[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical skills](https://durgical.com/en/clinical-skills/) › [Reading lab results](https://durgical.com/en/clinical-skills/areas/labs/) › Reading a full blood count

# Reading a full blood count

- Interpretation

A routine blood test counts the red cells, white cells and platelets and measures their size, which points to many common diseases.

Updated Nov 13, 2026

## Key points

- Read each line separately (red cells, white cells, platelets), then ask whether more than one line is affected.
- Classify anaemia by MCV: microcytic (iron deficiency, thalassaemia trait), normocytic (chronic disease, kidney disease, acute loss, haemolysis), macrocytic (B12 or folate deficiency, alcohol, liver disease, drugs).
- The reticulocyte count separates a marrow that is not producing (low) from bleeding or haemolysis (high).
- Use absolute counts for the differential: a high percentage of lymphocytes may only reflect a low neutrophil count.
- Pancytopenia, blasts or a sudden unexplained change needs a blood film and urgent senior or haematology review.

On this page

- The basics Key points
- What it is
- Reference values and concepts
- Reading it step by step
- Patterns and likely causes
- Patterns and common mistakes
- Next step
- Memory aid
- Elsewhere in DurgiPedia

- Study Test yourself

- Connections Connections

## What it is

The full (complete) blood count is the most requested blood test, and reading it well is a basic skill for every station and ward round. An automated analyser reports three cell lines. The red cell line gives haemoglobin, haematocrit, the red cell count and the indices: mean cell volume (MCV, the size of the cells), mean cell haemoglobin (MCH) and its concentration (MCHC), and the red cell distribution width (RDW, how much the sizes vary). The white cell line gives the total white count and the differential, best read as absolute counts of neutrophils, lymphocytes, monocytes, eosinophils and basophils rather than percentages. The platelet line gives the count and often the mean platelet volume. A good reading is systematic: one line at a time, then the combination, always against the patient's age, sex, pregnancy, altitude, previous results and clinical picture. Anaemia is classified by MCV into microcytic, normocytic and macrocytic, and the reticulocyte count then separates poor production from loss or destruction. When two or three lines are low together (bicytopenia or pancytopenia), or abnormal cells are flagged, the analyser's numbers are not enough: a blood film and often a senior or haematology opinion are needed.

## Reference values and concepts

- Haemoglobin, usual adult ranges: men about 13.5–17.5 g/dL, women about 12.0–15.5 g/dL; lower in pregnancy, higher at altitude (ranges vary by laboratory).
- MCV about 80–100 fL; MCH about 27–33 pg; MCHC about 32–36 g/dL; RDW about 11.5–14.5%.
- White cells about 4.0–11.0 × 10⁹/L; neutrophils about 2.0–7.5 × 10⁹/L; lymphocytes about 1.0–4.0 × 10⁹/L; eosinophils below about 0.5 × 10⁹/L.
- Platelets about 150–400 × 10⁹/L; reticulocytes about 0.5–2.5% of red cells.
- A high RDW means mixed cell sizes: early iron deficiency, a combined deficiency, or a recent transfusion.
- Neutropenia is graded by the absolute neutrophil count: below about 1.5 mild, below 1.0 moderate, below 0.5 × 10⁹/L severe.

## Reading it step by step

- Check the patient, date and sample, and find previous results — a trend is worth more than a single value.
- Read haemoglobin against age, sex, pregnancy and altitude — the same number can be normal for one patient and low for another.
- If anaemic, look at the MCV and then the RDW — size sorts the causes into microcytic, normocytic or macrocytic.
- Ask for or read the reticulocyte count — low means poor production; high means bleeding or haemolysis.
- Read the total white count, then each cell type as an absolute count — percentages mislead when the total is abnormal.
- Read the platelets and look for analyser flags such as clumps — clumping in the tube gives a falsely low count.
- Combine the three lines: one, two or all three abnormal — bicytopenia or pancytopenia points to the marrow or the spleen.
- Match the pattern to the history and examination, then decide on the film and further tests — numbers alone rarely give the diagnosis.

## Patterns and likely causes

| Pattern | Points towards | Next test |
| --- | --- | --- |
| Low Hb, low MCV, high RDW | Iron deficiency | Ferritin, iron studies, look for blood loss |
| Low Hb, low MCV, normal RDW, high red cell count | Thalassaemia trait | Haemoglobin electrophoresis, ferritin |
| Low Hb, normal MCV, low reticulocytes | Anaemia of chronic disease, kidney disease, early marrow failure | Creatinine, inflammatory markers, film |
| Low Hb, normal or high MCV, high reticulocytes | Bleeding or haemolysis | Bilirubin, LDH, haptoglobin, film, direct antiglobulin test |
| Low Hb, high MCV, hypersegmented neutrophils | B12 or folate deficiency | B12, folate, film |
| Low Hb, low white cells, low platelets | Marrow failure or infiltration, hypersplenism, severe deficiency | Film, reticulocytes, urgent haematology |
| Very high white cells with blasts | Acute leukaemia | Film, urgent haematology |
| Isolated low platelets, otherwise normal count | Immune thrombocytopenia, drugs, clumping artefact | Film, repeat in citrate tube |

## Patterns and common mistakes

- Microcytic with a low ferritin is iron deficiency; in an adult man or a woman after menopause, look for gastrointestinal blood loss.
- Microcytosis with a normal or high red cell count and a near-normal haemoglobin suggests thalassaemia trait rather than iron deficiency.
- Macrocytosis with hypersegmented neutrophils suggests B12 or folate deficiency; without them, think alcohol, liver disease, hypothyroidism or drugs.
- Neutrophilia: bacterial infection, steroids, stress or smoking; lymphocytosis: viral infection or chronic lymphocytic leukaemia; eosinophilia: allergy, parasites, drugs.
- Common mistakes: ignoring the trend, reading percentages instead of absolute counts, trusting a low platelet count without checking for clumps, missing a sample drawn above a drip.
- Haemoglobin may be normal in the first hours after acute bleeding: haemodilution comes later, so judge blood loss clinically.

