Complete Blood Count (CBC)
Last updated: April 10, 2026
A Complete Blood Count (CBC) is one of the most commonly ordered blood tests. It measures the major components of blood — red blood cells, white blood cells, hemoglobin, hematocrit, and platelets — to give healthcare providers a broad picture of overall health.
What Is a CBC?
A Complete Blood Count is a routine blood test that counts and measures the cells circulating in the bloodstream. It looks at three main types of blood cells: red blood cells that carry oxygen, white blood cells that fight infection, and platelets that help blood clot.
Healthcare providers order a CBC for many reasons. It may be part of a routine annual checkup, or it may be used to investigate symptoms such as fatigue, weakness, bruising, or fever. The test can also help monitor chronic conditions and track the effects of certain medications on blood cell counts.
No fasting is required before a CBC. The test involves a simple blood draw, and results are usually available within 24 hours. A CBC is often ordered alongside other panels like the Comprehensive Metabolic Panel (CMP) for a more complete health picture.
What Does a CBC Measure?
A CBC typically includes the following components. Each plays a different role in maintaining health.
Hemoglobin (Hgb)
The oxygen-carrying protein inside red blood cells. Hemoglobin is the primary marker used to diagnose anemia. Low levels may indicate iron deficiency, chronic disease, or blood loss. High levels could suggest dehydration or a bone marrow disorder.
Typical range: Men: 13.5 – 17.5 g/dL • Women: 12.0 – 16.0 g/dL
White Blood Cells (WBC)
White blood cells are the body's defense system. They fight bacteria, viruses, and other threats. A CBC measures the total WBC count, and a differential breaks it down into five types: neutrophils, lymphocytes, monocytes, eosinophils, and basophils.
Typical range: 4,500 – 11,000 cells/mcL
Platelets
Small cell fragments that help blood clot and stop bleeding. Too many platelets may increase the risk of blood clots, while too few may lead to excessive bleeding or bruising.
Typical range: 150,000 – 400,000 cells/mcL
Hematocrit (Hct)
The percentage of total blood volume made up of red blood cells. Like hemoglobin, hematocrit helps evaluate anemia and hydration status. It tends to rise with dehydration and drop with blood loss or fluid overload.
Typical range: Men: 38.3% – 48.6% • Women: 35.5% – 44.9%
Red Blood Cells (RBC)
The total number of red blood cells in a sample. Red blood cells carry oxygen from the lungs to every tissue in the body and return carbon dioxide to the lungs. The RBC count is often interpreted alongside hemoglobin and hematocrit.
Typical range: Men: 4.7 – 6.1 million cells/mcL • Women: 4.2 – 5.4 million cells/mcL
Mean Corpuscular Volume (MCV)
The average size of red blood cells. MCV helps classify the type of anemia. Small cells (microcytic) may suggest iron deficiency. Large cells (macrocytic) may suggest vitamin B12 or folate deficiency.
Typical range: 80 – 100 fL
Red Cell Distribution Width (RDW)
Measures how much red blood cells vary in size. A high RDW means red blood cells are more varied, which may indicate iron deficiency anemia, mixed nutritional deficiencies, or chronic inflammation.
Typical range: 11.5% – 14.5%
CBC Reference Ranges at a Glance
Reference ranges may vary by laboratory, testing method, age, sex, and other factors. Always use the ranges provided on the lab report.
| Component | Men | Women | Units |
|---|---|---|---|
| RBC | 4.7 – 6.1 | 4.2 – 5.4 | million cells/mcL |
| Hemoglobin | 13.5 – 17.5 | 12.0 – 16.0 | g/dL |
| Hematocrit | 38.3% – 48.6% | 35.5% – 44.9% | % |
| WBC | 4,500 – 11,000 | cells/mcL | |
| Platelets | 150,000 – 400,000 | cells/mcL | |
| MCV | 80 – 100 | fL | |
| RDW | 11.5% – 14.5% | % | |
When Is a CBC Ordered?
