Reticulocytes

Reticulocytes are young red blood cells just released from the bone marrow. Their number shows how many new red blood cells the marrow is producing at that moment. Testing most often investigates anaemia, distinguishing reduced production from loss, and monitors treatment response.

Who the test is for

• people whose complete blood count has shown anaemia, to identify whether the cause is reduced production, loss or breakdown of red blood cells;
• people with suspected bleeding or haemolysis, accelerated red blood cell breakdown;
• people treated for iron, vitamin B12 or folic acid deficiency, because rising reticulocytes are the first sign treatment is working;
• people during or after chemotherapy, to monitor bone marrow function.

What reticulocytes are and what they show

Red blood cells (erythrocytes) form in bone marrow. They first enter blood as reticulocytes, maturing in circulation within one to two days. Their number is therefore a good indicator of marrow function: after blood loss or accelerated red cell breakdown, healthy marrow increases production and reticulocytes rise. If the marrow lacks building materials such as iron, vitamin B12 or folic acid, or is itself damaged, reticulocytes are too few.

Results are expressed as an absolute number per litre of blood or a percentage of total red blood cells. The absolute count is more useful in anaemia because the percentage may look normal even when marrow is not meeting needs.

Preparation and sample

Reticulocytes are measured in venous blood and fasting is unnecessary. Testing is most often performed alongside a blood count from the same collection. Tell your doctor if taking iron, vitamin B12 or folic acid or if you recently received a transfusion, because these affect results.

How to interpret the result

In adults the reticulocyte count is usually approximately 22 to 97 ×10⁹/L, or 0,5 to 2,5 % of red blood cells. The reference interval depends on method and age and is printed on your report. Reticulocytes are always interpreted alongside haemoglobin and the rest of the blood count by a doctor.

Raised reticulocytes

A raised reticulocyte count (reticulocytosis) means the bone marrow is working harder. This occurs several days after bleeding, in haemolytic anaemias and as an expected treatment response: after starting iron, vitamin B12 or folic acid, reticulocytes begin rising after several days and usually peak after a week to ten days. Raised reticulocytes are then a good sign.

Low reticulocytes

A low reticulocyte count with anaemia means marrow is not producing enough new cells. The most common causes are untreated iron deficiency, vitamin B12 or folic acid, anaemia of chronic disease and chronic kidney disease. Less often, marrow damage is responsible, for example due to chemotherapy, some medicines, alcohol or blood disorders.

Reticulocytes and iron deficiency

In iron-deficiency anaemia reticulocytes are usually low or inappropriately normal relative to the anaemia. The result alone does not confirm iron deficiency: this requires ferritin and other iron indicators, included alongside a blood count in the Ferro (iron) panel. Reticulocytes are useful later for checking whether iron replacement is working.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed under Contact). Prices for all tests and panels are available in the service price list. Take the results to the doctor who recommended the test.

Frequently asked questions

What do raised reticulocytes mean?

That marrow is producing more red blood cells. This is expected after bleeding and during successful anaemia treatment, and can also suggest accelerated red cell breakdown (haemolysis). A doctor assesses the cause alongside the blood count and, if needed, additional tests such as bilirubin and LDH.

Are raised reticulocytes without anaemia a cause for concern?

A mildly raised value with a normal blood count often has no particular significance, especially if you recently started iron or vitamins. A doctor may suggest repeating testing and, if the elevation persists, investigating accelerated red cell breakdown.

Why does a doctor request reticulocytes after starting iron treatment?

Because reticulocytes rise before haemoglobin. Their rise within the first week or two shows the marrow is receiving enough iron and treatment is working.