Reticulocytes

A reticulocyte count 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 count 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 counted manually is compared to the total number of RBCs:

Reticulocyte (%) = number of reticulocytes X 100 / 1000 (RBCs counted)

For healthcare providers who want to have a definitive number of reticulocytes, they can order an Absolute Reticulocyte Count (ARC):

ARC = reticulocytes (%) x RBC count (x 1012/L) / 100

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
• You have 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
• You have been diagnosed and are 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)
• You are undergoing radiation or chemotherapy
• You have received a bone marrow transplant
• Occasionally when you have 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 along 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 you bleed, 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 (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

When you have 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 may be reported.

Corrected reticulocyte count (%) = reticulocyte (%) x [patient HCT(%)/45]
45 is considered the average normal hematocrit (HCT).

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 below.)

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 you do not have anemia, or have 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

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 can not be performed on everyone. It can, however, provide additional information, if necessary.

When you have anemia (the number of red blood cells (RBCs), hemoglobin and hematocrit are low), the percentage of reticulocytes may appear high compared to the overall number of RBCs. This is in part due to the immature cells’ early release from the bone marrow into circulation and the longer time they spend maturing in the blood (from the normal 1 day to 3 or 4 days). In order to get a more accurate assessment of bone marrow function, the reticulocyte percentage (%) is often corrected with a calculation called a corrected reticulocyte count (CRC). This calculation compares your hematocrit with a normal hematocrit value.

The reticulocyte production index (RPI) is a calculation to correct for the degree of reticulocyte immaturity, reflecting how early 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 Production Index = corrected reticulocyte count/maturation time

The maturation time is based on a patient’s hematocrit with correction factors varying from the typical one day to as much as three days.

The immature reticulocyte fraction (IRF) is calculated as a ratio of immature reticulocytes to the total number of reticulocytes. It is reported by an automated blood analyzer when a reticulocyte count test is performed. In certain conditions, IRF is a better indicator of bone marrow response than a total reticulocyte count.

The hemoglobin inside of reticulocytes can be measured and reported as either a mean reticulocyte hemoglobin content (CHr) or a reticulocyte hemoglobin equivalent (Ret-He), depending upon the test method used. This test result would be one of the values reported when blood is evaluated using an automated hematology analyzer.

Reticulocytes are “young” red blood cells that are released by the bone marrow before they become fully mature. The amount of hemoglobin inside of reticulocytes can help determine if there has been enough iron available, to be incorporated into hemoglobin production and then into red blood cell production in the bone marrow, within the past few days. This makes the test useful in identifying functional iron deficiency in certain clinical conditions and in assessing iron deficiency anemia in children.

Yes. If anemia is detected during a routine blood test, the healthcare practitioner may order additional testing (including a reticulocyte count) on the same tube of blood if it can be done on the same day.

Yes. Your healthcare provider will determine how long you should wait after a transfusion before having a reticulocyte count performed.

In some cases, a procedure called a bone marrow aspiration may be performed to obtain a sample of marrow to evaluate under the microscope. Sometimes this is the best way for a healthcare provider to determine how well the bone marrow is functioning.

People who move to higher altitudes may have slightly higher 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.