Reticulocyte Count
At a Glance
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 the return of normal marrow function following chemotherapy, bone marrow transplant, or post-treatment follow-up for iron deficiency anaemia
When to Get Tested?
When you have a decreased (or increased) RBC count, haemoglobin, and haematocrit and your 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
Test Preparation Needed?
No test preparation is needed
The Test Sample
What is being tested?
Reticulocytes are immature red blood cells (RBCs). They are produced in the bone marrow when stem cells differentiate and progress toward 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. 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.
Normal RBCs have a lifespan of about 120 days. The body attempts to maintain a stable number of RBCs in circulation by continually removing old RBCs 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 anaemia will develop. Increased loss of red blood cells may be due to severe and short term (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 a sufficient number of RBCs is present and 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, or because of deficiencies in certain nutrients such as iron, vitamin B12, or folate. This decreased production leads to fewer RBCs in circulation, decreased haemoglobin and oxygen-carrying capacity, a lower haematocrit, and to a reduction in the 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 (polycythemia vera), and cigarette 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.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
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 RBC 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.
When is it requested?
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.
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 occure 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 does the test result mean?
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:
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:
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.
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 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.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
