Reticulocytes

What is being analysed?
This test measures the number and percentage of reticulocytes in the blood and is used as an indicator of how efficiently the bone marrow is producing red blood cells. Reticulocytes are immature red blood cells. They are produced by the bone marrow when stem cells differentiate into the next stage of red cell development, forming reticulocytes and finally red blood cells. Most red blood cells are fully mature when released into the blood by the bone marrow, but about 0.5-2% of circulating red cells are reticulocytes. The body tries to keep a stable number of red blood cells in circulation by systematically removing old red cells (average lifespan about 120 days) and producing new ones in the bone marrow. If this balance is upset by excessive loss or reduced production of red cells, the patient will develop anaemia. Excessive loss of red cells may be due to acute or chronic bleeding or haemolysis (destruction of red cells). The body compensates for this loss by increasing the rate of red cell production. When this happens, the number and percentage of reticulocytes increase until balance is restored or until the bone marrow reaches its production capacity.

Reduced red cell production can occur when the bone marrow is not working normally (because of bone marrow disorders such as aplastic anaemia, or bone marrow suppression from various causes, including radiotherapy or chemotherapy for cancer), because of insufficient erythropoietin (a hormone produced by the kidneys that stimulates red cell production), or because of deficiencies of certain nutrients such as iron, vitamin B12 or folate. This reduced production leads to fewer red cells in circulation, a lower haemoglobin concentration (haemoglobin is the protein inside red cells that carries oxygen), a lower haematocrit (the percentage of cells relative to plasma in the blood) and a lower number of reticulocytes, which do not fully replace the old red cells removed from the circulation.

Sometimes both the red cell count and the reticulocyte count may be increased because of overproduction; this can be due to a variety of causes, including inappropriate erythropoietin production, disorders that cause a chronic increase in the number of red cells (polycythaemia vera) and sometimes smoking.

What samples are required?
The test is carried out on a blood sample taken with a needle from a vein in the arm, or by fingerprick (children and adults) or heel prick (newborns).

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
The reticulocyte count is ordered to determine whether the bone marrow is responding adequately to the body’s need for red cells and to help determine the cause and classify an anaemia. The number of reticulocytes can be compared with the number of red cells to calculate a percentage; the test is therefore ordered together with the red cell count. Haemoglobin and haematocrit are generally ordered to assess the severity of the patient’s anaemia. Red cells, haemoglobin and haematocrit are often ordered routinely as part of the full blood count. Some red cell characteristics, such as cell size, volume and shape, are generally assessed together with the full blood count. Based on these results, the reticulocyte count can be considered a further red cell characteristic. Reticulocytes can be distinguished from mature red cells because they retain residual genetic material (RNA), which is not found in mature red cells. Circulating reticulocytes generally lose their RNA within one or two days, becoming mature red cells.

In a healthy person, the percentage of reticulocytes is extremely stable. When the number of red cells decreases (and the haematocrit therefore falls), the percentage of reticulocytes should increase to restore the red cell count. To assess bone marrow function more accurately, the calculated reticulocyte percentage is usually corrected using a formula called the corrected reticulocyte count or reticulocyte index (RI). This formula compares the patient’s haematocrit with a normal haematocrit.

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

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

When is it requested?
The reticulocyte count is ordered when a patient has a reduced red cell count and/or reduced haemoglobin and haematocrit, and the doctor wants to assess bone marrow function. In a patient with no obvious symptoms, these findings are picked up by a full blood count. The test may also be ordered when a patient has symptoms such as pallor, tiredness, weakness, shortness of breath and/or blood in the stool. When a patient is known to have iron, vitamin B12 or folate deficiency, kidney failure or bone marrow suppression (following chemotherapy or a bone marrow transplant), or is being treated with erythropoietin, the reticulocyte count (together with red cell count, haematocrit and haemoglobin) may be ordered at intervals recommended by the doctor to monitor bone marrow function and response to treatment.

When a patient has an increased number of red cells and high haemoglobin and haematocrit, the reticulocyte count may be ordered to determine the extent and rate of red cell overproduction.

What does the test result mean?
There is no single reference range for this test. Reference values can differ between laboratories because they depend on many factors, including the patient’s age, sex, the reference population and the analytical methods. Your laboratory report should include specific reference ranges for each test. Lab Tests Online strongly recommends always discussing your test results with your doctor. For more information on reference ranges, we suggest reading the article “Reference ranges and what they mean”.

What the doctor wants to find out is whether the bone marrow is responding appropriately and reacting as it should to an increased need for red cells. The reticulocyte count is directly affected by recent bone marrow activity. If the bone marrow responds appropriately to the need for more red cells, it will release more young cells, and the reticulocyte count will rise accordingly. If a patient has a bleeding episode, the number of reticulocytes will rise within a few days in an attempt to make up for the loss of red cells. If a patient has chronic blood loss, the reticulocyte count will settle at a higher level as the bone marrow tries to restore a normal number of red cells. If the bone marrow cannot keep up with demand or is not working properly, the reticulocyte percentage stays normal or only slightly above normal despite the increased demand, and in some cases it may even fall because of inadequate production. If the reticulocyte count is not raised in an anaemic patient, there may be some degree of bone marrow dysfunction or poor erythropoietin production.

The reticulocyte count gives an indication of what may have happened but does not diagnose a particular disease on its own. It is a signal that further tests can clarify, and one of the tools available for monitoring treatment.

If the number of reticulocytes rises after chemotherapy, a bone marrow transplant or treatment with iron, vitamin B12 or folate, the bone marrow is recovering. With overproduction of red cells, the reticulocyte count, red cell count, haemoglobin concentration and haematocrit will all be increased.

Is there anything else I should know?
People staying at high altitude will have an increased reticulocyte count as their body tries to adapt to the lower oxygen levels. Smokers may also show increased red cells and reticulocytes. The reticulocyte count may be raised during pregnancy. Newborn babies have a high percentage of reticulocytes, but this falls to adult levels within a few weeks.

Traditionally, the reticulocyte count was done manually by examining a suitably stained peripheral blood smear under a microscope and counting the number of reticulocytes in a number of fields. This method is still used but is gradually being replaced by automated methods that count a large number of cells and therefore give a more accurate reticulocyte count.

1. Can reticulocytes be tested at home?
2. Can the same sample used for the red cell count or full blood count be used?
3. Does a blood transfusion affect the reticulocyte count result?
4. How else can the doctor assess whether the bone marrow is producing blood cells properly?

1. Can reticulocytes be tested at home?
No. This test is carried out by specialist laboratory staff.

2. Can the same sample used for the red cell count or full blood count be used?
Yes. If anaemia is found on a routine full blood count, the doctor may request additional tests (including a reticulocyte count) on the same sample used for the full blood count.

3. Does a blood transfusion affect the reticulocyte count result?
Yes. The doctor will decide how long after the transfusion the reticulocyte count can be done.

4. How else can the doctor assess whether the bone marrow is producing blood cells properly?
In some cases a test called a bone marrow aspirate is needed to assess the state of the bone marrow. In certain circumstances this test is considered the best way to assess bone marrow function.