Reticulocytes: what they are, what they tell you and when they’re tested

Reticulocytes are newborn red blood cells. Your bone marrow makes them and sends them into the blood, where within a day or two they become mature red cells. Counting them shows how much new blood your marrow is making.

When they’re tested

Doctors mostly order them to understand an anaemia: whether the marrow is making too little, or making plenty while red cells are lost or broken down. They also use them to see whether a treatment with iron or vitamin B12 is working.

They are not a routine test for people who feel well. If your blood count is normal, they usually add nothing.

That is why Rosa does not test them. Instead it measures the full blood count, with haemoglobin, MCV and RDW, plus ferritin, serum iron and vitamin B12, which show whether there is anaemia and often where it comes from.

What they tell you, and what they do not

They are the marrow’s answer. If you lose blood or red cells break down, a healthy marrow makes more and reticulocytes rise. If something needed to make them is missing, or the marrow works too little, they stay low.

On their own they do not say why. They are always read with haemoglobin: high reticulocytes with anaemia mean something different from high reticulocytes with a normal haemoglobin.

Many lab reports also give an absolute count, and sometimes an index corrected for anaemia. Doctors often look at those more than the percentage.

That is why they are read together with other tests:

• Haemoglobin: It shows whether there is anaemia
• Ferritin: Without iron the marrow makes too little
• Vitamin B12: A shortage also slows the marrow down
• Haptoglobin: If it is low, red cells are breaking down

Normal reticulocyte levels

The lab decides what is normal. Reticulocytes usually make up 0.5 to 2.5% of red cells. Your own report may print different limits: its range is what counts.

The absolute count has its own limits, which vary from lab to lab. Read the ones printed next to your result.

They are much higher in newborns. In pregnancy and at high altitude they can rise a little.

What your result means

Find the row for your result, as a percentage. Haemoglobin and the rest of the blood count matter too: a value out of range together with anaemia should be seen by a doctor.

• Below 0.5: Low. The marrow is making few new red cells. If your haemoglobin is normal, it often matters little. With anaemia, iron, B12 or folate may be short. If you also have anaemia, talk to your doctor.
• 0.5 to 2.5: Normal. Your marrow is renewing red cells at the right pace. Nothing to do, if the rest of your blood count is normal.
• Above 2.5: High. The marrow is making more red cells. It happens after bleeding, when red cells break down, or while a treatment for anaemia is working. If you are not being treated for anaemia, talk to your doctor.

If your reticulocytes are high or low

• High reticulocytes: The marrow is making more: after bleeding, when red cells break down, or while a treatment for anaemia is working.
• Low reticulocytes: The marrow is making little. With anaemia, iron, B12 or folate is often short, sometimes the kidneys make too little hormone. Without anaemia, it matters little.

The most common causes

The causes depend on the direction.

• High: bleeding: After blood loss, even hidden, the marrow speeds up.
• High: red cells breaking down: Haemolysis, inherited or acquired. LDH and bilirubin usually rise too and haptoglobin falls.
• High: a treatment that works: A few days into iron or vitamin B12, reticulocytes rise. It is the expected sign.
• Low: short of iron, B12 or folate: The marrow lacks the material to produce.
• Low: kidneys or marrow: Kidneys that work poorly make less erythropoietin, the hormone that drives the marrow. Less often the marrow itself is diseased or held back by medicines.

What can skew the result

A few things change them without any disease.

• A recent transfusion: It changes the make-up of your blood for weeks. Tell your doctor.
• High altitude: With less oxygen, your body makes more red cells.
• Giving blood: In the weeks after, your marrow replaces what you gave and reticulocytes rise.
• The method: Lab analysers are not all the same. To follow the value over time, it is best to use the same lab.

When to worry

Talk to your doctor if:

• They are out of range and your haemoglobin is low too.
• They are high and you have not lost blood, are not being treated for anaemia and have not been at high altitude.
• You feel tired and short of breath, your skin or eyes look yellow or your urine is dark.
• You see blood in your stool, have black stools or very heavy periods.
• You have bleeding that will not stop, you faint or you are suddenly short of breath: call 112 or go to the emergency department.

Frequently asked questions

What are normal reticulocyte levels? Usually 0.5 to 2.5% of red cells. The absolute count has its own limits. The range on your own report is what counts.

High reticulocytes: what does it mean? That your marrow is making more red cells. It happens after bleeding, when red cells break down, or while a treatment with iron or B12 is working. Your doctor reads it with your haemoglobin.

Low reticulocytes: is it serious? With a normal blood count, usually not. With anaemia it means the marrow is making too little, often because iron, B12 or folate is short. Work it out with your doctor.

Do I need to fast for a reticulocyte count? No. They are measured on the same tube as the blood count.

Does Rosa test reticulocytes? No. Rosa measures the full blood count, ferritin, serum iron and vitamin B12, which show whether there is anaemia and often why. If your doctor wants reticulocytes, they order them separately.

Sources

• MSD Manual, consumer version: laboratory tests for blood disorders. Revised 2025. Manuale MSD, esami di laboratorio per le malattie del sangue
• Piva and colleagues, clinical utility of reticulocyte parameters. Clinics in Laboratory Medicine, 2015. PMID 25676377
• Barcellini and Fattizzo, clinical applications of hemolytic markers in the differential diagnosis and management of hemolytic anemia. Disease Markers, 2015. PMID 26819490