Reticulocyte count: normal range and what the result means (RET)

Reticulocytes are the youngest form of red blood cell (erythrocyte): they mature in the bone marrow from precursor cells and circulate in the blood for a few days until they become fully mature red blood cells. Their number in the blood directly reflects how actively the bone marrow is producing new red blood cells.

Because of this, a reticulocyte count is used not so much to establish anaemia itself as to work out its cause. If the bone marrow is responding to anaemia appropriately, reticulocytes increase, because the body is trying to compensate for the shortage; if bone marrow function is disturbed or the raw materials are lacking (iron, vitamin B12, folic acid), reticulocytes stay low even when anaemia is present. The test is also used to follow how the body recovers after chemotherapy or a bone marrow transplant, and to assess how well treatment given for anaemia is working.

• To diagnose a particular type of anaemia, when a full blood count shows a reduced haemoglobin or haematocrit
• To follow how well treatment for anaemia is working (for example iron, vitamin B12 or folic acid supplementation)
• To assess bone marrow function and its ability to produce new red blood cells
• To follow recovery after chemotherapy or a bone marrow transplant
• When haemolytic anaemia is suspected (red blood cells being destroyed faster than they are renewed), or after acute bleeding

A raised reticulocyte count most often shows that the bone marrow is producing new red blood cells more intensively — usually in response to their loss or destruction. The causes: haemolytic anaemia, active bleeding, haemolytic disease of the newborn, kidney cysts or tumours producing too much erythropoietin, and also pregnancy.

• Haemolytic anaemia — red blood cells are destroyed faster than the bone marrow manages to restore them
• Active bleeding — the body compensates for the red blood cells it has lost
• Haemolytic disease of the newborn (erythroblastosis fetalis)
• Kidney cysts or tumours producing too much erythropoietin
• Pregnancy

What to discuss with a doctor:

• A full blood count, and how haemoglobin and haematocrit change over time
• Markers of haemolysis (bilirubin, LDH, haptoglobin), if haemolytic anaemia is suspected
• Whether there is any active or previous bleeding
• Kidney function tests, if increased erythropoietin production is suspected

A low reticulocyte count shows that the bone marrow is not producing enough new red blood cells — most often because a raw material is missing (iron, vitamin B12, folic acid), or because the function of the bone marrow itself is disturbed. This matters when the causes of anaemia are being told apart: bone marrow that is responding appropriately would raise the reticulocyte count, whereas a low count shows that the body is not managing to compensate.

• Iron, vitamin B12 or folic acid deficiency anaemia
• Bone marrow failure (aplastic anaemia)
• Chronic kidney disease — reduced erythropoietin production
• Cirrhosis of the liver

What to discuss with a doctor:

• Iron markers (ferritin, serum iron, transferrin saturation), vitamin B12 and folic acid
• An assessment of bone marrow function, if other causes have been ruled out
• Kidney and liver function tests
• A repeat full blood count, to follow the change over time

Reticulocytes and the full blood count

A reticulocyte count does not replace a full blood count — it adds to it. The full blood count shows whether anaemia is present, while the reticulocyte count helps to understand whether the bone marrow is responding to it appropriately, and so sets the direction of further testing. The exact cause is established by a doctor together with the other tests, and the exact laboratory interval is given on your own result sheet, because methods of assessment can differ. Which tests are used when anaemia is suspected is described separately.