Microcytic anemia is among the most common problems seen in the family physician’s office. If you are a young woman or child with anemia, most likely it’s microcytic anemia due to iron deficiency. I often see patients who just assume that anemia means iron deficiency.
When a physician sees a patient with anemia the first thing they look at is the size of the red blood cells. Microcytic denotes that the size of the red blood cells is smaller than normal red blood cells, and anemia is the name given to a reduced red blood cell hemoglobin concentration in the blood. The size of the red blood cells is measured as the mean corpuscular volume (MCV) and is normally between 80-100 microns. When the MCV is less than 80 a patient is said to have microcytosis, and when it’s > 100 they are said to have macrocytosis. Microcytic anemia is when the hemoglobin concentration (Hgb) is <12 in women or <13 in men, and the MCV is <80. For more information blood counts see a prior post Complete Blood Count (CBC).
Iron deficiency is by far the most common cause of microcytic anemia. When a patient is found to have a microcytic anemia on a CBC the evaluation usually begins with an assessment of the patient’s iron status, a reticulocyte (newly produced red blood cell) count to assess the percentage of the red blood cells that are newly produced, and an assessment of why the patient is iron deficient. If iron deficiency is confirmed next we try to find the cause of the iron deficiency. Finding the cause of the blood loss may be simple, as in women with excessive menstrual blood loss or patients where there is a clear history of intestinal bleeding. When it is not obvious, testing for occult blood loss in the stool is needed. Treatment of iron deficiency consists of stopping the blood loss if possible by treatment of the cause, and replacement of the iron supplies. This can usually be done with oral iron, but occasionally patients either do not absorb adequate iron from the GI tract or cannot tolerate the side effects of oral iron. Constipation and abdominal pain are common side effects of oral iron supplementation. In cases where iron deficiency is refractory to oral iron, iron can be infused IV. There are risks of severe allergic reactions, even shock and death with IV iron, and this is done only when absolutely needed. Other fairly common causes of microcytic anemia include:
- Thallasemia minor: a genetic mutation of the hemoglobin molecule that causes very small red blood cells (MCV typically 60-70) and low normal hemoglobin concentrations or mild anemia. This is usually an easy diagnosis when the patient is asymptomatic, has normal iron levels, and a very low MCV.
- Lead poisoning needs to be suspected in children with possible lead paint exposure, adults with occupational or recreational exposure (battery workers, ammunition reloaders). It can be confirmed with serum lead testing.
- Other hereditary conditions like hereditary spherocytosis or hereditary eliptocytosis. These are due to genetic mutations in red blood cell wall proteins that can lead to very small red blood cells that have fragile cell membranes. Testing for red blood cell fragility can confirm the diagnosis.
- Anemia of chronic disease is typically a normocytic anemia, although it is sometimes microcytic. This is when due to a chronic illness, most commonly renal disease, but sometimes general poor health, the bone marrow under produces red blood cells.
Other causes of microcytic anemia are quite uncommon, and if none of the above diagnoses is made, bone marrow aspiration or biopsy and specialty consultation with a hematologist may be needed for evaluation. Overall most patients with microcytic anemia have benign causes and good response to treatment with iron supplementation, but it is key in iron deficiency to figure out the cause of the iron deficiency. If the anemia does not respond to iron therapy to figure out if the iron is not absorbed, the patient is not taking the iron (non-compliance), or if there is a cause other than iron deficiency. Iron deficiency is so common that in a low risk patient, like a teenage girl with heavy menses, it may be appropriate just to give iron supplementation and get a follow up hemoglobin level without further assessment. In adults, especially adults over age 35-40, it’s key to assess for GI blood loss, and if present to evaluate the colon for polyps or cancer and if not found to consider upper GI sources of blood loss like peptic ulcer or gastritis.