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 red blood cells (RBCs) and to help determine the cause of the anaemia and identify which type. The number of reticulocytes is compared to the total number of RBCs to calculate the percentage of reticulocytes. Because of this, the test is ordered along with an RBC count. A haemoglobin and/or haematocrit test are also usually ordered to evaluate the severity of the 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 red cell characteristics such as cell size, volume and shape. Based on these results, a reticulocyte count may be requested to further examine RBCs. Reticulocytes can be distinguished from mature red blood cells because they still contain remnant genetic material () inside them, a characteristic not found in mature red blood cells. Circulating reticulocytes generally lose their RNA within one to two days, thus becoming mature RBCs.
In a healthy person, the reticulocyte percentage is very stable. In someone with anaemia, when the number of red blood cells decreases, the percentage of reticulocytes may appear increased compared to the overall number of red blood cells. In order to get a more accurate assessment of bone marrow function, a calculation - the calculated reticulocyte percentage (%) - is often corrected with a calculation called a corrected reticulocyte count or a reticulocyte index (RI). This 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 requested when someone has a decreased RBC count and/or a decreased haemoglobin and haematocrit and it is necessary to evaluate bone marrow function. In someone with no apparent symptoms, these findings may be found during routine blood tests. Testing may also be ordered when someone has symptoms such as paleness, fatigue, weakness, shortness of breath and/or blood in the stool.
When a person 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 by the doctor to monitor marrow function and response to treatment.
When someone has an increased number of red blood cells and elevated haemoglobin and haematocrit, the reticulocyte count may be requested to help determine the degree and rate of overproduction of red blood cells.
What does the test result mean?
The reticulocyte count 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 red blood cells and whether the bone marrow is able to respond to the extra requirement. Occasionally, results may indicate overproduction of red blood cells. If the bone marrow is responding appropriately to the demand for increased numbers of red blood cells, it will allow for the early release of more immature RBCs (reticulocytes) - thus the increased reticulocyte count. If someone haemorrhages (bleeds), the number of reticulocytes will rise a few days later in an attempt to compensate for the red cell loss. If they have chronic blood loss the number of reticulocytes will stay at an increased level as the marrow tries to keep up with the demand for new red blood cells. If the marrow is unable to keep up or is not functioning normally, 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 someone with anaemia 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 that can be used 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 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 red blood cells 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 has largely been replaced by automated instruments that allow for a greater number of cells to be counted, thus enhancing the accuracy of reticulocyte counts.