CBC Blood Test: What a Complete Blood Count Measures and What Your Results Mean

A CBC (complete blood count) is a blood test that measures the quantity and characteristics of three types of blood cells: red blood cells (which carry oxygen), white blood cells (which fight infection), and platelets (which help blood clot). It produces 15-20 values including hemoglobin, hematocrit, MCV, MPV, and a white cell differential. It's the single most commonly ordered blood test in medicine.

No. A CBC does not require fasting. You can eat and drink normally before having your blood drawn. However, if your doctor orders a CBC alongside other tests that do require fasting (like a metabolic panel or lipid panel), you'll need to fast for those other tests — the CBC will still be accurate regardless.

A CBC can detect anemia (low red blood cells or hemoglobin), infections and inflammation (elevated white blood cells), immune deficiency (low white blood cells), clotting disorders (abnormal platelet counts), and blood cancers like leukemia (abnormal cell counts or types). It also screens for nutritional deficiencies — iron, B12, and folate deficiency each produce characteristic CBC patterns.

RBC stands for red blood cell count — the number of red blood cells per microliter of blood. Normal range is 4.5-5.9 million/μL for men and 4.0-5.2 million/μL for women. Low RBC (anemia) causes fatigue, weakness, and shortness of breath. High RBC (polycythemia) can result from dehydration, chronic low oxygen, or rarely bone marrow disorders. RBC is always interpreted alongside hemoglobin and hematocrit.

WBC is the white blood cell count — the total number of immune cells in your blood. Normal range is 4,500-11,000 cells per microliter. High WBC (leukocytosis) usually indicates infection, inflammation, or stress. Low WBC (leukopenia) can be caused by viral infections, certain medications, autoimmune conditions, or bone marrow problems. The WBC differential breaks this into five cell types for more specific diagnosis.

Hemoglobin below 12 g/dL is the WHO threshold for anemia in women. The most common cause is iron deficiency, which produces a characteristic pattern of low MCV, low ferritin, and high TIBC (total iron binding capacity). Before supplementing iron, check a full iron panel — ferritin, serum iron, and TIBC — to confirm iron deficiency is the cause rather than B12 deficiency, folate deficiency, or chronic disease. Low hemoglobin in a woman of reproductive age is most commonly from menstrual blood loss; in post-menopausal women, GI blood loss should be considered.

A WBC count below 4,000–4,500 cells/μL is called leukopenia. The most common causes are viral infections (mono, HIV, hepatitis), certain medications (chemotherapy, some antibiotics, anticonvulsants, antithyroid drugs), and autoimmune conditions. Benign ethnic neutropenia — a normal variant more common in people of African, Middle Eastern, and West Indian descent — can cause a persistently low absolute neutrophil count without any pathology. A WBC below 2,000 cells/μL or an absolute neutrophil count below 500 cells/μL warrants prompt medical evaluation due to infection risk.