A complete blood count (CBC) is one of the most commonly ordered blood tests in medicine. It is used to screen for disease, monitor known conditions, follow treatment progress, and check overall health. The CBC measures the number, size, and other characteristics of the three main types of cells in your blood — red blood cells, white blood cells, and platelets — and the result is reported on a single page that can include ten or more individual measurements.
This article explains what each part of the CBC measures, why it is done, what the typical results mean, and what may need to happen next.
The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.
What is a complete blood count?
The complete blood count is a single blood test that measures and analyzes the cells circulating in your bloodstream. Blood is made up of a liquid component called plasma and three types of cells suspended in that liquid:
- Red blood cells. These cells carry oxygen from the lungs to every tissue in the body. They contain a protein called hemoglobin that binds oxygen and gives blood its red colour.
- White blood cells. These are the cells of the immune system. They fight infections and help the body recognize and destroy abnormal cells. There are several types of white blood cells, each with a different role.
- Platelets. These are tiny cell fragments that help the blood clot when a blood vessel is injured.
All three types of blood cells are produced in the bone marrow, the soft tissue inside many of the body’s larger bones. By measuring the cells circulating in the blood, the CBC provides indirect information about how well the bone marrow is working as well as how the body is carrying oxygen, fighting infection, and controlling bleeding.
Why is a CBC done?
A CBC is one of the most versatile blood tests in medicine and is ordered for many reasons, including:
- To investigate symptoms. Fatigue, weakness, fever, unexplained bruising, easy bleeding, recurrent infections, dizziness, or shortness of breath can all be investigated with a CBC.
- To screen for disease. A CBC is often included in routine medical check-ups because it can detect conditions before symptoms develop, including anemia, infections, and some blood cancers.
- To monitor a known condition. People with chronic illnesses such as kidney disease, cancer, autoimmune conditions, or bone marrow disorders often have CBCs done regularly to track changes over time.
- To follow treatment effects. Many treatments — particularly chemotherapy, radiation, and certain medications — can affect blood cell production. A CBC monitors these effects.
- Before surgery or a procedure. A CBC checks for anemia and ensures that platelet counts are adequate to prevent excessive bleeding.
- During pregnancy. A CBC is part of routine prenatal care to monitor for anemia and other changes.
How is the test performed?
A CBC is performed on a small sample of blood, usually drawn from a vein in the arm using a needle. In infants and young children, a heel-prick or finger-prick sample may be used instead. The blood is collected into a tube containing an anticoagulant — a substance that prevents the blood from clotting — and is then analyzed by automated laboratory equipment that counts and measures the cells.
For most CBCs, no special preparation such as fasting is needed. If your CBC is being done at the same time as other blood tests that require fasting, follow the instructions for those tests. Results are usually available within a few hours to a day.
If the automated count produces unusual results or specific abnormalities are suspected, a peripheral blood smear may also be performed. This is a separate test in which a drop of blood is spread on a glass slide and examined under the microscope by a pathologist or a trained laboratory technician.
What does a CBC report contain?
A CBC report typically includes 10 or more individual measurements. They are usually grouped into three sections — red blood cell measurements, white blood cell measurements, and platelet measurements — though the exact format varies between laboratories. The following sections explain what each component measures and what abnormal results may mean.
Red blood cell measurements
The CBC includes several measurements that together describe the red blood cells, the cells that carry oxygen throughout the body. These measurements work together to detect and characterize anemia (a low red blood cell count) or polycythemia (a high red blood cell count).
- RBC count (red blood cell count). The total number of red blood cells in a measured volume of blood. A typical reference range is 4.7–6.1 million cells per microlitre for adult men, 4.2–5.4 million for adult women, and 4.0–5.5 million for children. A low RBC count suggests anemia. A high RBC count can be seen in conditions where the body produces too many red blood cells, such as polycythemia vera, or in response to chronic low oxygen levels.
- Hemoglobin (HGB). The amount of hemoglobin in the blood. Hemoglobin is the protein inside red blood cells that binds and carries oxygen. A typical reference range is 13.5–17.5 grams per decilitre (g/dL) for adult men, 12.0–15.5 g/dL for adult women, and 11–13 g/dL for children. Hemoglobin is often the most clinically useful red blood cell measurement because it directly reflects the blood’s oxygen-carrying capacity. A low hemoglobin level is a defining feature of anemia.
