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 (Chapter 26). 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.
Therapeutic trial in folate (or B12) deficiency was discussed.
PYRIDOXINE
Pyridoxine (vitamin B6) is necessary for synthesis of D-aminolevulinic acid, a precursor
of heme. Pyridoxine-deficient patients develop anemia with microcytic-hypochromic RBCs that can simulate chronic iron deficiency. Both hereditary and acquired (secondary) forms exist. The hereditary form is rare and the acquired form is
uncommon, with the most frequently mentioned secondary type being due to tuberculin therapy with isoniazid (INH).
Sideroblastic Anemias*
HEREDITARY (SIDEROACHRESTIC) Acquired
Idiopathic
Alcoholism (most common etiology) Lead poisoning
Drug induced (isoniazid, cycloserine, chloramphenicol) Some patients with various diseases
Thalassemia
Other hemolytic anemias Megaloblastic anemia Rheumatoid-collagen
Myeloproliferative and myelodysplastic disorders Lymphomas/carcinomas/myeloma
Infection Uremia
Hypothyroidism or hyperthyroidism
*Marrow sideroblasts more than 20% of nucleated RBC.
Sideroblastic anemias. Pyridoxine deficiency anemia is included in the sideroblastic
anemias. These conditions by definition have conversion of at least 20% of all bone marrow nucleated RBC to ringed sideroblasts. Ring sideroblasts are normoblasts with abnormal numbers of iron-stainable cytoplasmic granules that appear to form a ring around the nucleus when stained with iron stains. The sideroblastic anemia group includes the rare hereditary form that frequently responds to pyridoxine and a secondary or acquired category that includes various conditions that may be
associated with sideroblastic marrows (see thebox above). Of these, the most likely to have sideroblastic marrows are alcoholism, thalassemia, and some of the
myelodysplastic syndromes. Hematologically, the sideroblastic anemias are characterized by hypochromic RBCs, sometimes predominant and sometimes
coexisting with a minority or majority population of nonhypochromic RBC (“dimorphic” RBC population). If the hypochromic microcytic RBC are predominant, the MCV may be decreased. There typically is an elevated serum iron level with increased saturation of iron-binding capacity.
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CHAPTER 4 PRODUCTION-DEFECT ANEMIA
Clinical Laboratory Medicine