Reticulocytes (RET%)

What is RET%

The reticulocyte percentage (RET%) is a relative indicator showing what proportion of all red blood cells in the blood are reticulocytes — young, immature red blood cells. Each reticulocyte circulates in the blood for about 1–2 days before it fully matures and becomes a fully functional red blood cell.

This indicator reflects the productive activity of the bone marrow — the more new cells released into the blood, the higher the RET%. Assessed together with the absolute reticulocyte count (RET#) and other red blood cell parameters, it allows a more accurate understanding of the cause and course of anaemia.

Reference ranges

The normal range for adults is typically 0.5–2.5%. In women, the value may be slightly higher due to the influence of the menstrual cycle. In newborns, RET% is higher and decreases to adult levels during the first month of life.

It is very important to assess RET% together with the absolute count: an elevated percentage in the presence of a low red blood cell count does not always indicate a true increase in bone marrow activity.

Why RET% may be elevated

An elevated RET% indicates that the bone marrow is actively producing new red blood cells — this may be a response to:

• Active bleeding — surgery, trauma, or gastrointestinal bleeding stimulates compensatory red blood cell production.
• Haemolytic anaemia — red blood cells are destroyed too quickly, and the body tries to compensate by increasing production.
• Recovery after anaemia treatment — after starting iron, vitamin B12, or folic acid, RET% rises within 5–7 days.
• Living at high altitude or intense physical exercise.

Why RET% may be reduced

A reduced RET% means that the bone marrow is not producing enough new red blood cells. Possible causes include:

• Iron, vitamin B12, or folic acid deficiency.
• Aplastic anaemia.
• Chronic kidney disease.
• Bone marrow infiltration by tumours.
• Chemotherapy or radiation therapy.

What to do next

Assess RET% together with haemoglobin, haematocrit, MCV, and RBC. The combination of these indicators makes it possible to distinguish, for example, iron deficiency anaemia from B12 deficiency or haemolysis.

If RET% is low and haemoglobin is also low, this indicates that the bone marrow is not responding appropriately.

Monitoring over time

RET% is especially useful for monitoring treatment effectiveness. When treating iron deficiency anaemia, the reticulocyte response — a rise in RET% — is observed as early as 5–7 days. The peak is typically reached within 7–10 days of starting treatment.

After bone marrow transplantation, RET% is one of the first indicators showing that the new marrow system has begun to function.

Questions for your doctor

• Does my RET% match the clinical picture and other blood parameters?
• Are additional tests needed to determine the cause of a reduced or elevated RET%?
• How quickly should I expect a reticulocyte response if I start treatment?
• When should the test be repeated to monitor progress?