What Is Complete Blood Count (CBC)?
A Complete Blood Count (CBC) is one of the most common and informative blood panels ordered by healthcare providers. It provides a quantitative and qualitative assessment of the cellular components in your blood: red blood cells (RBCs), white blood cells (WBCs), and platelets. By measuring the number, size, and volume of these cells, a CBC offers a crucial snapshot of your overall health and hematologic status.
This panel is routinely ordered as part of an annual physical, to investigate symptoms like fatigue, fever, bruising, or unexplained weight loss, to monitor chronic conditions (like anemia or blood disorders), and to track the effects of medications (like chemotherapy) that can impact blood cell production [1]. It screens for a wide range of conditions, including:
- Anemias (iron deficiency, vitamin B12/folate deficiency, hemolytic)
- Infections (bacterial, viral, parasitic)
- Inflammatory disorders
- Bone marrow disorders (leukemia, myelodysplastic syndromes)
- Clotting and bleeding disorders (thrombocytopenia)
- Dehydration
- Nutritional deficiencies
Tests Included in This Panel
A standard CBC panel includes the following individual tests. The normal ranges provided are for adults and may vary slightly between laboratories. Always interpret results using your lab's reference range.
| Test Name | What It Measures | Normal Range (Adult) | Unit |
|---|---|---|---|
| White Blood Cell Count (WBC) | Total number of white blood cells (leukocytes) in a volume of blood. | 4,500 - 11,000 | cells/µL |
| Red Blood Cell Count (RBC) | Number of red blood cells (erythrocytes) in a volume of blood. | Male: 4.7 - 6.1 Female: 4.2 - 5.4 | million cells/µL |
| Hemoglobin (Hgb) | Amount of oxygen-carrying protein within red blood cells. | Male: 13.5 - 17.5 Female: 12.0 - 15.5 | g/dL |
| Hematocrit (Hct) | Percentage of blood volume made up by red blood cells. | Male: 38.8% - 50.0% Female: 34.9% - 44.5% | % |
| Mean Corpuscular Volume (MCV) | Average size of an individual red blood cell. | 80 - 100 | fL |
| Mean Corpuscular Hemoglobin (MCH) | Average amount of hemoglobin inside a single red blood cell. | 27 - 31 | pg |
| Mean Corpuscular Hemoglobin Concentration (MCHC) | Average concentration of hemoglobin within red blood cells. | 32 - 36 | g/dL |
| Red Cell Distribution Width (RDW) | Variation in the size of red blood cells (anisocytosis). | 11.5% - 14.5% | % |
| Platelet Count (Plt) | Number of platelets (thrombocytes) in a volume of blood. | 150,000 - 450,000 | platelets/µL |
| Mean Platelet Volume (MPV) | Average size of platelets. | 7.5 - 12.5 | fL |
| Test Name | What It Measures | Normal Range (Adult) | Unit |
|---|---|---|---|
| Neutrophils | First responders to bacterial and fungal infections. | 40% - 60% (1,800 - 7,800 cells/µL) | % |
| Lymphocytes | Key for viral infections and adaptive immunity. | 20% - 40% (1,000 - 4,800 cells/µL) | % |
| Monocytes | Clean-up cells that become macrophages. | 2% - 10% (200 - 1,000 cells/µL) | % |
| Eosinophils | Involved in allergic responses and parasite fights. | 1% - 4% (100 - 400 cells/µL) | % |
| Basophils | Involved in inflammatory and allergic responses. | 0.5% - 1% (20 - 100 cells/µL) | % |
How to Read Your Results
Step-by-Step Interpretation
- Start with Red Blood Cell Indices (Anemia Check): Look at Hemoglobin (Hgb) and Hematocrit (Hct). Low values indicate anemia. Next, use MCV to classify it:
* Normal MCV (80-100 fL): Normocytic anemia. Common causes: Chronic disease, early iron deficiency, blood loss, kidney disease.
* High MCV (>100 fL): Macrocytic anemia. Common causes: Vitamin B12 or folate deficiency, certain medications.
* Check RDW: A high RDW indicates a mix of large and small RBCs, seen in mixed deficiencies (e.g., iron + B12) or ongoing blood loss.
- Assess White Blood Cells (Infection/Inflammation): An elevated WBC count (leukocytosis) often suggests infection, inflammation, stress, or a bone marrow disorder. A low WBC count (leukopenia) can indicate viral infection, autoimmune disease, or bone marrow suppression.
* High Lymphocytes: Suggests viral infection (like mononucleosis) or certain leukemias.
* High Eosinophils: Suggests allergic conditions, parasitic infections, or autoimmune disorders.
- Evaluate Platelets (Clotting Risk):
* High Platelet Count (Thrombocytosis): Can increase clotting risk. May be reactive (to infection, inflammation) or due to a bone marrow disorder.
* Mean Platelet Volume (MPV): High MPV with low platelet count can indicate active platelet destruction. High MPV with normal/high count may indicate a reactive or myeloproliferative process.
