Blood🩸Doctor · Patient guide
Anaemia explained:
a simple roadmap from blood test to cause
Understand the pattern, find the cause — a calm, practical guide to the questions behind a low haemoglobin result and the tests that help answer them.
Anaemia is common, but it is not a diagnosis in its own right. It is a signal that the blood is carrying less oxygen than it should. The useful question is not only “How low is the haemoglobin?” but also “Why has it fallen?”
“Treat the cause, not just the number.”
What anaemia can feel like
Some people have no symptoms, especially when anaemia develops slowly. Others notice tiredness, reduced exercise tolerance, breathlessness, palpitations, dizziness, headaches or paler-than-usual skin. Symptoms do not reliably tell us the cause, so a blood test and clinical assessment matter.
The simple roadmap: how clinicians work it out
A full blood count (FBC) confirms whether anaemia is present. The next tests are chosen to answer a small number of practical questions rather than ordering everything at once.
“The MCV gives the map; the reticulocyte count tells us whether the bone marrow is moving.”
Cell size: the first useful clue
| What the FBC shows | What it means in simple terms | Common directions for the next step |
|---|---|---|
| Small cells Microcytic pattern | Red cells are smaller than expected. | Iron deficiency is common. Thalassaemia trait and inflammation-related iron restriction are other important possibilities. |
| Usual-sized cells Normocytic pattern | Cell size is not the main clue, so the marrow response and clinical context become especially useful. | Consider recent bleeding, haemolysis, kidney disease, inflammation, medicines and bone-marrow disorders, among other causes. |
| Large cells Macrocytic pattern | Red cells are larger than expected. | Consider vitamin B12 or folate deficiency, alcohol/liver disease, thyroid disease, medicines, reticulocytosis and marrow disorders. |
The tests, translated
| Test | Plain-language purpose | What it may help distinguish |
|---|---|---|
| FBC | Counts red cells, white cells and platelets; measures haemoglobin and cell size. | Anaemia pattern and whether other blood-cell lines are affected. |
| Reticulocyte count | Counts newly made red cells. | Whether the marrow is responding appropriately. |
| Ferritin + iron studies | Looks at stored and available iron. | Iron deficiency versus iron being less available during inflammation. Ferritin can rise with inflammation, so it is interpreted in context. |
| Blood film | A laboratory professional examines cells under a microscope. | Shape, colour and maturity clues that automated counters cannot fully explain. |
| LDH, bilirubin, haptoglobin and DAT | Looks for red-cell breakdown and, when appropriate, an immune cause. | Haemolysis and possible autoimmune haemolysis. |
| B12, folate, kidney, liver and thyroid tests | Checks conditions needed for healthy red-cell production. | Potential reversible causes, selected according to the FBC pattern and clinical story. |
Four patterns worth recognising
1. Iron deficiency: replace iron and explain the deficit
Iron deficiency is a frequent cause of anaemia. It may follow blood loss (including heavy periods or gastrointestinal blood loss), increased demand in pregnancy, reduced iron intake or reduced absorption. A low ferritin often supports depleted iron stores, but no laboratory value should be interpreted alone. The key principle is to identify the cause of the iron deficit as well as restoring iron.
small cellslow ferritin often helpslook for the reason
2. Inflammation or kidney disease: the marrow may lack the right signal or iron access
During inflammation, iron can be held in storage and become less available for red-cell production. In chronic kidney disease, damaged kidneys may not make enough erythropoietin, a hormone that tells the bone marrow to produce red cells. These patterns can overlap with true iron deficiency, so clinicians often check iron stores before deciding on treatment.
usual-sized or mildly small cellsclinical context matterscheck kidney function and iron
3. B12 or folate deficiency: large cells can be an early warning
Vitamin B12 and folate are needed to make healthy red cells. Vitamin B12 deficiency can also affect the nervous system; pins and needles, balance change, memory change or new numbness should be mentioned promptly to a clinician. The cause may involve diet, absorption, medicines or an autoimmune condition, not simply food intake.
large cellsneurological symptoms matterdo not self-treat blindly
4. Haemolysis or bleeding: the body may be trying hard to catch up
When red cells are being lost through bleeding or destroyed too early (haemolysis), the marrow often increases output. Dark urine, jaundice, a sudden fall in haemoglobin, a high reticulocyte count or a raised bilirubin/LDH can prompt further assessment. Blood-film findings and a direct antiglobulin test (DAT) may help distinguish possible causes.
raised reticulocytes can be a cluelook for bleeding or breakdownmay need prompt assessment
Questions to bring to an appointment
- Has the haemoglobin changed quickly, or has it been low for a long time?
- Are the red cells small, usual-sized or large?
- Is the reticulocyte count showing a good marrow response?
- Do iron studies, B12/folate or kidney tests help explain the pattern?
- Is there any evidence of blood loss, inflammation, haemolysis, an inherited condition or a medication effect?
- What is the plan to treat the cause and confirm that the blood count recovers?
“A blood result starts a conversation; it should not end one.”
A final word
Anaemia is best approached calmly and systematically. The combination of symptoms, FBC pattern, reticulocyte count, blood film and carefully chosen tests usually narrows the answer quickly. If a cause is not obvious, if more than one blood-cell line is abnormal, or if symptoms are severe, prompt specialist assessment may be needed.
Reliable further reading
- NHS: Iron deficiency anaemia
- NHS: Vitamin B12 or folate deficiency anaemia
- National Kidney Federation: Anaemia in kidney failure
- British Society for Haematology: laboratory diagnosis of iron deficiency
Reviewed: August 2026 | Audience: adults, families and learners