A complete blood count (CBC) is a common blood test that provides important information about the types and numbers of cells in the blood. As part of a routine checkup or to help diagnose specific conditions, doctors may order a CBC to evaluate a person’s general health and screen for a variety of disorders, including anemia, infection, and leukemia.
What is checked in a CBC?
A CBC measures the following components in the blood:
- Red blood cells (RBCs)
- White blood cells (WBCs)
- Hemoglobin
- Hematocrit
- Platelets
Red blood cells (RBCs)
Red blood cells, also called erythrocytes, are the most common type of blood cell. Their primary role is to carry oxygen throughout the body. The CBC provides information about the number of RBCs as well as their size and shape:
| CBC Red Blood Cell Test | What It Measures | What Abnormal Results May Mean |
|---|---|---|
| RBC count | The number of RBCs | Low RBC count (anemia) or high RBC count (polycythemia) |
| MCV (mean corpuscular volume) | The average size of RBCs | Microcytic or macrocytic anemia |
| MCH (mean corpuscular hemoglobin) | The average amount of hemoglobin per RBC | Anemia or thalassemia |
| MCHC (mean corpuscular hemoglobin concentration) | The average concentration of hemoglobin per RBC | Anemia or thalassemia |
| RDW (red cell distribution width) | The variation in RBC size | Anemia, nutritional deficiencies, liver disease |
White blood cells (WBCs)
White blood cells, also called leukocytes, are cells of the immune system that help fight infection. The CBC provides an overall WBC count as well as numbers for the five major types of white blood cells:
| CBC White Blood Cell Test | What It Measures | What Abnormal Results May Mean |
|---|---|---|
| WBC count | Total number of white blood cells | Infection, leukemia, inflammation, allergies, stress |
| Neutrophil count | The most common type of white blood cell | Bacterial infection, inflammation, leukemia |
| Lymphocyte count | Help fight viral infections and produce antibodies | Viral infections, leukemia, immune disorders |
| Monocyte count | Largest type of white blood cell | Chronic infection, inflammation, autoimmune disorders |
| Eosinophil count | Respond to allergies and parasites | Allergies, asthma, parasitic infections, eczema |
| Basophil count | Involved in allergic reactions | Allergies, asthma, autoimmune disorders |
Hemoglobin
Hemoglobin is the iron-containing protein in red blood cells that carries oxygen. The CBC reports the amount of hemoglobin in the blood. Low hemoglobin, a condition called anemia, can indicate blood loss, nutritional deficiency, bone marrow problems, or other disorders.
Hematocrit
Hematocrit measures the percentage of blood volume made up of red blood cells. Low hematocrit indicates anemia, while high levels can suggest polycythemia vera, a disease that causes bone marrow to produce too many red blood cells.
Platelets
Platelets, also called thrombocytes, are cell fragments involved in blood clotting. The CBC reports the number of platelets. A low platelet count can lead to excessive bruising and bleeding, while an elevated count may signal thrombocythemia, a disorder of too many platelets.
When is a CBC drawn?
Doctors order a CBC as part of regular health screenings or when symptoms suggest infection, inflammation, bleeding disorders, anemia, or other blood-related conditions. Reasons a doctor may order a CBC test include:
- Part of a routine physical exam
- Unexplained fatigue, weakness, bruising, or bleeding
- Fever or signs of infection
- Inflammation or autoimmune disorders
- Bleeding disorders
- Anemia symptoms like pale skin, irregular heartbeat, dizziness
- Nutritional deficiencies
- Cancer screening or monitoring
- Side effects of medications that affect blood cells
Some examples of specific situations when a CBC may help identify issues include:
Signs of infection
An elevated white blood cell count indicated by the CBC may signal the body is fighting an infection. Doctors can look for increases in neutrophils and other white blood cell types to help pinpoint if the cause is bacterial, viral, or fungal.
Fatigue and weakness
When fatigue and weakness are present, the CBC can help check for anemia. Low red blood cell or hemoglobin levels confirm not enough oxygen is reaching tissues. Additional CBC results help categorize the cause as from blood loss, destruction, or lack of production.
Bruising or bleeding
Unexplained or excessive bruising or bleeding may be caused by low platelets or an underlying clotting disorder. The CBC provides platelet numbers and size to assist in diagnosis along with other screening tests for blood disorders.