Common Patterns
| Pattern | Possible Meaning | Next Steps |
|---|---|---|
| Low Hgb/Hct, Low MCV, High RDW | Classic pattern for iron deficiency anemia. | Check serum Ferritin, iron, TIBC. Investigate source of blood loss. |
| Low Hgb/Hct, High MCV | Suggests macrocytic anemia (B12/folate deficiency). | Check serum Vitamin B12 and folate levels. |
| Low Hgb/Hct, Normal MCV, High WBC/Plt | May indicate anemia of chronic disease or inflammation. | Check markers like C-Reactive Protein (CRP) and Erythrocyte Sedimentation Rate (ESR). |
| Very High WBC with immature cells (blasts) | Potential sign of acute leukemia. | Requires urgent hematology referral for bone marrow biopsy. |
| Low WBC, Low RBC, Low Platelets (Pancytopenia) | Indicates bone marrow failure or infiltration. | Evaluate for aplastic anemia, vitamin deficiencies, leukemia, or autoimmune disease. Requires hematology workup. |
| High Platelets, Normal/High MPV | Reactive thrombocytosis (to infection, inflammation) or essential thrombocythemia. | Check CRP/ESR for inflammation. If persistent and unexplained, hematology referral may be needed. |
| High Lymphocytes, low Neutrophils | Common in viral infections (e.g., influenza, COVID-19). | Supportive care, follow-up CBC to ensure normalization. |
| High Neutrophils, low Lymphocytes | Common in acute bacterial infections or physical stress. | Search for source of infection. |
What Abnormal Results Mean
Signs of Anemia & Nutritional Deficiencies:- Iron Deficiency: Low Hgb/Hct, Low MCV, High RDW. Often seen with low Ferritin.
- Vitamin B12/Folate Deficiency: Low Hgb/Hct, High MCV. May be accompanied by large, oval-shaped RBCs on smear.
- Anemia of Chronic Disease: Low Hgb/Hct, Normal/Low MCV, often with normal RDW. Associated with chronic inflammation, kidney disease, or cancer.
- Bacterial Infection: High WBC (Leukocytosis) driven by high Neutrophils. May see "band" cells (immature neutrophils).
- Viral Infection: Often normal or low WBC, with high Lymphocytes (lymphocytosis).
- Parasitic Infection/Allergy: High Eosinophils (eosinophilia).
- Leukemia: Often dramatically high WBC with abnormal or immature cells (blasts). Can also cause pancytopenia.
- Myelodysplastic Syndromes: May show cytopenias (low counts) with oddly shaped cells.
- Hemolytic Anemia: Low Hgb, High MCV or variable, High RDW, often with high bilirubin and low haptoglobin.
- Thrombocytopenia (Low Platelets): Risk of easy bruising, bleeding. Causes range from ITP to medication side effects.
- Reactive Thrombocytosis (High Platelets): Usually secondary to inflammation, infection, or iron deficiency.
How to Prepare for This Test
- Fasting: A standard CBC does not require fasting. However, if your CBC is part of a larger panel that includes glucose or lipids, you may be instructed to fast.
- Timing: Can be performed at any time of day.
- Medications/Hydration: Inform your doctor of all medications, supplements, and herbs you take, as some can affect results (e.g., chemotherapy, antibiotics, diuretics). Severe dehydration can falsely elevate RBC counts.
- Procedure: A blood sample is drawn from a vein in your arm. The process typically takes less than five minutes.
Individual Biomarker Deep Dives
- Hemoglobin (Hgb): The iron-containing protein that carries oxygen. Learn more about Hemoglobin.
- Hematocrit (Hct): The proportion of blood made of RBCs. A direct reflection of RBC number and size.
- Mean Corpuscular Volume (MCV): A key classifier for anemia types.
- White Blood Cell Count (WBC) & Differential: Your immune system's cellular army. Each type has a specific defensive role.
- Platelet Count (Plt): Essential for blood clotting and wound healing.
- Ferritin: While not part of a standard CBC, it is the most specific test to confirm iron deficiency anemia. Learn more about Ferritin.
Frequently Asked Questions
1. How often should I get a CBC test?For healthy adults, a CBC is typically part of a routine annual physical. Your doctor may order it more frequently if you have a chronic condition (like anemia), are undergoing treatment that affects blood cells, or are experiencing relevant symptoms.
2. Do I need to fast for a CBC?No, fasting is not required for a standalone CBC test.
3. What is the difference between a CBC and a CBC with differential?A standard CBC reports total counts. The "diff" breaks down the white blood cells into their five specific types (neutrophils, lymphocytes, etc.), providing more detailed information about your immune response.
4. How much does a CBC test cost?Costs vary widely. Without insurance, it can range from $30 to $100. With insurance, you typically pay a copay or coinsurance, and it is often fully covered as part of preventive care.
5. Does insurance cover a CBC?Yes, most health insurance plans, including Medicare and Medicaid, cover a CBC when it is medically necessary (ordered by a physician for symptoms, diagnosis, or monitoring).
6. Can a CBC detect cancer?A CBC can suggest the possibility of blood cancers like leukemia or lymphoma by showing abnormal, very high, or very low counts of specific cells, often with immature forms present. It is not a definitive test for solid tumors, though abnormalities can be a clue. Diagnosis requires further testing like a bone marrow biopsy.
7. What does it mean if my RBC, hemoglobin, and hematocrit are all high?This is called polycythemia. It can be due to dehydration (relative), chronic low oxygen levels (e.g., lung disease, sleep apnea), or a bone marrow disorder (polycythemia vera).
8. Why would my platelet count be low?Causes include immune system destruction (ITP), medications, viral infections, autoimmune diseases, liver cirrhosis, and bone marrow problems.
9. Are there risks to getting a CBC?The risks are minimal and associated with the blood draw, including slight pain, bruising, or lightheadedness. Serious complications are extremely rare.
10. What should I do if my CBC results are abnormal?Do not panic. Many abnormalities are temporary (e.g., from an infection). Discuss the results with your healthcare provider, who will interpret them in the context of your health, symptoms, and other tests to determine if further investigation is needed.
Disclaimer: This content is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or lab results. Normal ranges may vary between laboratories. Sources:
- American Association for Clinical Chemistry (AACC), Lab Tests Online. "Complete Blood Count (CBC)."
- Mayo Clinic. "Complete Blood Count (CBC)."
- National Institutes of Health (NIH), U.S. National Library of Medicine. "Blood Differential Test."