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Folic Acid (Vitamins B9) - Deficiency Test

The folic acid test is a quantitative analysis of serum levels of folic acid (also called pteroylglutamic acid, folacin, or folate) by radioisotopic assay of competitive binding. It's often performed concomitantly with measurement of serum vitamin B12 1evels. Like vitamin B12 folic acid is a water-soluble vitamin that influences hematopoiesis, deoxyribonucleic acid synthesis, and overall body growth.

Normally, diet supplies folic acid in liver, kidney, yeast, fruits, leafy vegetables, eggs, and milk. Inadequate dietary intake may cause a deficiency, especially during pregnancy. Because of folic acid's vital role in hematopoiesis, the usual indication for this test is a suspected hematologic abnormality.

Purpose

• To aid differential diagnosis of megaloblastic anemia, which may result from deficiency of folic acid or vitamin B12
• To assess folate stores in pregnancy

Patient preparation

• Explain to the patient that this test determines the folic acid level in the blood.
• Instruct him to observe an overnight fast before the test.
• Tell him that the test requires a blood sample. Explain who will perform the venipuncture and when.
• Reassure him that although he may experience some discomfort from the needle puncture and the pressure of the tourniquet, collecting the sample takes only a few minutes.
• Check the patient's medication history for drugs that may affect test results, such as phenytoin or pyrimethamine.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml royal-blue-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.
• Remove diet restrictions.

Precautions

• Handle the sample gently to prevent hemolysis.
• Protect it from light.
• Send the sample to the laboratory immediately.

Reference values

Normally, serum folic acid values are greater than or equal to 3.5 ug/L.

Abnormal findings

Low serum levels may indicate hematologic abnormalities, such as anemia (especially megaloblastic anemia), leukopenia, and thrombocytopenia. The Schilling test is often performed to rule out vitamin B12 deficiency, which also causes megaloblastic anemia. Decreased folic acid levels can also result from hypermetabolic states (such as hyperthyroidism), inadequate dietary intake, small-bowel malabsorption syndrome, chronic alcoholism, or pregnancy.

Serum levels greater than normal may indicate excessive dietary intake of folic acid or folic acid supplements. Even when taken in large doses, this vitamin is nontoxic.

Interfering factors

• Hemolysis due to rough handling of the sample
• Sample deterioration caused by exposure to light (possible decrease)
• Alcohol, phenytoin, pyrimethamine, anticonvulsants such as primidone, antineoplastics, antimalarials, and oral contraceptives (possible decrease)