Reading a complete blood count: red cells, white cells and platelets
- Topic
The most ordered blood test: how to read the red cell, white cell and platelet lines in order and what the common changes mean.
See it in 3D: Spleen
What it is
The complete blood count measures hemoglobin, hematocrit, red cell indices (mean corpuscular volume, red cell distribution width), the white cell count with its differential, and platelets. Reading it in a fixed order, always with the patient's clinical picture and previous results, turns numbers into a short list of causes. Reference ranges vary with the laboratory, age, sex, pregnancy and altitude, which matters in high cities of the Andes such as Bogotá.
Key facts
- Anemia (WHO thresholds at sea level): hemoglobin below 13 grams per deciliter in men, below 12 in non-pregnant women, below 11 in pregnancy; values rise at altitude.
- Mean corpuscular volume: normal about 80 to 100 femtoliters; below is microcytic, above is macrocytic.
- Red cell distribution width: high when cells vary in size (iron deficiency, mixed deficiencies); often normal in thalassemia trait.
- White cells: normal adult total about 4,000 to 11,000 per microliter; interpret each line as an absolute count, not a percentage.
- Neutropenia: absolute neutrophils below about 1,500 per microliter; below about 500 is severe, with high infection risk.
- Platelets: normal about 150,000 to 450,000 per microliter; above 450,000 is thrombocytosis; reactive counts usually stay below 1,000,000.
- Two or three low lines at once (pancytopenia) or blasts on the smear point to the bone marrow.
Step by step
- Check the patient's name, date and sample; compare with any previous count.
- Read hemoglobin and hematocrit: is there anemia or a high red cell mass?
- Classify any anemia by mean corpuscular volume, then look at red cell distribution width and the reticulocyte count.
- Read the total white count and then each type as an absolute number: neutrophils, lymphocytes, monocytes, eosinophils, basophils.
- Read the platelets; if low and unexpected, ask for a smear to look for clumping.
- Ask whether one line or several lines are abnormal; several lines suggest a marrow or systemic process.
- Review the peripheral smear when anything is unexplained, and link the result to the history and exam.
Common complete blood count patterns
| Finding | Common meaning | Next step |
|---|---|---|
| Low hemoglobin, low MCV, high RDW | Iron deficiency | Ferritin; look for blood loss |
| Low MCV, high red cell count, normal ferritin | Thalassemia trait | Hemoglobin electrophoresis |
| Low hemoglobin, high MCV | Vitamin B12 or folate deficiency, alcohol, liver disease | B12 and folate levels, smear for hypersegmented neutrophils |
| Low absolute neutrophils | Medicines, viral infection, marrow disease, benign ethnic neutropenia | Repeat, review medicines, smear |
| Blasts or several low lines | Acute leukemia or marrow failure | Urgent smear and hematology referral |
| High hemoglobin and hematocrit | Dehydration, hypoxia, smoking, altitude, polycythemia vera | Repeat when hydrated, oxygen saturation, JAK2 testing if unexplained |
| High platelets | Reactive causes much more often than essential thrombocythemia | Ferritin, inflammatory markers, look for infection or bleeding |
| Isolated low platelets | Immune thrombocytopenia, medicines, infection, clumping | Smear, review medicines, HIV and hepatitis C tests |
Clinical notes
- Fever with severe neutropenia is a medical emergency (febrile neutropenia).
- Very low platelets with bleeding from mucous membranes, or with anemia and confusion or kidney injury, need same-day evaluation.
- Blasts on the smear or pancytopenia with fever, bleeding or bone pain: urgent hematology referral.
- Microcytic anemia in an adult man or a postmenopausal woman: look for gastrointestinal blood loss.
- A very high white count with immature cells may be leukemia rather than infection; look at the smear.
In the lab
- Peripheral blood smear and reticulocyte count.
- Ferritin and iron studies; vitamin B12 and folate.
- Hemoglobin electrophoresis when thalassemia is suspected.
- Bone marrow examination when the cause of cytopenias or blasts is not clear.
Test yourself
Think of the answer, then tap the question to see it.
How is anemia classified by mean corpuscular volume?
Microcytic (below about 80 femtoliters), normocytic (about 80 to 100) and macrocytic (above about 100).
What does the reticulocyte count add to the reading of an anemia?
It separates low production by the marrow (low count) from loss or destruction of red cells (high count).
What is the most likely cause of a platelet count of 600,000 per microliter in a patient with iron deficiency?
Reactive thrombocytosis due to the iron deficiency; essential thrombocythemia is considered only after reactive causes are excluded.
Why should white cells be read as absolute counts rather than percentages?
A normal percentage can hide a low absolute neutrophil count when the total white count is low.
Connections
See it in 3D1
The structures of Durgical's 3D anatomy where it is found.
Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis.
Sources: Complete Blood Count (CBC) (MedlinePlus Medical Tests, U.S. National Library of Medicine, 2025); Reactive Thrombocytosis (Secondary Thrombocythemia) (MSD Manual Professional Edition, 2025).
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Representative model, for educational purposes only.