Folic acid (pteroylmonoglutamate) is needed for normal RBC and WBC function and for the production of cellular genes. Folic acid is a more potent growth promoter than VB₁₂, although both depend on the normal functioning of intestinal mucosa for their absorption. Folic acid, like VB₁₂, is required for DNA production. Folic acid is formed by bacteria in the intestines; is stored in the liver; and is present in eggs, milk, leafy vegetables, yeast, liver, fruits, and other elements of a well-balanced diet.

This test is indicated for the differential diagnosis of megaloblastic anemia and in the investigation of folic acid deficiency, iron deficiency, and hypersegmented granulocytes. Measurement of both serum and RBC folate levels constitutes a reliable means of determining the existence of folate deficiency. The finding of low serum folate means that the patient’s recent diet was subnormal in folate content, that the patient’s recent absorption of folate was subnormal, or both. Low RBC folate can mean either that there is tissue folate depletion owing to folate deficiency requiring folate therapy or, alternatively, that the patient has primary VB₁₂ deficiency that is blocking the ability of cells to take up folate. Serum levels are commonly high in patients with VB₁₂ deficiency because this vitamin is needed to allow incorporation of folate into tissue cells. For thoroughness, the serum VB₁₂ should also be determined because >50% of all patients with significant megaloblastic anemia have VB₁₂ deficiency rather than folate deficiency.

Serum:

• Adults: 2.5–20 ng/mL or 4.5–45.3 nmol/L

• Children: 5–21 ng/mL or 11.3–47.6 nmol/L

• Infants: 14–51 ng/mL or 31.7–115.5 nmol/L

RBC folate:

• Adults: 140–628 ng/mL or 317–1422 nmol/L

• Children: >160 ng/mL or >362 nmol/L

1. If a serum sample is ordered, obtain a fasting venous sample of 3 mL using a red-topped tube or SST. Protect the sample from light. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered.

2. If RBC folate is ordered, draw up to two 5-mL tubes of venous blood using a lavender-topped tube with EDTA anticoagulant. An Hct determination is also required. Patient should not have had a folic acid supplement (e.g., in a daily vitamin) for at least 3–5 days.

1. Decreased folic acid levels are associated with:

   1. Inadequate intake owing to alcoholism, chronic disease, malnutrition, diet devoid of fresh vegetables, or anorexia

   2. Malabsorption of folic acid (e.g., small bowel disease)

   3. Excessive use of folic acid by the body (e.g., pregnancy, hypothyroidism)

   4. Megaloblastic (macrocytic) anemia caused by VB₁₂ deficiency

   5. Hemolytic anemia (sickle cell, phenocytosis, PNH)

   6. Liver disease associated with cirrhosis, alcoholism, hepatoma

   7. Adult celiac disease, sprue

   8. Vitamin B₆ deficiency

   9. Carcinomas (mainly metastatic), acute leukemia, myelofibrosis

   10. Crohn disease, ulcerative colitis

   11. Infantile hyperthyroidism

   12. Intestinal resection, jejunal bypass procedure

   13. Drugs that are folic antagonists (interfere with nucleic acid synthesis)

       1. Anticonvulsants (phenytoin)

       2. Aminopterin and methotrexate

       3. Antimalarials

       4. Alcohol (ethanol)

       5. Oral contraceptives

       6. Heavy usage of antacids

2. Increased folic acid levels are associated with:

   1. Blind loop syndrome

   2. Vegetarian diet

   3. Pernicious anemia, VB₁₂ deficiency

3. Decreased RBC folate occurs with:

   1. Untreated folate deficiency

   2. VB₁₂ deficiency (60% of uncomplicated cases)

1. Drugs that are folic acid antagonists, among others (see Appendix E)

2. Hemolyzed specimens (false elevation)

3. Iron-deficiency anemia (false increase)