- Hematocrit (HCT). The proportion of the blood made up of red blood cells, expressed as a percentage. A typical reference range is 45%–52% for adult men, 37%–48% for adult women, and 29%–65% for children. The hematocrit moves up and down with the hemoglobin and the RBC count, providing a similar but complementary measure.
- Mean corpuscular volume (MCV). The average size of the red blood cells. A typical reference range is 80–96 femtolitres (fL) for adults. MCV is one of the most useful measurements when investigating anemia because the size of the red blood cells helps narrow down the cause:
- Microcytic means the cells are smaller than normal (low MCV). The most common cause is iron deficiency. Other causes include thalassemia and chronic disease.
- Normocytic means the cells are normal in size. Anemia with a normal MCV can be caused by acute blood loss, kidney disease, or many chronic conditions.
- Macrocytic means the cells are larger than normal (high MCV). The most common causes are vitamin B12 deficiency, folate deficiency, alcohol-related liver disease, hypothyroidism, and certain medications.
- Mean corpuscular hemoglobin (MCH). The average amount of hemoglobin inside each red blood cell. A typical reference range is 27–33 picograms per cell (pg/cell) for adults. MCH usually moves in the same direction as MCV, so it provides similar information about the cause of anemia.
- Mean corpuscular hemoglobin concentration (MCHC). The average concentration of hemoglobin in a given volume of red blood cells. A typical reference range is 33–36 g/dL for adults. MCHC is most useful for identifying conditions in which red blood cells are particularly pale (low MCHC) or particularly dense (high MCHC).
- Red cell distribution width (RDW). A measure of how much variation there is in the size of the red blood cells. A typical reference range is 11.5%–14.5% for adults. A high RDW means that the red blood cells vary widely in size, which is often an early sign of nutritional deficiency anemia (iron, B12, or folate). RDW can also help distinguish between different types of anemia when the MCV is normal or borderline.
White blood cell measurements
The CBC measures the total number of white blood cells in the blood, which is a broad indicator of immune system activity. Most CBC reports also include a “differential” — a breakdown of the major types of white blood cells. Some laboratories report the differential automatically with every CBC; others perform it only when ordered separately or when abnormal results are detected.
- White blood cell count (WBC). The total number of white blood cells per microlitre of blood. A typical reference range for adults is 4,500–11,000 cells per microlitre. A high WBC count (called leukocytosis) is most commonly caused by infection, but can also be seen with inflammation, physical or emotional stress, certain medications, and some blood cancers including leukemia. A low WBC count (called leukopenia) can be caused by viral infections, certain medications including chemotherapy, autoimmune diseases, severe infections that have overwhelmed the bone marrow’s ability to produce cells, and bone marrow disorders.
- Differential. The breakdown of the WBC count into its specific cell types. The five main types of white blood cells, each with a different function, are neutrophils, lymphocytes, monocytes, eosinophils, and basophils. The differential is described in detail in the companion article, Understanding your white blood cell differential.
- Absolute neutrophil count (ANC). A specific measurement of the number of neutrophils in the blood. Neutrophils make up about 40%–60% of all white blood cells and are usually the first immune cells to respond to a bacterial infection. A typical reference range for adults is 1,500–8,000 cells per microlitre. The ANC is particularly important in patients receiving chemotherapy because a very low ANC (called neutropenia) significantly increases the risk of serious infection and may prompt delaying treatment or starting protective measures.
Platelet measurements
Platelets are tiny cell fragments produced in the bone marrow that play a critical role in stopping bleeding. When a blood vessel is injured, platelets gather at the site and form a plug that helps seal the wound.
- Platelet count (PLT). The number of platelets per microlitre of blood. A typical reference range is 135,000–400,000 per microlitre for adult men, 157,000–400,000 for adult women, and 150,000–300,000 for children. A low platelet count (called thrombocytopenia) increases the risk of bleeding and bruising. Common causes include certain medications, viral infections, autoimmune conditions, liver disease, and bone marrow disorders. A high platelet count (called thrombocytosis) can be seen with inflammation, iron deficiency, infection, and certain bone marrow conditions called myeloproliferative neoplasms.
- Mean platelet volume (MPV). The average size of the platelets. A typical reference range is 9.4–12.3 femtolitres for adults. MPV provides additional information about platelet production. A high MPV with a low platelet count suggests that the bone marrow is making new platelets rapidly to replace ones being destroyed or used up. A low MPV with a low platelet count suggests that the bone marrow is not producing platelets normally.
