexate, penicillin, aminopterin, phenobarbital, phenytoin, and antimalarials.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

McPherson RA and Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: WB Saunders; 2007:381.

Hoffman R, Benz Jr. EJ, Shattil SJ, et al., eds. Hematology: Basic Principles and Practice. 4th ed. Philladelphia, Pa: Churchill Livingston; 2005:529-32.