Blood Tests for Anemia: How to Find the Cause
Anemia is a symptom, not a diagnosis — it has many causes that require completely different treatments. The pattern of CBC, iron studies, B12, folate, and reticulocyte count together identify the cause and direct the correct management.
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Anemia is defined as haemoglobin below 13.5 g/dL in men and below 12.0 g/dL in women. The CBC (complete blood count) detects anemia — but it also provides critical clues to the cause through the MCV (mean corpuscular volume), the size of red blood cells.
| MCV (cell size) | Type of Anemia | Common Causes |
|---|---|---|
| Low MCV (below 80 fL) — Microcytic | Small red cells | Iron deficiency (most common), thalassaemia, anaemia of chronic disease |
| Normal MCV (80–100 fL) — Normocytic | Normal-sized red cells | Anaemia of chronic disease, haemolysis, kidney disease, blood loss, hypothyroidism |
| High MCV (above 100 fL) — Macrocytic | Large red cells | B12 deficiency, folate deficiency, alcohol, hypothyroidism, liver disease, some medications |
Classic Symptom Picture Below Hb 10 g/dL
Iron Deficiency Anemia: The Most Common Type
Iron deficiency is the most common cause of anemia worldwide. The blood pattern: low haemoglobin, low MCV (microcytic), low ferritin (the most sensitive early marker), low serum iron, high TIBC (total iron-binding capacity), and transferrin saturation below 16%. Ferritin falls before haemoglobin drops — checking ferritin catches iron depletion before clinical anemia develops. In women of reproductive age, ferritin below 30 ng/mL is abnormal even with a normal haemoglobin.
B12 and Folate Deficiency: The Macrocytic Pattern
Both B12 and folate deficiency produce large red blood cells (high MCV) and, in more advanced deficiency, low haemoglobin with hypersegmented neutrophils on the blood smear. Critically, B12 and folate must both be tested simultaneously — treating folate without testing B12 can resolve the anaemia while allowing neurological B12 deficiency to progress silently. Serum B12 below 300 pg/mL warrants methylmalonic acid (MMA) testing to confirm functional deficiency.
Anemia of Chronic Disease
In chronic inflammation, infection, cancer, autoimmune conditions, or kidney disease, the body deliberately sequesters iron and reduces red cell production as part of an immune response. The blood pattern: mildly low haemoglobin, normal or low MCV, normal or elevated ferritin (ferritin is an acute-phase reactant and rises during inflammation), low serum iron, and low or normal TIBC. Treatment targets the underlying condition rather than iron supplementation — giving iron in anaemia of chronic disease rarely helps and can be harmful.
MCV Is the Key to Finding the Cause
Small red cells (MCV <80 fL) = iron deficiency. Large red cells (MCV >100 fL) = B12 or folate deficiency. Normal-sized cells (MCV 80–100 fL) with low haemoglobin suggests chronic disease anaemia, kidney disease, or haemolysis. MCV guides which deficiency to investigate and treat first.
When Anemia Is Found: What to Do Next
The immediate next step after finding anemia on a CBC is identifying the cause — not starting iron supplements. Iron supplementation in macrocytic anemia or anaemia of chronic disease is ineffective or potentially harmful. The reticulocyte count (immature red cells) distinguishes between reduced red cell production (low reticulocytes) and increased red cell destruction/loss (high reticulocytes), narrowing the differential significantly. Discuss any confirmed anaemia with your healthcare provider before starting supplements.
Medical Disclaimer
This article is for educational purposes only and does not constitute medical advice. Reference ranges, supplement dosages, and nutritional information mentioned are general educational guidance from published research—not personalised recommendations. Do not use this content to self-diagnose or self-treat any condition. Always consult a qualified healthcare provider before making any changes to your health regimen, medications, or supplements.
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