Complete Blood Count (CBC)

What It Is

Blood contains three main types of cells. Red blood cells carry oxygen using hemoglobin. White blood cells help fight infection. Platelets help blood clot.

A CBC counts these cells and measures features such as their size and hemoglobin content. Together these values can give clues about anemia, infection, inflammation, bleeding or clotting problems, and some bone marrow or blood disorders.

Because a CBC is broad rather than specific, unusual results often lead to further testing rather than a diagnosis by themselves.

Common Terminology

Hemoglobin (Hgb)

The oxygen-carrying protein in red blood cells.

Hematocrit (Hct)

The percentage of blood volume made up of red blood cells.

WBC

White blood cell count, the number of infection-fighting cells in a volume of blood.

Platelets (PLT)

Small cell fragments that help blood form clots.

MCV

Mean corpuscular volume, the average size of red blood cells.

Differential

A breakdown of white blood cells into types: neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

RDW

Red cell distribution width, a measure of how much red blood cells vary in size.

Why the Test Is Done

A CBC is ordered for many reasons.

• As part of a routine health checkup.
• To look for causes of symptoms such as tiredness, weakness, fever, bruising, or bleeding.
• To help evaluate infection or inflammation.
• To screen for or follow anemia and other blood conditions.
• To monitor the effects of certain treatments, such as chemotherapy or some medications, that can affect blood cells.
• Before surgery or certain procedures.

What the Results Generally Describe

Typical adult reference ranges are listed below as general reference only. Each lab sets its own ranges, values differ by age and sex, and a clinician interprets individual results.

• White blood cell count: often about 4,500 to 11,000 cells per microliter; higher counts can be seen with infection or inflammation, and lower counts with some infections, medications, or bone marrow conditions.
• Hemoglobin: roughly 13.5 to 17.5 g/dL in adult men and 12.0 to 15.5 g/dL in adult women in many labs; low values are described as anemia.
• Hematocrit: generally moves in the same direction as hemoglobin.
• Platelets: often about 150,000 to 450,000 per microliter; low counts may be associated with bleeding or bruising, and high counts with inflammation or other conditions.
• Red cell indices such as MCV help describe the type of anemia, for example small cells (often linked to iron deficiency) or large cells (sometimes linked to low vitamin B12 or folate).

How the Test Is Done

A CBC uses a small blood sample.

• Blood is drawn from a vein in the arm into a tube containing an anti-clotting agent.
• In infants, a heel-prick sample is often used.
• An automated analyzer counts and measures the cells.
• Sometimes a lab specialist examines a blood smear under a microscope for more detail.

Preparation and What to Expect

A CBC needs little preparation.

• Fasting is not generally needed for a CBC alone, though it may be if other tests are drawn at the same time.
• The blood draw takes a few minutes and may cause brief stinging or a small bruise.
• Recent illness, intense exercise, dehydration, pregnancy, and high altitude can shift some values.
• Results are often available within hours to a day.

When Results Are Discussed With a Clinician

CBC results are generally reviewed together with symptoms, history, and any earlier results.

• Values slightly outside the reference range are common and are often rechecked or compared with past results.
• Larger changes or several abnormal values together are commonly followed up with additional tests.
• Low hemoglobin is often explored further to look for its cause, such as iron deficiency or blood loss.
• Very low or very high white cell or platelet counts are generally reviewed promptly.

Questions to Ask a Healthcare Professional

General questions people often find useful to bring to an appointment:

• Were any values outside this lab's reference range, and how much?
• How do these results compare with my previous CBCs?
• Could recent illness or medications have affected the numbers?
• Is a differential or blood smear part of this test?
• Are any follow-up tests usually considered for results like these?

Sources

This guide was written in plain language from the following authoritative references.

1. Complete Blood Count (CBC) — MedlinePlus (U.S. National Library of Medicine)
2. Blood Count Tests — MedlinePlus (U.S. National Library of Medicine)
3. Blood tests — NHS

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