Overview.
A Complete Blood Count (CBC) is the most frequently ordered laboratory test, providing a comprehensive snapshot of your blood's cellular components. It measures red blood cells (oxygen delivery), white blood cells (immune function), and platelets (clotting). The CBC is often the first test to reveal anemia, infection, immune disorders, and even certain cancers.
The CBC analyzes the three main types of cells in your blood: red blood cells (RBCs) that carry oxygen, white blood cells (WBCs) that fight infection, and platelets that enable blood clotting. It also measures hemoglobin (the oxygen-carrying protein), hematocrit (the proportion of blood made up of red cells), and several indices that describe the size and hemoglobin content of your red cells.
What's included.
- Hemoglobin (Hgb) — normal: 12.0-17.5 g/dL · Oxygen-carrying capacity of blood
- Hematocrit (Hct) — normal: 36-50 % · Percentage of blood volume that is red cells
- RBC Count — normal: 4.0-5.5 million/µL · Number of red blood cells
- WBC Count — normal: 4,500-11,000 cells/µL · Immune system activity level
- Platelet Count — normal: 150,000-400,000 cells/µL · Blood clotting capability
- MCV — normal: 80-100 fL · Average red cell size (microcytic vs macrocytic)
- RDW — normal: 11.5-14.5 % · Variation in red cell sizes
Preparation.
No fasting required.
When: Can be drawn any time of day. Results typically available within 24 hours.
- No fasting required for a standard CBC
- Stay well-hydrated (dehydration can affect hematocrit)
- Inform your provider of recent illness or inflammation
- Note any medications that may affect blood counts
When it's ordered.
- Annual wellness exams and routine health screenings
- Unexplained fatigue, weakness, or pallor
- Signs of infection (fever, inflammation)
- Unusual bleeding or bruising
- Before surgery to assess baseline blood health
- Monitoring chemotherapy or radiation effects
- Investigating suspected anemia or blood disorders
Interpretation.
What normal means
Your blood cells are being produced in normal quantities and functioning properly. Red cells are delivering adequate oxygen, your immune system has appropriate surveillance capacity, and your clotting system is balanced.
Abnormal patterns
Low hemoglobin + Low MCV (microcytic anemia)
Possible causes
- Iron deficiency
- Chronic disease
- Thalassemia trait
Next steps
- Iron studies (ferritin, serum iron, TIBC)
- Reticulocyte count
- Review diet and GI health
Low hemoglobin + High MCV (macrocytic anemia)
Possible causes
- B12 deficiency
- Folate deficiency
- Liver disease
- Hypothyroidism
Next steps
- B12 and folate levels
- Thyroid panel
- Liver function tests
High WBC count (leukocytosis)
Possible causes
- Acute infection
- Inflammation
- Stress response
- Leukemia (rare)
Next steps
- WBC differential
- Review symptoms and recent illness
- Repeat in 2-4 weeks if persistent
Low WBC count (leukopenia)
Possible causes
- Viral infection
- Autoimmune conditions
- Bone marrow suppression
- Certain medications
Next steps
- WBC differential
- Review medications
- Consider autoimmune workup
Low platelet count (thrombocytopenia)
Possible causes
- Viral infection
- Autoimmune destruction
- Liver disease
- Bone marrow issues
Next steps
- Peripheral blood smear
- Liver function tests
- Consider hematology referral
Cost & access.
The CBC is typically covered by insurance as part of routine care. Without insurance, costs range from $10-50 at most labs. It's often bundled with metabolic panels in annual checkups.
FAQs.
Do I need to fast for a CBC?
No, fasting is not required for a standard CBC. However, if your CBC is being drawn along with metabolic panels or lipids, you may need to fast for those tests.
What causes low hemoglobin?
The most common cause is iron deficiency, especially in women of childbearing age. Other causes include B12/folate deficiency, chronic diseases, blood loss, and less commonly, bone marrow disorders.
Should I be worried about a slightly high WBC?
Mildly elevated WBC is common and usually reflects recent infection, stress, or inflammation. A truly concerning elevation is typically much higher (>15,000-20,000) or persistent over multiple tests.
How often should I get a CBC?
For healthy adults, a CBC every 1-2 years as part of routine screening is sufficient. More frequent testing may be needed if you have anemia, are on certain medications, or have chronic conditions.