Table 2.6 Ferritin, Iron, and Iron Saturation Changes in Anemias N, no change; D, decrease; I, increase. Anemia Ferritin Iron Iron Saturation Hemorrhage, acute N D D Hemorrhage, chronic D D D Iron-deficiency D D D Aplastic D I I Megaloblastic I D D Hemolytic I I I Sideroblastic I I I Thalassemia major I I I Thalassemia minor I N/I N/I Bone marrow neoplasia N/I I I Uremia, nephrosis, or nephrotic syndrome N/I D/I D Liver disease N/I N/I N/I Chronic diseases I D D
Iron is contained in several components
Transferrin (also called siderophilin),
a transport protein largely synthesized
by the liver, regulates iron absorption.
Total iron-binding capacity (TIBC)
correlates with serum transferrin,
but the relation is not linear.
A serum iron test without a TIBC and transferrin
determination has very limited value
The combined results of transferrin, iron, and TIBC
tests are helpful in the differential diagnosis
of anemia, in assessment of iron-deficiency
anemia, and in the evaluation of thalassemia,
sideroblastic anemia, and hemochromatosis
Ferritin, a complex of ferric (Fe 2+ ) hydroxide and a protein, apoferritin, originates in the reticuloendothelial system. Ferritin reflects the body iron stores and is the most reliable indicator of total-body iron status. A bone marrow examination is the only better test. Bone marrow aspiration may be necessary in some cases, such as low-normal ferritin and low serum iron in the anemia of chronic disease. The ferritin test is more specific and more sensitive than iron concentration or TIBC for diagnosing iron deficiency.
Vitamin B 12 (VB 12 ) Vitamin B 12 (VB 12 ), also known as the antipernicious anemia factor, is necessary for the production of RBCs. It is obtained only from ingestion of animal protein and requires an intrinsic factor for absorption. Both VB 12 and folic acid depend on a normally functioning intestinal mucosa for their absorption and are important for the production of red blood cells. Levels of VB 12 and folate are usually tested in conjunction with one another because the diagnosis of macrocytic anemia requires measurement of both. This determination is used in the differential diagnosis of anemia and conditions marked by high turnover of myeloid cells, as in the leukemias. When binding capacity is measured, it is the unsaturated fraction that is determined. The measurement of unsaturated VB 12 –binding capacity (UBBC) is valuable in distinguishing between untreated polycythemia vera and other conditions in which there is an elevated Hct .
Folic acid is needed for normal RBC and WBC function and for the production of cellular genes. Folic acid is a more potent growth promoter than VB 12 , although both depend on the normal functioning of intestinal mucosa for their absorption. Folic acid, like VB 12 , 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 hypersegmental 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 VB 12 deficiency that is blocking the ability of cells to take up folate. Serum levels are commonly high in patients with VB 12 deficiency because this vitamin is needed to allow incorporation of folate into tissue cells. For thoroughness, the serum VB 12 should also be determined because more than 50% of all patients with significant megaloblastic anemia have VB 12 deficiency rather than folate deficiency.