DoctorAIA complete blood count (CBC) is a common blood test that counts the red blood cells, white blood cells and platelets in a small blood sample and measures hemoglobin, the protein that carries oxygen. Doctors use it to check general health and to look into symptoms such as tiredness, infections, bruising or bleeding.
What the CBC measures
Conventional US units come first, with SI units in brackets. These are typical adult ranges. Your report will print its own.
| Marker | What it is | Typical adult range | If higher | If lower |
|---|---|---|---|---|
| White blood cells (WBC) | Cells that help fight infection | 4,500 to 11,000 cells/µL (4.5 to 11.0 ×10⁹/L) | Can be a sign of infection, inflammation or physical stress. Some medicines, such as corticosteroids, raise it. Less often it reflects a blood disorder. | May occur with some viral infections, certain medicines, autoimmune conditions or reduced bone marrow activity. |
| Red blood cells (RBC) | Cells that carry oxygen | Men: 4.7 to 6.1 million/µL (4.7 to 6.1 ×10¹²/L). Women: 4.2 to 5.4 million/µL (4.2 to 5.4 ×10¹²/L) | Dehydration, smoking, living at high altitude, lung or heart conditions, or a bone marrow condition that makes too many red cells. | Anemia from iron, vitamin B12 or folate shortage, blood loss, long-term illness, or red cells breaking down early. |
| Hemoglobin (Hgb) | The oxygen-carrying protein inside red cells | Men: 13.5 to 17.5 g/dL (135 to 175 g/L). Women: 12.0 to 15.5 g/dL (120 to 155 g/L) | Dehydration, smoking, high altitude, lung disease or increased red cell production. | Anemia. Common causes include iron deficiency and blood loss. A mild fall is also normal in pregnancy. |
| Hematocrit (Hct) | The share of your blood volume made up of red cells | Men: 41 to 50% (0.41 to 0.50 L/L). Women: 36 to 44% (0.36 to 0.44 L/L) | Usually moves with hemoglobin: dehydration, smoking, high altitude or lung disease. | Usually moves with hemoglobin: anemia, blood loss or extra fluid in the blood. |
| Mean corpuscular volume (MCV) | The average size of your red cells | 80 to 100 fL | Larger red cells can go with low vitamin B12 or folate, alcohol use, an underactive thyroid, liver disease or some medicines. | Smaller red cells can go with iron deficiency, thalassemia trait or long-term illness. |
| Platelets | Small cell fragments that help blood clot | 150,000 to 450,000/µL (150 to 450 ×10⁹/L) | Can follow iron deficiency, inflammation, infection, surgery or injury. Rarely it comes from a bone marrow condition. | May occur with viral infections, some medicines, liver disease, autoimmune conditions or bone marrow problems. Very low counts raise the risk of bleeding. |
Many reports also include a white cell differential. This splits the white count into neutrophils, lymphocytes, monocytes, eosinophils and basophils, each with its own range. The pattern can help your doctor see which type of white cell is raised or lowered. Ranges for children, teenagers and older adults differ from those above.
Why your lab's range may differ
Labs use different equipment and different groups of healthy people to set a reference range. A value just outside the range in the table can therefore sit inside the range on your report. Use the range printed beside your result.
A reference range is usually set so that about 95 out of 100 healthy people fall inside it. That means a small share of healthy people sit slightly outside it, so a result marked H or L is a reason to look closer, not a diagnosis.
What can affect your results
- Hydration: dehydration can concentrate the blood and push hemoglobin, hematocrit and red cell count up. Extra fluid has the opposite effect.
- Altitude and smoking: both can raise hemoglobin.
- Pregnancy: blood volume expands, so hemoglobin and hematocrit often read lower.
- Recent illness or stress: infections, injury, surgery and intense exercise can change the white cell count for a short time.
- Medicines: corticosteroids can raise neutrophils, while some antibiotics, anti-seizure medicines, chemotherapy and other drugs can lower blood counts.
How the results fit together
Doctors rarely read one number alone. Some common patterns:
- Low hemoglobin with small red cells (low MCV) often points toward iron deficiency, and your doctor may check ferritin next. See ferritin and iron tests.
- Low hemoglobin with large red cells (high MCV) can go with low vitamin B12 or folate, alcohol use, thyroid problems or some medicines.
- A raised white cell count alongside fever or other symptoms may suggest infection or inflammation. The differential helps narrow it down.
- Several cell types running low at once may lead your doctor to repeat the test or look at the cells under a microscope.
When to contact a doctor
A single mildly abnormal value with no symptoms is often rechecked rather than treated. Still, tell your doctor about any flagged result, and book an appointment promptly if you also notice:
- unusual tiredness, paleness, a fast heartbeat or shortness of breath on light activity
- fevers or infections that keep coming back or do not clear
- easy bruising, frequent nosebleeds or bleeding gums
- blood in your stool or urine, or very heavy periods
Seek urgent care for chest pain, fainting, severe shortness of breath, black tarry stools or bleeding that will not stop. In an emergency, call your local emergency number.
Frequently asked questions
What is a normal CBC result?
Typical adult values are a white cell count of 4,500 to 11,000 per microliter, hemoglobin of about 13.5 to 17.5 g/dL in men and 12.0 to 15.5 g/dL in women, and platelets of 150,000 to 450,000 per microliter. Ranges differ between labs, so compare your result with the range printed on your own report.
What does a high white blood cell count mean?
A high white cell count can be a sign of infection, inflammation or physical stress, and some medicines such as corticosteroids raise it. Less often it relates to a blood disorder. The differential, which shows which type of white cell is raised, helps your doctor decide what to check next.
What does low hemoglobin mean?
Low hemoglobin is called anemia. Common causes include iron deficiency, blood loss, low vitamin B12 or folate, and long-term illness. The size of your red cells (MCV) and other tests such as ferritin help your doctor look for the cause. Mild drops are also normal in pregnancy.
Do I need to fast for a CBC?
A CBC on its own does not usually need fasting. If it is drawn together with tests such as glucose or cholesterol, you may be asked to fast for several hours. Follow the instructions you were given when the test was ordered, and ask the lab if you are unsure.