The reticulocyte count is a test for determining bone marrow function and evaluating erythropoietic activity. This test is also useful in classifying anemias. A reticulocyte is an immature red blood cell (RBC) that can be readily identified under a microscope by staining the peripheral blood smear with Wright or Giemsa stain. It is an RBC that still has some microsomal and ribosomal material left in the cytoplasm. It sometimes takes a few days for that material to be cleared from the cell. Normally there are a small number of reticulocytes in the bloodstream.
The reticulocyte count gives an indication of RBC production by the bone marrow. Increased reticulocyte counts indicate the marrow is releasing an increased number of RBCs into the bloodstream, usually in response to anemia. A normal or low reticulocyte count in a patient with anemia indicates that the marrow response to the anemia by way of production of RBCs is inadequate and perhaps is contributing to or is the cause of the anemia (as in aplastic anemia, iron deficiency, vitamin B12 deficiency, depletion of iron stores). An elevated reticulocyte count found in patients with a normal hemogram indicates increased RBC production compensating for an ongoing loss of RBCs (hemolysis or hemorrhage).
Because the reticulocyte count is a percentage of the total number of RBCs, a normal to low number of reticulocytes can appear high in the anemic patient, because the total number of mature RBCs is low. To determine if a reticulocyte count indicates an appropriate erythropoietic (RBC marrow) response in patients with anemia and a decreased hematocrit, the reticulocyte index is calculated by multiplying the patient’s Reticulocyte Count by his or her Hematocrit Level and dividing the result by Normal Hematocrit Level.
The reticulocyte index in a patient with a good marrow response to the anemia should be 1.0. If it is below 1.0, even though the reticulocyte count is elevated, the bone marrow response is inadequate in its ability to compensate (as seen in iron deficiency, vitamin B12 deficiency, marrow failure). In these clinical situations, if iron or vitamin B12 is administered, the reticulocyte count will rise significantly to the point that the index equals or exceeds 1.0.
Causes of False Reticulocyte Count
- Pregnancy may cause an increased reticulocyte count.
- Howell-Jolly bodies are blue stippling material in the RBC that occurs in severe anemia or hemolytic anemia. The RBCs containing these Howell-Jolly bodies look like reticulocytes and can be miscounted by some automated counter machines as reticulocytes; this gives a falsely high number of reticulocytes.
Causes of High Reticulocyte Count
- Hemorrhage (3 to 4 days later): In response to significant blood loss, the marrow attempts to compensate by producing large numbers of RBCs, some of which are immature RBCs called reticulocytes.
- Hemolytic disease of the newborn: Immune-mediated destruction of RBCs reduces RBC survival. The marrow attempts to compensate for the shortened RBC survival by producing large numbers of RBCs, some of which are immature RBCs called reticulocytes.
- Treatment for iron, vitamin B12, or folate deficiency: After replacement treatment for anemia caused by nutritional deficiency, the marrow responds by increasing production of RBCs, some of which are immature RBCs called reticulocytes.
Hemolytic Anemia causes the Red Blood Cell destroyed at a faster rate than normal which decrease the RBCs survival rate. The marrow attempts to compensate for the shortened RBC survival by producing large numbers of RBCs, some of which are immature RBCs called reticulocytes. Examples of Hemolytic Anemia include:
- Immune Hemolytic Anemia.
- Trauma from a Prosthetic Heart Valve.
Causes of Low Reticulocyte Count
Nutritional deficiencies suppress marrow production of RBCs and reticulocytes as well. Low Reticulocyte associated conditions that are caused by nutritional deficiencies include Iron-deficiency Anemia, Pernicious Anemia, and Folic Acid Deficiency Anemia.
Chronic Diseases cause the bone marrow production of Red Blood Cells and reticulocytes to be reduced.
Bone Marrow also fails to produce Red Blood Cells and reticulocytes because of :
- Aplastic Anemia.
- Radiation Therapy.
- Bone Marrow Failure.
- Adrenocortical Hypofunction.
- Anterior Pituitary Hypofunction.