At a glance

Also known as

Retic count; reticulocyte index; corrected reticulocyte

Why get tested?

To help evaluate the bone marrow's ability to produce red blood cells (RBCs) and to help distinguish between anaemia related to blood loss or destruction and anaemia related to decreased RBC production; to help monitor bone marrow response and return of normal marrow function following chemotherapy treatment, bone marrow transplant, or post-treatment follow-up for iron deficiency anaemia

When to get tested?

If a patient has a decreased (or increased) RBC count, haemoglobin, and haematocrit and the doctor wants to evaluate bone marrow function

Sample required?

A blood sample obtained by inserting a needle into a vein in the arm or sometimes from pricking a finger (or the heel of an infant)

What is being tested?

This test measures the number and percentage of reticulocytes in the blood and serves as an indicator of the adequacy of bone marrow red blood cell (RBC) production. Reticulocytes are immature red blood cells. They are produced in the bone marrow when stem cells differentiate to the next stage of RBC development, eventually forming reticulocytes and finally mature RBCs. Most RBCs are fully mature before they are released from the bone marrow into the blood, but about 0.5 - 2% of the RBCs in circulation will be reticulocytes.

The body attempts to maintain a stable number of RBCs in circulation by continually removing old RBCs (approximately 120 days old) and producing new ones in the bone marrow. If this steady state is disrupted by an increased loss of RBCs or by decreased production, then the affected patient will develop anaemia. Increased loss of red blood cells may be due to acute or chronic bleeding (haemorrhage) or haemolysis. The body compensates for this loss by increasing the rate of RBC production. When this happens the number and percentage of reticulocytes in the blood increases until the balance is restored or until the production capacity of the marrow is reached.

Decreased RBC production may occur when the bone marrow is not functioning normally (due to a bone marrow disorder such as aplastic anaemia or due to marrow suppression from a variety of causes including radiation and chemotherapy treatments for cancer), because of insufficient erythropoietin (a hormone produced by the kidneys to stimulate RBC production) or because of deficiencies in certain nutrients such as iron, vitamin B12, or folate. This decreased production leads to decreased numbers of RBCs in circulation, decreased amounts of haemoglobin (an oxygen-carrying protein inside the RBC), a decreased haematocrit (the amount of cells versus plasma in the blood) and a decreasing number of reticulocytes as old RBCs are removed from the bloodstream but not fully replaced.

Occasionally, both the reticulocyte count and the RBC count will be increased because of excess RBC production. This may be due to a variety of causes including inappropriately increased production of erythropoietin, disorders that chronically produce increased numbers of RBCs (polycythaemia vera), and even smoking.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or sometimes from pricking a finger or the heel of an infant.

The Test

How is it used?

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 anaemia. 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 haemoglobin and/or haematocrit are also usually ordered in order to evaluate the severity of the patient's anaemia.

The RBC, haemoglobin, and haematocrit are often ordered routinely as part of a full blood count (FBC). Often included with the FBC 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 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.

Reticulocyte (%) = [number of reticulocytes / number of red blood cells] x 100

Reticulocyte Index = reticulocyte count (%) x [measured haematocrit / normal haematocrit]

When is it requested?

A reticulocyte count may be ordered when a patient has a decreased RBC count and/or a decreased haemoglobin and haematocrit 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, haematocrit, and haemoglobin) 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 haemoglobin and haematocrit the reticulocyte count may be ordered to help determine the degree and rate of overproduction of RBCs.

What does the test result mean?

Looking for reference ranges?

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 haemorrhages (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 anaemic 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 haemoglobin, and percentage of haematocrit will all be increased.

Is there anything else I should know?

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.

Common Questions

Can the reticulocyte count be done at home?

No, it must be performed and interpreted by trained personnel in the doctor’s office or laboratory.

Can the reticulocyte count be done on the same tube of blood as the RBC count or FBC?

Yes. If anaemia is detected during a routine blood test the doctor can frequently order additional testing (including a reticulocyte test) on the same tube of blood if it can be done on the same day.

Will a blood transfusion affect reticulocyte results?

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

How else might my doctor evaluate the cell production in my bone marrow?

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 doctor to determine how well the bone marrow is functioning.