Low MCV Blood Test: What It Means and Common Causes

You're reviewing your blood test results and there it is—MCV is low. The arrow points down, and maybe your doctor mentioned "microcytosis" or "small red blood cells." What does this mean, and should you be worried?

This guide will explain what MCV is, why it might be low, what the most common causes are, and what steps you should take next.

Note: Normal ranges vary by lab. Always use the reference range on your specific lab report.

MCV stands for Mean Corpuscular Volume—it's a measure of the average size of your red blood cells.

Think of your red blood cells like delivery trucks carrying oxygen throughout your body. MCV tells you whether those trucks are:

• Normal size (normocytic)
• Too small (microcytic)—low MCV
• Too large (macrocytic)—high MCV

Red blood cells carry hemoglobin, the protein that holds oxygen. When cells are smaller than normal (low MCV), they typically have less hemoglobin and can't carry as much oxygen—this is why you might feel tired or weak.

Why Does MCV Matter?

The size of your red blood cells is a major diagnostic clue. Different conditions cause cells to be small in characteristic ways, so MCV helps your doctor narrow down what's happening.

Low MCV always means something is affecting red blood cell production. The job is to figure out what.

1. Iron Deficiency (Most Common!)

By far, the most common cause of low MCV is not having enough iron to make normal-sized red blood cells.

How it works:

• Your body needs iron to make hemoglobin
• Without enough iron, each red blood cell has less hemoglobin
• The cells end up smaller than normal (microcytic)
• Eventually, you develop iron deficiency anemia

Common causes of iron deficiency:

• Not eating enough iron-rich foods (common in vegetarians/vegans)
• Blood loss (heavy menstrual periods, ulcers, colon polyps)
• Pregnancy (your body needs more iron for the baby)
• Poor iron absorption (celiac disease, gastric bypass surgery)
• Frequent blood donation

Red flags for iron deficiency:

• Fatigue and weakness
• Pale skin
• Shortness of breath with activity
• Craving ice or other non-food substances (pica)
• Brittle nails
• Headaches
• Cold hands and feet

2. Thalassemia (Common, Often Missed!)

Thalassemia is a genetic condition affecting hemoglobin production. It's incredibly common—about 5% of the world's population carries a thalassemia gene, though many don't know it.

Types:

• Thalassemia minor (trait): Mild, usually no symptoms
• Thalassemia major: Severe, requires lifelong treatment

Thalassemia trait vs. iron deficiency: Both cause low MCV, but there's a key difference in blood work:

Important: Many people with thalassemia trait are mistakenly treated for iron deficiency for years—when they don't actually need iron supplements! This is why getting the right blood tests matters.

3. Anemia of Chronic Disease

Chronic inflammation can block iron use, leading to smaller red blood cells.

Conditions that can cause this:

• Autoimmune diseases (rheumatoid arthritis, lupus)
• Chronic infections (HIV, hepatitis, tuberculosis)
• Cancer
• Kidney disease
• Long-term inflammatory conditions

How it's different from iron deficiency: Your ferritin (iron stores) might be normal or even high because your body is hoarding iron—it just can't use it properly.

4. Lead Poisoning (Less Common but Serious)

Lead interferes with the body's ability to make hemoglobin, causing microcytic anemia.

Who's at risk:

• Children in older homes with lead paint
• People who work with batteries, construction, or metal refining
• People who use traditional remedies or cosmetics containing lead
• People who drink water from lead pipes

Other signs of lead poisoning:

• Abdominal pain
• Constipation
• Neurological symptoms (in children: developmental delays)
• Gray-blue line along gums (Burton's line)

This requires immediate medical attention and treatment to remove lead from the body.

5. Sideroblastic Anemia (Rare)

This is a rare condition where your bone marrow can't properly use iron to make hemoglobin. Iron accumulates in the developing red blood cells in a characteristic ring pattern.

Can be:

• Genetic (inherited)
• Acquired (from certain medications, alcohol use, or copper deficiency)

• Copper deficiency: Rare, but can happen after gastric surgery or with excessive zinc supplementation
• Vitamin B6 deficiency: Needed for hemoglobin synthesis
• Medications: Some chemotherapy drugs, certain antibiotics

Low MCV never appears in isolation. Your doctor will look at your complete blood count (CBC) to understand the full picture:

The RDW clue: RDW (Red Cell Distribution Width) is often the key to telling iron deficiency from thalassemia. In iron deficiency, your body is making new small cells while you still have some older normal cells—so RDW is high. In thalassemia, all the cells are uniformly small—so RDW is normal.

Low MCV itself isn't an emergency, but certain situations require prompt attention:

Seek Prompt Care If You Have:

• Chest pain or shortness of breath
• Fainting or severe dizziness
• Rapid or irregular heartbeat
• Black or bloody stools (suggests GI bleeding)
• Heavy menstrual bleeding (soaking through a pad/tampon every hour)
• Extreme fatigue making it hard to function
• Possible lead exposure

Schedule Prompt Follow-Up For:

• Any unexplained low MCV (even if you feel fine)
• Low MCV that's new or getting worse over time
• Family history of thalassemia or anemia
• Chronic medical conditions that could cause anemia

To find the cause of low MCV, your doctor may order:

Essential Tests

• Iron studies panel: Ferritin, serum iron, TIBC, transferrin saturation
• CBC with differential: Detailed look at all blood cells
• RDW: How much your red blood cells vary in size

Additional Tests

• Hemoglobin electrophoresis: To check for thalassemia or other hemoglobin disorders
• Reticulocyte count: How many young red blood cells you're making
• Lead level: If lead exposure is possible
• Vitamin B6 and copper levels: If deficiency is suspected
• Thyroid tests: Thyroid problems can affect blood counts
• Inflammatory markers (CRP, ESR): To check for chronic inflammation
• Occult blood test: To check for hidden GI bleeding

Women-Specific Testing

• Pap smear and pelvic exam: To check for heavy menstrual bleeding
• Pregnancy test: Pregnancy increases iron needs
• Ferritin trend: Over time, especially if menstruating

Treatment depends entirely on the underlying cause.