## Next step

- Unexplained anaemia: order ferritin and iron studies, B12 and folate, reticulocytes and a blood film, guided by the MCV.
- Call a senior now for haemoglobin falling with signs of bleeding, a neutrophil count below 0.5 × 10⁹/L with fever, platelets below about 20 × 10⁹/L, or blasts.
- Fever with severe neutropenia is treated as neutropenic sepsis: cultures and broad-spectrum antibiotics without waiting for results.
- Repeat a result that does not fit the patient (clotted, diluted or wrong-patient sample) before acting on it.
- Mild isolated abnormalities in a well patient: repeat in a few weeks and document the plan.

## Memory aid

Microcytic causes, TAILS: Thalassaemia, Anaemia of chronic disease, Iron deficiency, Lead poisoning, Sideroblastic anaemia.

## Test yourself

Think of the answer, then tap the question to see it.

## Why should the white cell differential be read as absolute counts rather than percentages?

Because percentages depend on the total: a high lymphocyte percentage may only mean the neutrophils are low. Absolute counts show which cell line is truly abnormal.

## What does the reticulocyte count add when a patient has a normocytic anaemia?

It tells whether the marrow is responding. A low count points to poor production (chronic disease, kidney disease, marrow failure); a high count points to bleeding or haemolysis.

## A 34-year-old woman has Hb 10.1 g/dL, MCV 71 fL and a high RDW. What is the most likely cause and the next test?

Iron deficiency anaemia, often from heavy menstrual loss. The next test is ferritin, with iron studies, and a history of bleeding and diet.

## A 68-year-old man has Hb 8.2 g/dL, white cells 1.9 × 10⁹/L and platelets 45 × 10⁹/L. How do you describe this and what comes next?

Pancytopenia. It suggests marrow failure or infiltration, hypersplenism or a severe deficiency; a blood film, reticulocytes, B12 and folate and an urgent haematology opinion are next.

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## Elsewhere in DurgiPedia

- [Anemia physiology and compensation](https://durgical.com/en/physiology/anemia-physiology-and-compensation/)
- [Hematopoiesis](https://durgical.com/en/physiology/hematopoiesis/)
- [White blood cells: types and functions](https://durgical.com/en/physiology/white-blood-cells-types-and-functions/)
- [Anaemia: classification](https://durgical.com/en/general-pathology/anaemia-classification/)
- [Anemia syndrome](https://durgical.com/en/clinical-exam/anemia-syndrome/)
- [Blood cells on a smear: erythrocytes, leukocytes, platelets](https://durgical.com/en/histology/blood-cells-on-a-smear-erythrocytes-leukocytes-platelets/)

## Connections

## Related 5

- [The peripheral blood smear: what to look for](https://durgical.com/en/clinical-skills/the-peripheral-blood-smear-what-to-look-for/)
- [Coagulation tests: PT/INR, aPTT, fibrinogen and D-dimer](https://durgical.com/en/clinical-skills/coagulation-tests-pt-inr-aptt-fibrinogen-and-d-dimer/)
- [Electrolytes, urea and creatinine](https://durgical.com/en/clinical-skills/electrolytes-urea-and-creatinine/)
- [Liver tests: hepatocellular or cholestatic?](https://durgical.com/en/clinical-skills/liver-tests-hepatocellular-or-cholestatic/)
- [CRP, ESR and procalcitonin](https://durgical.com/en/clinical-skills/crp-esr-and-procalcitonin/)

## Related conditions 10

- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)
- [Anaemia of chronic disease (anaemia of inflammation)](https://durgical.com/en/conditions/anaemia-of-chronic-disease/)
- [Thalassemias (α and β)](https://durgical.com/en/conditions/thalassemias/)
- [Megaloblastic anaemia (B12 or folate deficiency)](https://durgical.com/en/conditions/megaloblastic-anaemia/)
- [Aplastic anaemia](https://durgical.com/en/conditions/aplastic-anaemia/)
- [Immune thrombocytopenia (ITP)](https://durgical.com/en/conditions/immune-thrombocytopenia/)
- [Acute leukaemia (AML and ALL)](https://durgical.com/en/conditions/acute-leukaemia/)
- [Polycythaemia vera](https://durgical.com/en/conditions/polycythaemia-vera/)
- [Febrile neutropenia](https://durgical.com/en/conditions/febrile-neutropenia/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: [Normal and Abnormal Complete Blood Count With Differential](https://www.ncbi.nlm.nih.gov/books/NBK604207/) (StatPearls (NCBI Bookshelf), 2024).

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