Reticulocyte Count

The RBC, hemoglobin, and hematocrit are frequently ordered as part of a complete blood count (CBC). The CBC usually includes an evaluation of RBC characteristics, such as cell size, volume, and shape. Based on these results, a reticulocyte count may be ordered to further examine the RBCs. Reticulocytes can be distinguished from mature RBCs because they still contain remnant genetic material (RNA), a characteristic not found in mature RBCs. Circulating reticulocytes generally lose their RNA within one to two days, thus becoming mature RBCs.

When you have a known iron or vitamin B12 or folate deficiency, known kidney disease, known bone marrow suppression – such as that due to chemotherapy or bone marrow transplant - and/or are receiving erythropoietin, the reticulocyte count may be ordered along with a RBC count, hematocrit, and hemoglobin at intervals recommended by your doctor to monitor marrow function and response to treatment.

When you have an increased number of RBCs and elevated hemoglobin and hematocrit, the reticulocyte count may be ordered to help determine the degree and rate of overproduction of RBCs.

In a healthy patient, the reticulocyte percentage is essentially stable. When the number of RBCs and hematocrit decreases, the percentage of reticulocytes may appear increased compared to the overall number of RBCs. In order to get a more accurate assessment of bone marrow function, the calculated reticulocyte percentage (%) is often corrected with a calculation called a corrected reticulocyte count or a reticulocyte index (RI). This calculation compares the patient’s hematocrit with a normal hematocrit value. An additional calculation called the reticulocyte production index (RPI) is sometimes calculated to correct for the degree of reticulocyte immaturity – reflecting how quickly the reticulocytes were released from the bone marrow and how long it will take them to mature in the bloodstream. The RPI and maturation time vary with the hematocrit.

Reticulocyte (%) = [Number of Reticulocytes / Number of Red Blood Cells] X 100

Reticulocyte Index = Reticulocyte count (%) X [Measured hematocrit / Normal hematocrit]

Reticulocyte Production Index = (Reticulocyte Index) X (1/maturation time)

Some automated reticulocyte counts may have an immature reticulocyte fraction (IRF) and a mean reticulocyte volume (MRV) reported. They are primarily research parameters at this time. The reticulocyte count is a reflection of recent bone marrow activity. If your bone marrow is responding appropriately to the demand for increased numbers of RBCs, then the bone marrow will allow for the early release of more immature RBCs, increasing the number of reticulocytes in the blood.

An increased reticulocyte percentage may indicate conditions such as:

• Bleeding: If you bleed (hemorrhage), then the number of reticulocytes will rise a few days later in an attempt to compensate for the red cell loss. If you have chronic blood loss, then the number of reticulocytes will stay at an increased level as the marrow tries to keep up with the demand for new RBCs.
• Hemolytic anemia
• Hemolytic disease of the newborn

If your marrow is unable to keep up or is not functioning normally, then the number of reticulocytes may be normal or only slightly elevated despite demand but will eventually decrease due to lack of adequate production. If the number of reticulocytes is not elevated when you are anemic, then it is likely that there is some degree of bone marrow dysfunction or failure and/or a deficiency of erythropoietin.

Decreased reticulocyte percentages may be seen, for example, with:

• Iron deficiency anemia
• Pernicious anemia or folic acid deficiency
• Aplastic anemia
• Radiation therapy
• Bone marrow failure caused by infection or cancer

The reticulocyte count gives an indication of what may be happening but is not directly diagnostic of any one particular disease. It is a sign that further investigation may be necessary and a tool that can be used to monitor the effectiveness of therapy.

If reticulocyte numbers rise following chemotherapy, a bone marrow transplant, or treatment of an iron or vitamin B12 or folate deficiency, then bone marrow RBC production is beginning to recover. In conditions causing RBC overproduction, the number of reticulocytes and RBCs, the concentration of hemoglobin, and percentage of hematocrit will all be increased.

Reticulocyte counts may be increased during pregnancy. Newborns have a higher percentage of reticulocytes, but the number drops to near adult levels within a few weeks.

Traditionally, reticulocyte counts have been done manually by looking at a specially stained slide under the microscope and counting the number of reticulocytes in a number of fields of view. This method is still in use, but it is in the process of being replaced by automated methods that allow for a greater number of cells to be counted, thus enhancing the accuracy of reticulocyte counts.