A reticulocyte test is used to determine the number and/or percentage of reticulocytes in the blood to help evaluate conditions that affect red blood cells (RBCs), such as anemia or bone marrow disorders. Reticulocytes are newly produced, relatively immature red blood cells. They form and mature in the bone marrow before being released into the blood.

The reticulocyte test may be used:

• As a follow up to abnormal results on a complete blood count (CBC), RBC count, hemoglobin or hematocrit, to help determine the cause
• To determine if the bone marrow is functioning properly and responding adequately to the body's need for red blood cells
• To help detect and distinguish between different types of anemia
• To monitor response to treatment, such as that for iron-deficiency anemia
• To monitor bone marrow function following treatments such as chemotherapy
• To monitor function following a bone marrow transplant

Most often, a reticulocyte count is performed with an automated instrument (hematology analyzer) and can be done simultaneously with a CBC, which includes an RBC count, hemoglobin and hematocrit. Either an absolute number of reticulocytes and/or a percentage of reticulocytes can be reported. For a percentage, the number of reticulocytes is compared to the total number of RBCs:

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

Several other tests may be used in conjunction with a reticulocyte count to further evaluate someone for a condition affecting RBC production. Some examples include:

• Iron studies
• Vitamin B12 and Folate
• Haptoglobin
• G6PD
• Erythropoietin

Sometimes a bone marrow aspiration and biopsy may be done in follow up to abnormal results on initial tests. This procedure is invasive and is not done on everyone. It can, however, provide additional information, if necessary.

A reticulocyte count may be ordered when:

• CBC results show a decreased RBC count and/or a decreased hemoglobin and hematocrit
• A healthcare practitioner wants to evaluate bone marrow function
• An individual has signs and symptoms of anemia or chronic bleeding, such as paleness, lack of energy, fatigue, weakness, shortness of breath, and/or blood in the stool
• A person has been diagnosed and is being treated for a condition known to affect RBC production, such as iron deficiency anemia, vitamin B12 or folate deficiency, or kidney disease (which can affect the production of erythropoietin, a hormone produced by the kidneys that stimulates RBC production by the bone marrow)
• An individual is undergoing radiation or chemotherapy
• A person has received a bone marrow transplant
• Occasionally when a person has an increased number of RBCs and elevated hemoglobin and hematocrit, to help determine the degree and rate of overproduction of RBCs

Results must be interpreted carefully and in conjunction with results of other tests, such as a red blood cell (RBC) count, hemoglobin (Hb), hematocrit (Hct), or full CBC. In general, the reticulocyte count (absolute number or percentage) is a reflection of recent bone marrow activity. Results may indicate whether a disease or condition is present that is generating an increased demand for new RBCs and whether the bone marrow is able to respond to the extra requirement. Occasionally, results may indicate overproduction of RBCs.

When anemia is present (i.e., low RBCs, low hemoglobin, low hematocrit) and the bone marrow is responding appropriately to the demand for increased numbers of RBCs, then the bone marrow will produce more and allow for the early release of more immature RBCs, increasing the number of reticulocytes in the blood.

A high reticulocyte count with low RBCs, low hemoglobin, and low hematocrit (anemia) may indicate conditions such as:

• Bleeding: If an individual bleeds (hemorrhage), then the number of reticulocytes will rise a few days later in an attempt to compensate for the red cell loss. If someone 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 (although it may not be high if the blood loss leads to iron deficiency).
• Hemolytic anemia: In this condition, anemia is caused by increased destruction of RBCs. The bone marrow increases RBC production to compensate, resulting in a high reticulocyte count.
• Hemolytic disease of the newborn: This condition causes increased RBC destruction, similar to hemolytic anemia described above.

A low reticulocyte count with low RBCs, low hemoglobin, and low hematocrit (anemia) 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
• Severe kidney disease; this may cause a low level of erythropoietin.
• Alcoholism
• Endocrine disease

When an individual has anemia, the percent of reticulocytes present in the blood may appear high compared to the overall number of RBCs. In order to get a more accurate assessment of bone marrow function, a calculation called a corrected reticulocyte count or reticulocyte index (RI) may be reported. The RI is calculated by comparing the person's hematocrit with a normal hematocrit value. Other calculations that may also be reported include the reticulocyte production index (RPI) and an immature reticulocyte fraction (IRF). The IRF was previously referred to as the reticulocyte maturity index; however, IRF is now the most common term used to quantify the younger fraction of reticulocytes. (For more on these, see the Common Questions section.)

The reticulocyte test gives an indication of the presence of a disease or condition 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.

When an individual does not have anemia, or has a high RBC count (polycythemia), a high reticulocyte count may indicate an overproduction of RBCs. Some conditions that may cause this include:

• Polycythemia vera
• Tumor that produces excess erythropoietin

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

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

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. Although it is used occasionally, the manual method has been replaced by automated methods that allow for a greater number of cells to be counted, thus enhancing the accuracy of reticulocyte counts. The automated method is considered to be more accurate in identifying reticulocytes.