Reticulocyte count: checking how fast your marrow makes red cells

Red cells start life in the bone marrow, and just before they leave they pass through a stage called the reticulocyte. Lab Tests Online UK explains that reticulocytes still contain a remnant of genetic material (RNA), which mature red cells do not, and that circulating reticulocytes generally lose it within one to two days, becoming mature red cells.

Counting them is therefore a snapshot of recent marrow output. A healthy, steady-state adult has a small, stable fraction of reticulocytes among circulating red cells; Lab Tests Online UK puts this at about 0.5 to 2 per cent. The body keeps the number of red cells stable by removing old cells, which live for around 120 days, and replacing them with new ones.

It is requested when the red cell count, haemoglobin, haematocrit or platelet count is abnormal and the doctor wants to judge bone marrow activity. Lab Tests Online UK says it helps distinguish anaemia related to blood loss or destruction from anaemia related to decreased production, and helps monitor marrow response and return of normal function after chemotherapy, bone marrow transplant, or follow-up after iron deficiency treatment.

It may also be used with symptoms such as paleness, tiredness, weakness, shortness of breath or blood in the stool, after a recent episode of blood loss, when red cell lifespan is shortened as in some haemolytic anaemias, or in people with known iron, B12 or folate deficiency, kidney disease or marrow suppression.

No preparation is needed. Blood can be taken at any time of day, before or after a meal. The sample goes into a tube with EDTA preservative. It can be run on the same tube as a full blood count, but it must be analysed the same day, before the reticulocytes mature.

A blood transfusion affects the result, so your doctor will decide how long to wait after a transfusion. Lab Tests Online UK also notes that people who move to higher altitude, smokers, and pregnant women may have raised counts, and that newborns have a higher percentage that drops to near adult levels within a few weeks.

Results are given as a percentage of red cells and as an absolute number. Because a low red cell count makes the percentage look inflated, laboratories often correct it. Lab Tests Online UK describes the corrected reticulocyte count, or reticulocyte index, which compares the patient's haematocrit with a normal value, and a reticulocyte production index that adjusts for how long the cells take to mature.

Some automated analysers also report the immature reticulocyte fraction and mean reticulocyte volume, which Lab Tests Online UK describes as primarily research parameters at present.

An increased percentage may indicate bleeding, because the count rises a few days after blood loss, or long-term loss where it stays raised, as well as haemolytic anaemia and haemolytic disease of the newborn. In red cell over-production conditions such as polycythaemia vera and in smokers, both the reticulocyte and red cell counts can be raised.

A decreased percentage may be seen in iron deficiency anaemia, pernicious anaemia or folic acid deficiency, aplastic anaemia, radiation therapy, and bone marrow failure caused by infection or cancer. If the reticulocyte number is not raised when someone is anaemic, there is likely to be some degree of marrow disease or failure or a deficiency of erythropoietin.

The test points in a direction but does not diagnose a particular disease. It shows whether further investigation is needed and helps monitor treatment.

A raised count leads to tests for bleeding or haemolysis, such as bilirubin, LDH and a blood film. A low count leads to iron, B12 and folate studies, kidney function tests and, if needed, a haematology opinion. In some cases a bone marrow aspiration is used to look at the marrow directly, which Lab Tests Online UK describes as sometimes the best way to judge how well the marrow is functioning.

If you are being treated for deficiency, a rise in reticulocytes after treatment starts is an early sign that the marrow is recovering, often before the haemoglobin has risen noticeably.