Reticulocyte Count

The reticulocyte count is used to help determine if the  bone marrow is responding adequately to the body’s need for red blood cells (RBCs) and to help determine the cause of and classify different types of anaemia. The number of reticulocytes must be compared to the number of RBCs to calculate a percentage of reticulocytes and haemoglobin and/or haematocrit are also usually requested to help evaluate the severity of anaemia.

The RBC, haemoglobin, and haematocrit are frequently measured as part of a full blood count (FBC). The FBC usually includes an evaluation of red blood cells (RBCs), White blood cells (WBCs) and Platelets (PLT) characteristics, such as cell size, volume, and shape. Based on these results, a reticulocyte count may be requested 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 which do not contain genetic material. Circulating reticulocytes generally lose their RNA within one to two days, thus becoming mature RBCs.

A reticulocyte count may be requested when you have a decreased RBC count and/or a decreased haemoglobin and haematocrit and your doctor wants to evaluate bone marrow function. If you have no apparent symptoms, these findings may be found during routine blood testing. Reticulocyte count may also be used when you have symptoms such as paleness, tiredness, weakness, shortness of breath, and/or blood in the stool.

A Reticulocyte count is useful after a recent episode of blood loss or in cases where the lifespan of red cell is shortened as seen in some haemolytic anaemia.

Reticulocyte count may also be used when you have a known iron or vitamin  vitamin B12 or folate deficiency, known kidney disease, known bone marrow suppression as may occur during chemotherapy or bone marrow transplant. Reticulocyte count may be requested with a RBC count, haematocrit, and haemoglobin at intervals recommended by your doctor to monitor marrow function and response to treatment.

When you have an increased number of RBCs and elevated haemoglobin and haematocrit, the reticulocyte count may be used to help work out the degree and rate of overproduction of RBCs.

What your doctor is looking for is an appropriate response from the bone marrow, to confirm that your bone marrow is working properly to an increased demand for red blood cells. In a healthy patient, the reticulocyte percentage is stable. When the number of RBCs and haematocrit 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 haematocrit with a normal haematocrit 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 haematocrit.

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

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

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 ((haemorrhage), then the number of reticulocytes will rise a few days later in an attempt to compensate for the red cell loss. If you have long-term (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.
• Haemolytic anaemia
• Haemolytic 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 anaemic, then it is likely that there is some degree of bone marrow disease or failure and/or a deficiency of erythropoietin. Decreased reticulocyte percentages may be seen, for example, with:

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

The reticulocyte count gives an indication of what may be happening but cannot diagnose of any one particular disease. Reticulocyte count can show whether further investigations may be necessary and can help 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 haemoglobin, and percentage of haematocrit 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 an 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.

Yes. If anaemia is detected during a routine blood test, the doctor may request additional testing (including a reticulocyte count) on the same tube of blood, but it must be done on the same day, before the reticulocytes mature.