A healthcare provider may order a CBC for many reasons, including:
- Routine health screenings as part of an annual physical exam
- Investigating symptoms such as fatigue, weakness, fever, bruising, or unexplained weight loss
- Monitoring chronic conditions like kidney disease, autoimmune disorders, or cancer
- Pre-surgical evaluation to check blood counts before a procedure
- Medication monitoring for drugs that may affect blood cells, such as chemotherapy
- Following up on infections to track the immune response over time
- Screening for blood disorders like anemia, polycythemia, or thrombocytopenia
What Abnormal Results May Mean
A single abnormal value does not necessarily indicate a problem. Results are usually interpreted together and in the context of symptoms, medical history, and other test results.
High Values May Indicate
- High RBC/Hgb/Hct: Dehydration, polycythemia vera, lung disease, or living at high altitude
- High WBC: Bacterial infection, inflammation, stress response, allergic reaction, or leukemia
- High platelets: Iron deficiency, inflammation, infection, or a bone marrow disorder
- High MCV: Vitamin B12 or folate deficiency, liver disease, or hypothyroidism
- High RDW: Mixed nutritional deficiencies or early iron deficiency
Low Values May Indicate
- Low RBC/Hgb/Hct: Iron deficiency anemia, chronic blood loss, kidney disease, or bone marrow failure
- Low WBC: Viral infection, autoimmune disorder, bone marrow suppression, or certain medications
- Low platelets: Autoimmune conditions (ITP), viral infections, liver disease, or medication side effects
- Low MCV: Iron deficiency anemia or thalassemia
Frequently Asked Questions
What does a CBC blood test measure?
A Complete Blood Count measures red blood cells (RBC), hemoglobin, hematocrit, white blood cells (WBC), platelets, mean corpuscular volume (MCV), and red cell distribution width (RDW). It provides a broad overview of blood health and can help detect anemia, infection, clotting disorders, and blood cancers.
Do fasting requirements apply to a CBC?
No, fasting is generally not required for a CBC. However, if a CBC is ordered alongside other tests that do require fasting (like a metabolic panel or lipid panel), the healthcare provider may ask the patient to fast.
How often should a CBC be performed?
For healthy adults, a CBC is typically included as part of an annual physical exam. People with chronic conditions, those on medications that affect blood cells, or those experiencing symptoms like unexplained fatigue or bruising may need more frequent testing as recommended by a healthcare provider.
What does a low hemoglobin level mean?
Low hemoglobin may indicate anemia, which can result from iron deficiency, vitamin B12 or folate deficiency, chronic disease, blood loss, or bone marrow problems. A healthcare provider considers the full CBC results along with symptoms and medical history to determine the cause.
What causes a high white blood cell count?
A high WBC count may indicate infection, inflammation, allergic reactions, stress, or certain medications like corticosteroids. In rare cases, it could suggest a bone marrow disorder. The WBC differential helps narrow the cause by showing which types of white blood cells are elevated.
What is the difference between a CBC and a CBC with differential?
A standard CBC measures total white blood cell count, red blood cells, hemoglobin, hematocrit, and platelets. A CBC with differential adds a breakdown of the five types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. This helps pinpoint the type of infection or immune response.
Can dehydration affect CBC results?
Yes. Dehydration can concentrate the blood, making red blood cell counts, hemoglobin, and hematocrit appear falsely elevated. Conversely, overhydration can dilute the blood and make these values appear lower than expected. Adequate hydration before the blood draw helps ensure accurate results.
What does a low platelet count mean?
A low platelet count (thrombocytopenia) may indicate an autoimmune condition, viral infection, liver disease, certain medications, or bone marrow disorders. It can increase the risk of bleeding and bruising. A healthcare provider may order additional tests to find the underlying cause.
Related Tests and Conditions
CBC Biomarkers
Related Panels
Related Conditions
Sources
- MedlinePlus. Complete Blood Count (CBC). U.S. National Library of Medicine.
- American Society of Hematology. Blood Basics.
- Mayo Clinic. Complete blood count (CBC). Mayo Foundation for Medical Education and Research.
- Lab Tests Online. Complete Blood Count (CBC). American Association for Clinical Chemistry.
- Cleveland Clinic. Complete Blood Count (CBC).
Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges vary by laboratory, age, sex, and testing method. Always discuss lab results with a qualified healthcare provider who can interpret them in the context of individual health history.