Iron Deficiency Anemia - Diagnosis

Diagnosis

Anemia may be diagnosed from symptoms and signs, but when it is mild, it may not be diagnosed from mild nonspecific symptoms. Pica, an abnormal craving for dirt, ice, or other "odd" foods occurs variably in iron and zinc deficiency, but is neither sensitive or specific to the problem, so is of little diagnostic help.

Anemia is often first shown by routine blood tests, which generally include a complete blood count (CBC) which is performed by an instrument which gives an output as a series of index numbers. A sufficiently low hemoglobin (Hb) by definition makes the diagnosis of anemia, and a low hematocrit value is also characteristic of anemia. Further studies will be undertaken to determine the anemia's cause. If the anemia is due to iron deficiency, one of the first abnormal values to be noted on a CBC, as the body's iron stores begin to be depleted, will be a high red blood cell distribution width, reflecting an increased variability in the size of red blood cells (RBCs). In the course of slowly depleted iron status, an increasing RDW normally appears even before anemia appears.

A low mean corpuscular volume (MCV) often appears next during the course of body iron depletion. It corresponds to a high number of abnormally small red blood cells. A low MCV, a low mean corpuscular hemoglobin and/or mean corpuscular hemoglobin concentration, and the appearance of the RBCs on visual examination of a peripheral blood smear narrows the problem to a microcytic anemia (literally, a "small red blood cell" anemia). The numerical values for these measures are all calculated by modern laboratory equipment.

The blood smear of a patient with iron deficiency shows many hypochromic (pale and relatively colorless) and rather small RBCs, and may also show poikilocytosis (variation in shape) and anisocytosis (variation in size). With more severe iron-deficiency anemia, the peripheral blood smear may show target cells, hypochromic pencil-shaped cells, and occasionally small numbers of nucleated red blood cells. Very commonly, the platelet count is slightly above the high limit of normal in iron deficiency anemia (this is mild thrombocytosis). This effect was classically postulated to be due to high erythropoietin levels in the body as a result of anemia, cross-reacting to activate thrombopoietin receptors in the precursor cells that make platelets; however, this process has not been corroborated. Such slightly increased platelet counts present no danger, but remain valuable as an indicator even if their origin is not yet known.

The diagnosis of iron-deficiency anemia will be suggested by appropriate history (e.g., anemia in a menstruating woman or an athlete engaged in long-distance running), the presence of occult blood (i.e., hidden blood) in the stool, and often by other history. For example, known celiac disease can cause malabsorption of iron. A travel history to areas in which hookworms and whipworms are endemic may be helpful in guiding certain stool tests for parasites or their eggs.

Body-store iron deficiency is diagnosed by diagnostic tests, such as a low serum ferritin, a low serum iron level, an elevated serum transferrin and a high total iron binding capacity. A low serum ferritin is the most sensitive lab test for iron deficiency anemia. However, serum ferritin can be elevated by any type of chronic inflammation and so is not always a reliable test of iron status if it is within normal limits (i.e., this test is meaningful if abnormally low, but less meaningful if normal).

Serum iron levels (i.e., iron not part of the hemoglobin in red cells) may be measured directly in the blood, but these levels increase immediately with iron supplementation (the patient must stop supplements for 24 hours), and pure blood-serum iron concentration in any case is not as sensitive as a combination of total serum iron, along with a measure of the serum iron-binding protein levels (TIBC). The ratio of serum iron to TIBC (called iron saturation or transferrin saturation index or percent) is the most specific indicator of iron deficiency, when it is sufficiently low. The iron saturation (or transferrin saturation) of < 5% almost always indicates iron deficiency, while levels from 5% to 10% make the diagnosis of iron deficiency possible but not definitive. Saturations over 12% (taken alone) make the diagnosis unlikely. Normal saturations are usually slightly lower for women (>12%) than for men (>15%), but this may indicate simply an overall slightly poorer iron status for women in the "normal" population.

Iron-deficiency anemia and thalassemia minor present with many of the same lab results. It is very important not to treat a patient with thalassemia with an iron supplement, as this can lead to hemochromatosis (accumulation of iron in various organs, especially the liver). A hemoglobin electrophoresis provides useful evidence for distinguishing these two conditions, along with iron studies.