The reticulocyte count is ordered to help determine if the bone marrow is responding adequately to the body’s need for RBCs and to help determine the cause of and classify different types of anemia. The number of reticulocytes must be compared to the number of RBCs to calculate a percentage of reticulocytes; thus, the test is ordered along with a RBC count. A hemoglobin and/or hematocrit are also usually ordered in order to evaluate the severity of the patient’s anemia.

The RBC, hemoglobin, and hematocrit are often ordered routinely as part of a complete blood count (CBC). Often included with the CBC is 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 RBCs. Reticulocytes can be distinguished from mature RBCs because they still contain remnant genetic material (RNA) inside them, a characteristic not found in mature RBCs. Circulating reticulocytes generally lose their RNA within one to two days, thus becoming mature RBCs.

In a healthy patient, the reticulocyte percentage is very stable. When the number of RBCs decreases (thus the 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.

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

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

A reticulocyte count may be ordered when a patient has a decreased RBC count and/or a decreased hemoglobin and hematocrit and the doctor wants to evaluate bone marrow function. In a patient with no apparent symptoms, these findings may be found during routine blood work. Testing may also be ordered when a patient has symptoms such as paleness, fatigue, weakness, shortness of breath, and/or blood in the stool.

When a patient has 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 is undergoing erythropoietin treatment, the reticulocyte count (and RBC count, hematocrit, and hemoglobin) may be ordered at intervals recommended by the doctor to monitor marrow function and response to treatment.

When a patient has 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.

What the doctor is looking for is an appropriate response from the bone marrow, to see that the bone marrow is reacting as it should to an increased demand for RBC production.

The reticulocyte count is a reflection of recent bone marrow activity. If the 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 (reticulocytes) – thus the increased reticulocyte count. If a patient hemorrhages (bleeds), then the number of reticulocytes will rise a few days later in an attempt to compensate for the red cell loss. If a patient has 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. If the 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 in an anemic patient, then it is likely that there is some degree of bone marrow dysfunction or failure and/or a deficiency of erythropoietin.

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 to monitor 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.

Patients who move to higher altitudes may have increased reticulocyte counts as their body adapts to the lower oxygen content of their new location. Smokers also may demonstrate increased number of RBCs and reticulocytes.

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.