For Iron Deficiency

Iron supplements are the standard treatment:

• Oral iron tablets (ferrous sulfate, ferrous gluconate)
• Taken with vitamin C for better absorption
• Can take 3-6 months to replenish stores
• May cause constipation or black stools (normal)

Dietary changes:

• Eat more iron-rich foods (red meat, liver, spinach, fortified cereals)
• Pair iron-rich foods with vitamin C (citrus, bell peppers)
• Avoid calcium with iron (calcium blocks absorption)
• Cooking in cast iron pans adds iron to food

Treating the cause:

• Menstrual management for heavy periods
• Testing for and treating GI bleeding (ulcers, polyps)
• Treating celiac disease if present

For Thalassemia

Thalassemia minor (trait):

• Usually requires no treatment
• Avoid unnecessary iron supplements (ferritin is often normal or high)
• Genetic counseling before having children
• You can live a completely normal, healthy life

Thalassemia major:

• Regular blood transfusions
• Iron chelation therapy to remove excess iron from transfusions
• Comprehensive medical care

For Anemia of Chronic Disease

• Treat the underlying condition
• Iron supplements typically don't help (iron is present but not usable)
• Sometimes erythropoietin injections to stimulate red blood cell production

For Lead Poisoning

• Remove from lead exposure
• Chelation therapy to remove lead from the body
• Nutritional support (calcium, iron) to reduce lead absorption

While waiting for your appointment or test results:

1. Review Your Diet

Are you getting enough iron?

• Good sources: Red meat, poultry, fish, eggs, beans, lentils, spinach, fortified cereals
• Vegetarian/vegan? Plant-based iron (non-heme) is less well absorbed—pair with vitamin C
• Avoid tea, coffee, and calcium with iron-rich meals (they block absorption)

2. Assess Your Risk Factors

• Do you have heavy menstrual periods?
• Any family history of anemia or thalassemia?
• Any chronic medical conditions?
• Any chance of lead exposure?
• Recent surgeries (especially stomach or weight-loss surgery)?
• Frequent blood donation?

3. Track Your Symptoms

Keep a note of:

• Energy levels
• Shortness of breath with activity
• Any unusual cravings (especially ice)
• Headaches or dizziness
• Menstrual bleeding patterns

4. Don't Start Iron Supplements Yet!

Unless your doctor has confirmed iron deficiency with blood tests, don't start iron. If you have thalassemia, extra iron can be harmful. Always test before supplementing.

When discussing your low MCV results:

1. "What's causing my low MCV?"
2. "Do I have iron deficiency, or could it be something else like thalassemia?"
3. "What tests do I need to find the cause?"
4. "Do I need iron supplements?"
5. "Could any of my medications be causing this?"
6. "Should I be tested for thalassemia?"
7. "Do I need to see a hematologist (blood specialist)?"
8. "When should we recheck my levels?"
9. "Are there any symptoms I should watch for?"
10. "What dietary changes do you recommend?"

During Pregnancy

Low MCV in pregnancy is often due to:

• Iron deficiency (very common in pregnancy)
• Physiologic anemia of pregnancy (blood volume expands faster than red cell production)

Pregnant women need more iron—prenatal vitamins contain iron for this reason. Your doctor will monitor your levels throughout pregnancy.

In Children

Low MCV in children is most often due to:

• Iron deficiency from milk consumption (too much milk displaces iron-rich foods)
• Lead exposure (children are more vulnerable)
• Thalassemia trait

Children need prompt evaluation—iron deficiency affects development and cognitive function.

In Older Adults

Low MCV in older adults may indicate:

• GI bleeding (ulcers, polyps, colon cancer)
• Nutritional deficiencies
• Chronic disease
• Myelodysplastic syndromes (bone marrow disorders)

GI bleeding should always be ruled out in older adults with low MCV.

Low MCV means your red blood cells are smaller than normal. Iron deficiency is the most common cause, but thalassemia trait, chronic inflammation, and other conditions can also cause it.

The key is finding the cause—this is done through additional blood tests like iron studies and hemoglobin electrophoresis.

Most common scenarios:

• Premenopausal woman: Iron deficiency from menstrual blood loss
• Mediterranean/Asian/African ancestry: Thalassemia trait
• Older adult: GI bleeding, needs evaluation
• Anyone with chronic inflammation: Anemia of chronic disease

The right approach:

1. Don't panic—low MCV is very common and usually treatable
2. Get the recommended follow-up tests to identify the cause
3. Treat the underlying condition
4. Recheck to confirm improvement

Your body is telling you something needs attention—listen to it, work with your healthcare team, and get to the bottom of what's going on. With the right diagnosis and treatment, you'll be feeling like yourself again soon.

Related articles on WellAlly:

• Complete Guide to Iron Deficiency Anemia
• Thalassemia vs. Iron Deficiency: What's the Difference?
• Understanding Your Complete Blood Count (CBC)
• Ferritin Blood Test: What It Means