Low Haemoglobin with Low MCV: What Microcytic Anaemia Means

Not all anaemia is the same. When your CBC report shows both low haemoglobin (Hb) and low MCV (Mean Corpuscular Volume), this combination - called microcytic anaemia - tells you something specific about what is causing the anaemia and how to treat it.

Getting the cause right matters enormously: giving iron supplements to the wrong type of anaemia will not help, and in some cases (like thalassaemia) can cause harm.

What Is MCV?

MCV is the average size (volume) of your red blood cells, measured in femtolitres (fL).

Normal MCV: 80-100 fL

When MCV is low (below 80 fL) and haemoglobin is also low, this is microcytic anaemia - small, often pale red cells that carry less oxygen.

The Main Causes of Low Hb + Low MCV

1. Iron Deficiency Anaemia (Most Common)

The most common cause of microcytic anaemia worldwide, and especially prevalent in India - particularly in women of reproductive age, vegetarians, and children.

Iron deficiency develops in stages:

• Stage 1: Ferritin falls (iron stores depleted) - no blood count change yet
• Stage 2: Serum iron falls, TIBC rises - no anaemia yet but functional deficiency
• Stage 3: Haemoglobin falls, MCV drops - frank iron-deficiency anaemia

CBC pattern for iron deficiency: Low Hb + Low MCV + Low MCH + High RDW

Confirm with: Serum ferritin (below 30 ng/mL = depleted stores; below 12 = deficient)

Treatment: Oral iron supplements (ferrous sulphate or ferrous ascorbate), typically 100-200 mg elemental iron daily for 3-6 months. Take with vitamin C, away from calcium and tea.

2. Thalassaemia Trait (Carrier State)

Thalassaemia is a genetic condition that affects haemoglobin production. In thalassaemia major, both gene copies are affected and the condition is severe. In thalassaemia minor (or trait), only one gene copy is affected - and this is very common in India, particularly in Gujarat, Maharashtra, Punjab, Sindh, and parts of South India.

People with thalassaemia minor typically have:

• Mildly low haemoglobin (often 10-12 g/dL) throughout their life
• Low MCV (often 60-70 fL) - notably lower than in typical iron deficiency
• Normal ferritin - iron stores are fine

Why it matters: Thalassaemia trait is often mistaken for iron deficiency anaemia, especially in routine checkups. The patient is repeatedly given iron supplements that provide no benefit. The red cell count is usually normal or high in thalassaemia trait - a clue that distinguishes it from iron deficiency, where RBC count is low.

Confirm with: Haemoglobin electrophoresis or HPLC - this is the definitive test. Shows elevated HbA2 (above 3.5%) in beta-thalassaemia trait.

Treatment: None required for the carrier state itself. However, it is important for family planning - two carriers have a 25% chance of having a child with thalassaemia major.

3. Anaemia of Chronic Disease (with Low MCV)

Usually causes normocytic anaemia, but can produce microcytic anaemia in long-standing cases. Seen in chronic kidney disease, active tuberculosis, rheumatoid arthritis, and cancer.

The distinguishing feature: ferritin is normal or elevated (inflammation raises ferritin even when iron stores are functional), and serum iron is low due to inflammatory sequestration.

4. Sideroblastic Anaemia

Rare. The body has iron but cannot incorporate it into haemoglobin properly. Causes include lead poisoning, chronic alcohol use, and some genetic conditions. Very low MCV with high ferritin.

How to Tell Iron Deficiency from Thalassaemia Trait

These two conditions look very similar on a basic CBC. Here are the distinguishing clues:

The Mentzer Index (MCV divided by RBC count) is a simple bedside clue:

• Below 13 = thalassaemia more likely
• Above 13 = iron deficiency more likely

This is a guide, not a definitive test. Ferritin and electrophoresis are needed to confirm.

What to Do If You Have Low Hb + Low MCV

1. Check serum ferritin first. If ferritin is low (below 30 ng/mL), iron deficiency is very likely - start iron supplements and retest in 3 months.

2. If ferritin is normal despite low MCV and low Hb: suspect thalassaemia trait. Order haemoglobin electrophoresis.

3. If you do not respond to iron supplements after 6-8 weeks: reconsider the diagnosis - thalassaemia trait, chronic disease, or absorption problem.

4. If you are pregnant or planning pregnancy and have microcytic anaemia: early diagnosis of thalassaemia trait is important for partner screening.

Frequently Asked Questions

Can I have both iron deficiency and thalassaemia trait at the same time? Yes, and this is a common and tricky scenario. A thalassaemia carrier who develops iron deficiency will have both contributing to anaemia. Treating the iron deficiency will partially improve the haemoglobin, but MCV will remain low because of the thalassaemia component.

Do thalassaemia carriers need special diet or supplements? Generally no, unless they also have iron deficiency. They do not need extra iron supplementation routinely. They should make sure their doctor knows the diagnosis so it is not confused with iron deficiency in future checkups.

Is thalassaemia trait dangerous? Not by itself. Carriers live normal, healthy lives. The concern is reproductive - two carriers can have a child with thalassaemia major, which is a serious condition. Pre-marital or pre-pregnancy screening is strongly recommended.

Must Read

• Low Haemoglobin: Causes, Symptoms, and When to Worry - The broader picture of anaemia - all causes, severity grades, and when to act
• Iron Deficiency Anaemia and Your Ferritin Panel - The full iron workup explained - ferritin, serum iron, TIBC, and transferrin saturation

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