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Complete Blood Count (CBC)
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Complete Blood Count (CBC)

A complete blood count (CBC) is a routine test that measures the different types of cells in your blood to help diagnose and monitor various health conditions.

What is a Complete Blood Count?

A complete blood count (CBC) measures the number and size of various blood components, including:

  • Red blood cells: Carry oxygen from the lungs to the body.
  • White blood cells: Fight infections and diseases.
  • Platelets: Help blood clot to stop bleeding.
  • Hemoglobin: An iron-rich protein in red blood cells that carries oxygen.
  • Hematocrit: The proportion of your blood made up of red blood cells.
  • Mean corpuscular volume (MCV): The average size of your red blood cells.

What is it used for?

The CBC is a standard part of routine checkups. It is also used to monitor conditions or treatments that affect blood cell counts, such as anemia, infections, immune system disorders, and blood cancers.

Why do I need a complete blood count?

Your provider may order a CBC to assess your overall health, help diagnose a specific medical condition, or track changes in an existing blood disorder.

What happens during the test?

A health care professional will draw a small amount of blood from a vein in your arm using a needle. The procedure is quick, typically taking less than five minutes.

Preparation and Risks

Usually, no special preparation is needed. However, if your provider has ordered additional tests, you may be required to fast (avoid eating or drinking) for several hours. There is very little risk, though you may experience minor pain or bruising at the puncture site.

What do the results mean?

Levels outside the normal range do not always indicate a condition requiring treatment, as factors like diet, activity level, medications, and hydration can influence results. Generally:

  • Red blood cell/hemoglobin/hematocrit abnormalities: May suggest dehydration, anemia, heart disease, or iron deficiency.
  • Low white cell count: May indicate an autoimmune disorder, bone marrow issue, or cancer.
  • High white cell count: May indicate an infection or a reaction to medication.

Your provider will interpret your CBC results in the context of your medical history and symptoms to determine if further testing is necessary.

Frequently Asked Questions

nemia shows as low hemoglobin (Hgb) and hematocrit (Hct) with a reduced red‑cell count. The mean corpuscular volume (MCV) helps narrow the cause: low MCV suggests iron deficiency or thalassemia; high MCV points to B12/folate deficiency or chronic disease. After confirming anemia, doctors typically order iron studies, vitamin B12/folate levels, reticulocyte count, and sometimes a stool occult blood test to identify the underlying etiology.
Inform the lab and your provider about all current medications. Some drugs can suppress bone‑marrow production, leading to low neutrophils or platelets. Your clinician may schedule the CBC at a specific point in the medication cycle (e.g., just before the next dose) to obtain the most informative result, and they may order additional tests such as a differential count or reticulocyte count to interpret drug‑related changes accurately.
Yes. Vaccinations and acute viral infections can cause a transient rise in WBCs (especially neutrophils or lymphocytes) and a mild drop in platelet count. These changes usually normalize within a week to ten days. If the CBC was drawn within a few days of a vaccine or illness, the provider may repeat the test after the acute phase to confirm baseline values.
high WBC count usually signals infection, inflammation, stress, or less commonly a bone‑marrow disorder such as leukemia. Mild elevations (10‑15 ×10⁹/L) often resolve with a simple infection; persistent counts above 20 ×10⁹/L, especially with abnormal differential patterns, warrant further work‑up (e.g., repeat CBC, peripheral smear, and possibly bone‑marrow biopsy).
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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