Reading a full blood count

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

1. Check the patient, date and sample, and find previous results — a trend is worth more than a single value.
2. Read haemoglobin against age, sex, pregnancy and altitude — the same number can be normal for one patient and low for another.
3. If anaemic, look at the MCV and then the RDW — size sorts the causes into microcytic, normocytic or macrocytic.
4. Ask for or read the reticulocyte count — low means poor production; high means bleeding or haemolysis.
5. Read the total white count, then each cell type as an absolute count — percentages mislead when the total is abnormal.
6. Read the platelets and look for analyser flags such as clumps — clumping in the tube gives a falsely low count.
7. Combine the three lines: one, two or all three abnormal — bicytopenia or pancytopenia points to the marrow or the spleen.
8. 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

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.

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Elsewhere in DurgiPedia

• Anemia physiology and compensation
• Hematopoiesis
• White blood cells: types and functions
• Anaemia: classification
• Anemia syndrome
• Blood cells on a smear: erythrocytes, leukocytes, platelets

Connections

← All clinical skills

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 (StatPearls (NCBI Bookshelf), 2024).

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Representative model, for educational purposes only.