Reticulocyte Count: What It Says About Your Bone Marrow

A reticulocyte count measures young, newly released red blood cells in your blood, which gives clinicians a window into how actively your bone marrow is making red cells. A high count may suggest the marrow is responding to blood loss or red cell breakdown. A low count may suggest the marrow is not keeping up. This article explains how the test works, what high and low results may mean, and which symptoms need urgent care.

What a reticulocyte count measures

Reticulocytes are immature red blood cells. Your bone marrow makes them and releases them into the bloodstream, where they typically mature into fully formed red cells over about a day or so. A reticulocyte count shows how many of these young cells are circulating. Research on human reticulocyte maturation, such as the PubMed Central paper on reticulocyte maturation, describes how these cells gradually remodel themselves as they become mature red cells.

Think of it as a production report. A hemoglobin or red cell count tells you how many red cells are in the blood right now. The reticulocyte count adds information about whether the marrow is actively replacing them. Clinicians use the two together, because a low hemoglobin can have very different explanations depending on whether the marrow is responding.

How the test is done

The test uses a standard blood draw from a vein, often alongside a complete blood count (CBC). MedlinePlus (NIH) describes blood count tests as a group of measurements of the cells in your blood. Laboratories count reticulocytes by staining the cells and then using either a microscope or an automated analyzer. Flow cytometry, a laser-based method, is one way instruments count them, as described in the PubMed Central article Reticulocyte counting using flow cytometry.

You usually do not need to prepare. Tell the clinician about recent blood loss, a transfusion, pregnancy, or medicines you take, since these may affect the result.

How results are reported

You may see the result in more than one form:

• Reticulocyte percentage: the share of red cells that are reticulocytes.
• Absolute reticulocyte count: the number of reticulocytes in a given volume of blood.
• Corrected reticulocyte count or reticulocyte index: an adjusted value that accounts for how low the red cell level is, so that a percentage is not misleading when there are fewer red cells overall. Laboratories and clinicians may calculate it in slightly different ways, so ask your clinician how your report was derived.
• Immature reticulocyte fraction or reticulocyte hemoglobin: extra measurements that some analyzers report.

Reference ranges differ between laboratories and analyzers, so compare your result with the range printed on your own report. The percentage alone can be misleading in anemia, which is why clinicians often look at the absolute or corrected value.

What a high reticulocyte count may suggest

A high count may suggest the marrow is working hard and sending out young cells. Several situations can contribute:

• Bleeding. After blood loss, the marrow may speed up production. The cause of bleeding still needs to be found.
• Hemolysis. In hemolytic anemia, red cells break down faster than usual, and the marrow may respond by releasing more reticulocytes. Causes can include inherited conditions, autoimmune processes, some medicines, and infections.
• Recovery from a deficiency. When someone starts treatment for iron, vitamin B12, or folate deficiency, the reticulocyte count can rise as the marrow responds. Some clinicians use this response as a sign that treatment is working.
• Newborn and pregnancy-related situations. Newborns normally have higher counts, and blood group incompatibility between mother and baby (see MedlinePlus on Rh incompatibility) can cause red cell breakdown in the baby.
• Living at high altitude. The body may make more red cells in response to lower oxygen.

A high count can also be a laboratory artifact. A study on the Sysmex XE-2100 analyzer, listed in Sources, reported that on some analyzers a very high white blood cell count can cause "pseudoreticulocytosis," an overcount of reticulocytes. This is one example, and findings may differ between instruments. Your clinician considers the whole picture before interpreting an unexpected value.

What a low reticulocyte count may suggest

A low or inappropriately normal count in someone with anemia may suggest the marrow is not making enough red cells. Several factors can contribute:

• Nutrient deficiency. Not enough iron, vitamin B12, or folate can limit red cell production. The related articles on iron studies and borderline B12 results cover those tests.
• Chronic disease or kidney disease. Long-term inflammation or reduced kidney function can lower red cell production. The kidneys make a hormone, erythropoietin, that signals the marrow.
• Medicines and treatments. Chemotherapy, radiation, and some other drugs can suppress the marrow.
• Infections. Some viral infections can temporarily slow red cell production.
• Bone marrow conditions. MedlinePlus describes bone marrow diseases and aplastic anemia, in which the marrow does not make enough blood cells. These are much less common than everyday causes such as nutrient deficiency, and one low reticulocyte count alone does not establish them.

Putting the count together with other tests

The reticulocyte count rarely stands alone. Clinicians combine history, examination and testing. Common companions include:

• A CBC, including hemoglobin and mean corpuscular volume (the average size of your red cells). Our article on understanding a complete blood count walks through those values.
• A peripheral blood smear, in which a specialist looks at your cells under a microscope.
• Iron studies, vitamin B12 and folate levels.
• Markers of hemolysis, such as bilirubin and lactate dehydrogenase, which a clinician may order if red cell breakdown is suspected.
• Kidney function tests.

If results still do not explain the anemia, a clinician may discuss a bone marrow examination. That decision depends on the whole picture, not on one value.

What happens after an abnormal result

Treatment depends on the cause, not on the reticulocyte count itself. Nutrient deficiencies are often treated with supplements or dietary changes under a clinician's guidance. Bleeding requires finding its source. Hemolysis and marrow disorders are typically managed by a blood specialist (hematologist). Do not start iron or other supplements on your own based on this result, since too much iron can cause problems and a clinician can tell you whether it is appropriate.

Your clinician may repeat the test after treatment starts to see whether the marrow is responding. MedlinePlus has a general overview of anemia, and our article on low hemoglobin covers next steps in more detail.

Symptoms to watch for

Anemia can cause tiredness, paleness, shortness of breath with activity, a fast heartbeat, dizziness, and headaches. Symptoms may develop slowly and be easy to dismiss. Tell your clinician about them, along with any dark urine, yellowing of the skin or eyes, black or bloody stools, heavy periods, or easy bruising, because these can point toward the cause. Severe or sudden symptoms are listed in the emergency box below.

One situation needs emergency attention. If you are receiving or have recently had chemotherapy, or you know your white blood cell count is low or you have a bone marrow problem, a fever of 100.4°F (38°C) or higher can be a sign of a serious infection, because the body may have fewer infection-fighting cells. Call your oncology team right away or go to the emergency room rather than waiting for a routine appointment.

Questions to bring to your appointment

• Was my reticulocyte count high, low, or in range for the level of anemia I have?
• Is this the absolute or corrected value?
• Which other tests might help explain the result?
• Could any of my medicines or recent illnesses have affected it?
• When should the test be repeated?

When to Seek Medical Care