Lab test5 min read
Complete blood count (CBC)
A group of blood tests that measure the number and size of blood cells — red cells, white cells, platelets and haemoglobin. It is often part of a routine check-up and helps diagnose anaemia and infections.
A complete blood count measures red cells, which carry oxygen; white cells, which fight infection; platelets, which help blood clot; and haemoglobin, haematocrit and the average red cell size (MCV). It is also called a full blood count.
NHLBI notes that red cell count, haemoglobin and MCV above or below normal may indicate anaemia, and that a low haematocrit may be a sign of it. A CBC with differential is a separate test that counts each of the five main types of white blood cell. Results are affected by diet, activity, medicines, menstrual periods and hydration.
What is it?
According to MedlinePlus, a complete blood count is a group of tests that measure the number and size of different blood cells; it is also called a full blood count. It measures:
| Component | What it is |
|---|---|
| Red blood cells | Carry oxygen from the lungs to the body. |
| White blood cells | Fight infections and disease; the standard test gives their total count. |
| Platelets | Help the blood clot to stop bleeding. |
| Haemoglobin | An iron-rich protein in red cells that carries oxygen. |
| Haematocrit | The share of whole blood made up of red cells. |
| Mean corpuscular volume (MCV) | The average size of the red cells. |
Why is it done?
It is often part of a routine check-up of your overall health, and helps diagnose blood diseases, infections and immune disorders, and monitor conditions or treatments that affect blood cell counts, such as anaemia and blood cancers.
Preparation
Usually no preparation is needed; you may be asked to fast for a few hours if other tests are run on the same sample.
How is it done?
A health professional takes a blood sample from a vein in your arm with a small needle. It usually takes less than five minutes, and you may feel a brief sting.
Understanding the result
According to MedlinePlus, abnormal red cell, haemoglobin or haematocrit levels may point to dehydration, anaemia, heart disease or low iron. A low white cell count may point to an autoimmune disorder, a bone marrow disorder or cancer, and a high count to an infection or a reaction to a medicine.
NHLBI publishes these reference ranges for adults:
| Component | Men | Women |
|---|---|---|
| Red blood cells | 5 to 6 million cells/mcL | 4 to 5 million cells/mcL |
| Haemoglobin | 14 to 17 g/dL | 12 to 15 g/dL |
| Haematocrit | 41% to 50% | 36% to 44% |
| Mean corpuscular volume (MCV) | 80 to 95 femtolitres | 80 to 95 femtolitres |
NHLBI says these ranges vary with sex, age, altitude and race, so rely on the reference range in your laboratory report and on your doctor's interpretation.
Risks and side effects
There is very little risk: you may have slight pain or bruising where the needle went in, which usually fades quickly.
Limits of the test
An abnormal result doesn't always mean a condition that needs treatment: diet, activity, medicines, menstrual periods and hydration all affect it. It is one tool among many that your doctor interprets with your history and symptoms, and more tests may be needed.
After the test
Depending on the likely cause, a CBC with differential or iron tests such as ferritin may follow. For anaemia, NHLBI lists further tests such as colonoscopy or upper endoscopy to look for bleeding, urine tests and bone marrow tests.
Common questions
Do I need to fast before a CBC?
Usually not, unless other tests are run on the same sample.1
Does an abnormal result mean I am ill?
Not always: diet, activity, medicines, periods and hydration affect the result, and your doctor reads it with your symptoms.1
What is the difference between a CBC and a CBC with differential?
The standard test gives the total white cell count; the differential counts each of the five main types.1
Questions for your doctor
- Which values in my result are outside the range, and why?
- Do I need iron tests or other tests?
- Could my medicines or condition affect the result?
- When should I repeat the test?
References
- 1MedlinePlus / NLM. Complete Blood Count (CBC). medlineplus.gov/lab-tests/complete-blood-count-cbc/Health agencies & guidelines · Accessed 2026-10-11
- 2NHLBI / NIH. Anemia — Diagnosis. www.nhlbi.nih.gov/health/anemia/diagnosisHealth agencies & guidelines · Accessed 2026-10-11
- 3MedlinePlus / NLM. Iron Tests. medlineplus.gov/lab-tests/iron-tests/Health agencies & guidelines · Accessed 2026-10-11
Review status: Edited content · Last updated:
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Educational content only — no diagnosis, and no substitute for a clinician.