What conditions can affect a CBC?
Because the CBC reflects so many aspects of body function, an enormous range of conditions can affect its results. The following are some of the most common causes of abnormal CBC results, organized by which component is affected.
Causes of low red blood cells, hemoglobin, or hematocrit (anemia)
- Iron, vitamin B12, or folate deficiency
- Acute blood loss (such as from injury, surgery, or gastrointestinal bleeding) or chronic slow blood loss (such as from heavy menstrual periods or a slow gastrointestinal bleed)
- Chronic kidney disease, which reduces the production of erythropoietin, the hormone that signals the bone marrow to make red blood cells
- Chronic inflammatory conditions, infections, or cancer
- Bone marrow disorders such as myelofibrosis, aplastic anemia, or multiple myeloma
- Inherited conditions such as sickle cell disease and thalassemia
- Medications including chemotherapy
Causes of high red blood cells, hemoglobin, or hematocrit
- Dehydration (which concentrates the blood)
- Chronic lung disease causing low blood oxygen
- Living at high altitude
- Smoking
- Polycythemia vera, a type of myeloproliferative neoplasm
- Testosterone therapy
Causes of high white blood cell count
- Bacterial, viral, or fungal infections
- Inflammation, including conditions such as rheumatoid arthritis
- Physical or emotional stress, including after surgery or trauma
- Certain medications, including corticosteroids
- Smoking
- Leukemia and other blood cancers
Causes of low white blood cell count
- Viral infections
- Certain medications, including chemotherapy and some antibiotics
- Autoimmune diseases such as lupus
- Severe infections that overwhelm bone marrow production
- Vitamin B12 or folate deficiency
- Bone marrow disorders
Causes of low platelet count
- Viral infections
- Certain medications
- Immune thrombocytopenic purpura (ITP), an autoimmune condition
- Liver disease and an enlarged spleen
- Bone marrow disorders
- Pregnancy
Causes of high platelet count
- Iron deficiency
- Inflammation or infection
- Recent blood loss or recovery from surgery
- Following the removal of the spleen
- Essential thrombocythemia and other myeloproliferative neoplasms
What happens after the CBC?
If your CBC results are within the reference ranges and your doctor has no other concerns, no further investigation is usually needed. Your doctor may recommend a routine repeat CBC if you have a chronic condition that requires monitoring, or no repeat at all if the test was done as a one-time screening.
If a result is abnormal, the next steps depend on which measurement is abnormal, by how much, and what symptoms or other findings are present. Some possibilities include:
- Repeat the CBC. Mildly abnormal results — particularly small variations in the WBC or platelet count — sometimes resolve on their own. A repeat test in a few days or weeks can confirm whether the abnormality is real and persistent.
- Add additional blood tests. Specific patterns of abnormality often prompt follow-up testing. For example, low hemoglobin and low MCV often prompt iron studies; low hemoglobin and high MCV often prompt vitamin B12 and folate testing. A high WBC count may prompt a peripheral blood smear or additional testing for infection.
- Order imaging or other investigations. Persistent unexplained abnormalities — particularly involving anemia or low platelets — may prompt imaging, endoscopy, or other tests to look for an underlying cause.
- Refer to a specialist. Persistent or significant abnormalities may prompt referral to a hematologist, a doctor who specializes in blood diseases.
- Perform a bone marrow biopsy. If multiple components of the CBC are abnormal, or if abnormal cells are seen on the peripheral blood smear, a bone marrow biopsy may be needed to evaluate blood cell production directly. Our companion article, Understanding your bone marrow biopsy report, explains this procedure in detail.
An abnormal CBC result is not in itself a diagnosis. The CBC is a starting point. Your doctor will interpret the results in the context of your symptoms, medical history, physical examination, and any other test results before deciding whether further investigation is needed.
Questions to ask your doctor
- Were any of my CBC results outside the reference range?
- If a result is abnormal, how significant is the abnormality?
- Could any of my medications be affecting my CBC results?
- Do I need any follow-up tests to investigate the cause of an abnormal result?
- Should I have a repeat CBC, and if so, when?
- Should I be referred to a hematologist or another specialist?
- Are there lifestyle changes — such as diet or stopping smoking — that could improve my results?
- What symptoms should prompt me to contact you between visits?
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