Folic acid (folate) is necessary for adequate synthesis of certain purines and pyrimidines, which, in turn, are precursors of cell DNA. Folate is also necessary for methionine synthesis, histadine catabolism, and metabolism of serine and glycine. Vitamin B12 converts inactive 5-methyltetrahydrofolate to tetrahydrofolate, which is able to transfer one-carbon groups.

Folic acid deficiency causes a megaloblastic anemia that may be indistinguishable from pernicious anemia in every laboratory test except the Schilling test without IF. It may also be indistinguishable clinically, except that neurologic symptoms do not occur from folic acid deficiency. Folic acid therapy improves most hematologic abnormalities of PA, even though the PA defect is a deficiency of vitamin B12, not folic acid, but folic acid therapy alone can worsen PA neurologic damage. Therefore, it is necessary to differentiate B12 from folic acid problems.

Causes of folic acid deficiency. The most frequent cause of folic acid deficiency is dietary deficiency. This is especially common in chronic alcoholics. However, some investigators report that alcohol can inhibit folate absorption and interfere with folate metabolism. Another important cause is malabsorption, especially that category due to primary small bowel disease. Ten percent to 25% of pregnant women are reported to have some degree of folic acid deficiency, although by far the most common cause of deficiency anemia in pregnancy is iron deficiency. Folic acid deficiency in pregnancy may be due to dietary defect plus fetal demands; sometimes no good explanation is available. Uncommonly (<5%) a more severe folate deficiency state can occur in the last half of the third trimester. Some reports suggest that oral contraceptive pills can be associated with folic acid and vitamin B6 deficiency, but this is disputed by others. Drug-induced folate deficiency includes several drug categories. Certain cytotoxic medications such as methotrexate exert an antitumor effect by interfering with folate metabolism. Anticonvulsant drugs, especially phenytoin (about 30% of cases, range 14%-50%) and primidone (Mysoline), frequently show macrocytosis without anemia, but in a few patients induce a macrocytic megaloblastic anemia that responds best to folic acid. Phenytoin is associated with some degree of folate deficiency in about 40% of patients (27%-76%). It should be noted that megaloblastic anemia due to diet, pregnancy, or anticonvulsant drugs shows normal Schilling test results. Sulfasalazine (Azulfidine), used in therapy of ulcerative colitis, is also sometimes associated with macrocytosis due to folic acid deficiency. Colchicine, para-aminosalicylic acid (PAS), and neomycin interfere with folate absorption in some patients.

Serum folate assay. Folic acid deficiency can be proved by serum folic acid measurement. If the test is done by the original microbiologic assay system, any antibiotic therapy must cease for a full week before the serum is drawn. Immunoassay (EIA or RIA) is less complicated than bacterial methods and is not affected by antibiotics; therefore, RIA has made folate measurement more practical. Unfortunately, because serum folate measurement is not ordered frequently, smaller laboratories will probably not do the test for economic reasons. Serum folate levels fall below normal limits 3-4 weeks after dietary or absorption-induced deficiency begins. Tissue folate levels (measured by RBC folate assay) become abnormal about 3 months later than serum folate and also return to normal after therapy somewhat later than serum folate. Anemia may not develop until 5 months after onset of deficiency in folate. In some patients with folate deficiency from deficient diet, a few meals with adequate folic acid may elevate serum folate values into the folate reference range, but RBC folate levels may still be low. My personal experience, as well as that of some others, indicates that RBC folate levels are more frequently low than serum folate levels in patients with suspected folate deficiency. Another problem with serum folate is a decreased level sometimes found in patients with severe liver or kidney disease. However, the RBC folate method also has its difficulties. Some manufacturers have kits that permit B12 and folate assay to be performed simultaneously on the same serum specimen (thus saving time and money), whereas RBC folate assay requires a different specimen and different processing than does serum B12 assay. Another problem is that B12 deficiency interferes with incorporation of folate into RBC. Therefore, B12 deficiency without folate deficiency can produce low RBC folate levels even though the serum folate level is normal; the combination of low serum B12 and low RBC folate levels might be misinterpreted as the combined B12 and folate deficiency seen in malabsorption.