Low MCV and MCH Blood Test: What Does It Mean?

Low MCV (mean corpuscular volume) and low MCH (mean corpuscular hemoglobin) mean your red blood cells are, on average, smaller and carry less hemoglobin than usual. This is called microcytosis. The most common causes are iron deficiency and thalassaemia trait, and the usual first test is ferritin.

A review in American Family Physician defines microcytosis in adults as an MCV below 80 femtoliters (fL). Cleveland Clinic gives a typical adult MCH range of 27 to 33 picograms per cell. Laboratories set their own ranges, so compare your numbers with those printed on your report.

MCV and MCH usually fall together, because smaller cells hold less hemoglobin. Cleveland Clinic notes that a low MCH can mean cells look paler than normal (hypochromic) as well as smaller.

This guide is for adults who have seen low MCV and MCH on a full blood count. It explains the main causes, how clinicians tell them apart and what to ask. It cannot tell you which cause applies to you.

Small red cells cause no symptoms by themselves. Symptoms come from anemia, if present, or from its cause. Healthdirect lists tiredness, a fast heartbeat, dizziness, shortness of breath, pale skin and chest pain among anemia symptoms, and some people have none.

Use local emergency services for chest pain, severe breathlessness, fainting, or heavy bleeding that does not stop. The NHS advises an urgent appointment for black or dark red stools or bloody diarrhoea. Iron tablets can also darken stools, so mention them if you take them.

The 2010 review names iron deficiency as the most common cause in the United States. Thalassaemia trait is the other common cause, and it is more frequent in some parts of the world. Other causes include:

• Long-term inflammation or illness, sometimes called anemia of chronic disease, including kidney disease, liver disease and some cancers.
• Other inherited hemoglobin conditions.
• Lead exposure and sideroblastic anemia, which are less common.

The two can look alike on a blood count, so further tests are needed. Ferritin, a marker of iron stores, is the recommended first test. A low ferritin points to iron deficiency. The same review notes that ferritin can be raised by inflammation, liver disease, cancer and kidney disease, so a normal value does not always rule iron deficiency out.

A 2022 American Family Physician review advises suspecting thalassaemia when microcytosis comes with a normal or high ferritin. Clues on the count include a red cell count that is high or normal despite small cells. A normal RDW (red cell distribution width) also leans toward thalassaemia, but both reviews say it is not reliable enough to separate the two on its own.

If iron deficiency is found, the 2010 review advises correcting it before hemoglobin electrophoresis is interpreted.

Hemoglobin electrophoresis measures the types of hemoglobin in your blood. In beta thalassaemia trait it typically shows a raised HbA2. In alpha thalassaemia trait it is usually normal, so a normal result does not rule alpha trait out. The 2022 review notes that genetic testing is needed to confirm the type.

The NHS explains that a carrier has one altered gene, does not have thalassaemia and never will. Carriers may have mild anemia because their red cells are smaller than usual, but it needs no treatment. It is more common in people with family origins around the Mediterranean, South Asia, the Middle East, China and Southeast Asia.

If both partners carry beta thalassaemia trait, each child has a 1 in 4 chance of having thalassaemia. The NHS suggests partner testing when planning a pregnancy, with results discussed with a genetic counsellor.

Small red cells can appear before, or without, anemia. Thalassaemia trait often causes small cells with a normal or only slightly low hemoglobin, and sometimes it is found only by a blood test. Iron stores can also run low before hemoglobin falls; the SameCase guide on low ferritin with normal hemoglobin covers that situation.

A normal hemoglobin does not mean the cause can be ignored. Knowing whether you carry a trait matters for family planning, and finding iron deficiency means looking for its cause.

The NHS says the underlying cause of iron deficiency anaemia needs to be found before treatment. Causes include heavy periods, pregnancy and bleeding from the stomach or bowel. The 2010 review advises checking the stomach and bowel for hidden blood loss when iron deficiency is found in adult men and women who no longer have periods.

If the cause is not clear, the NHS notes that further tests may follow, such as a stool test for hidden blood (FIT) or a referral to a specialist. Repeat blood tests are then used to check that iron levels are recovering.

Do not start iron supplements to fix the numbers before testing. The NHS warns that extra iron can be harmful if your body already has enough, which is why thalassaemia trait should not be treated with iron unless iron deficiency is also confirmed.

Bring your report, earlier blood counts, and any family history of anemia or thalassaemia.

• How low are my MCV and MCH, and is this new or long-standing?
• What does my ferritin show, and could inflammation be affecting it?
• Is iron deficiency, thalassaemia trait or both the more likely explanation?
• Do I need hemoglobin electrophoresis or genetic testing, and when?
• If I am iron deficient, how will the cause be checked?
• Should my partner or family be tested?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with low MCV and MCH described their ferritin, follow-up tests and conversations can help you plan what to ask.

Another person's cause may not be yours, and a similar case cannot diagnose you. We suggest using cases only to prepare questions for your own clinician.