Inflammation
Higher numbers of white blood cells or certain subgroups like neutrophils indicate inflammation. Doctors can use the CBC results to look for inflammatory diseases like arthritis or bowel diseases. Specific causes like bacterial infections can also be identified.
Cancer
The CBC provides broad screening for blood cancers like leukemia which directly impact the cells in the marrow and blood. Both high and low cell counts can indicate cancerous changes and be used to monitor treatments. Solid tumor cancers may also affect cell counts.
How to prepare for a CBC
No special preparation is required before a CBC test. However, the following tips can help ensure accurate results:
- Inform your doctor about any medications you take, as some drugs may impact CBC results.
- Avoid strenuous exercise the day before your test, which may temporarily impact cell counts.
- Drink plenty of fluids before the blood draw to avoid concentration of cells.
- Consider delaying your test if you have a cold, infection, or allergy flare up.
Understanding your CBC results
Reference ranges provide a standardized comparison for CBC results, although normal values may vary slightly between labs:
| CBC Test | Normal Reference Range |
|---|---|
| White blood cells (WBCs) | 4,500-11,000 per microliter |
| Neutrophils | 40-60% of total WBCs |
| Lymphocytes | 20-40% of total WBCs |
| Monocytes | 2-10% of total WBCs |
| Eosinophils | 1-4% of total WBCs |
| Basophils | 0.5-1% of total WBCs |
| Red blood cells (RBCs) | Male: 4.7-6.1 million per microliter |
| Female: 4.2-5.4 million per microliter | |
| Hemoglobin (Hgb) | Male: 13.5-17.5 g/dL |
| Female: 12.0-15.5 g/dL | |
| Hematocrit (Hct) | Male: 40-54% |
| Female: 36-48% | |
| Platelets | 150,000-450,000 per microliter |
Your doctor will interpret your CBC based on your results, symptoms, and medical history. Even if a value is outside the reference range, it may not always indicate a problem. Your doctor can explain what your CBC results mean and possible next steps.
When are CBC results abnormal?
CBC test results outside the normal range may warrant additional testing based on the degree and type of abnormality. Some examples include:
- Mildly low RBCs: Slight anemia may be from temporary causes like nutrients deficiency and resolve after supplements.
- Very low RBCs: Significant anemia requires further evaluation of underlying causes like internal bleeding.
- Elevated WBCs: Levels 2-3 times above normal indicate infection or inflammation to pinpoint, while extremely high levels over 25,000-50,000 suggest leukemia.
- Low platelets: Moderate thrombocytopenia around 50,000 may be monitored, while very low levels under 20,000 increase bleeding risk.
Your doctor will take your specific CBC results into account along with your health history in assessing any abnormal test values.
What do CBC results mean?
Some examples of what CBC test results may indicate:
- Enlarged RBCs: Nutrient deficiencies, liver or kidney disorders, alcoholism, autoimmune disease
- Small RBCs: Iron deficiency anemia, thalassemia, chronic disease
- Variation in RBC size (high RDW): Anemia, malnourishment, vitamin deficiencies
- Low white blood cell count: Viral infection, autoimmune disorder, bone marrow damage, chemotherapy side effects
- High neutrophil percentage: Bacterial infection, inflammation, tissue damage, metabolic disease
- High monocyte percentage: Chronic infection like tuberculosis or endocarditis
- High eosinophil count: Allergies, asthma, parasitic infection, adrenal gland disease
- Low platelet count: ITP (autoimmune disorder), leukemia, anemia, viral infection, medication side effects
These examples demonstrate how CBC results can provide clues about potential causes of abnormalities. Your doctor will interpret the test taking into account your health history and symptoms.
Conclusion
The complete blood count is one of the most commonly ordered blood tests providing health screening and helping diagnose diseases. CBC results give comprehensive data on counts, size, and morphology of the cellular components of blood. Doctors use the CBC to check for anemia, infection, blood disorders, cancers, inflammation, allergies, and other conditions. Understanding your CBC results and communicating effectively with your doctor can lead to proper treatment and monitoring when abnormalities occur. Staying informed enables you to take an active role in your health.