Complete blood count (CBC)
Lab test that counts and sizes blood cells and measures hemoglobin and hematocrit
Also known as
Definition
A complete blood count (CBC) is a group of lab values from one blood sample. The basic CBC covers red blood cells (erythrocytes), white blood cells (leukocytes, as a total count), platelets (thrombocytes), hemoglobin, hematocrit and the red cell indices MCV, MCH and MCHC. A CBC with differential also counts each of the five main types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils and basophils. In German-speaking countries the two are called the small and the large blood count.
| Parameter | Value |
|---|---|
| Unit | varies by value (e.g. /µl, g/dl, %, fl, pg) |
| Reference Range | Typical adult ranges: white blood cells 4,500–11,000/µl; red blood cells men 4.6–6.2 million/µl, women 4.2–5.4 million/µl; hemoglobin men 13–18 g/dl, women 12–16 g/dl; hematocrit men 40–55%, women 36–48%; platelets 150,000–400,000/µl; MCV 80–100 fl; MCH 27–32 pg; MCHC 32–36 g/dl. Labs differ; the range on your own report is what counts. Discuss it with your doctor. |
Agree on your personal target range with your doctor.
SI conversion: 4,500/µl equals 4.5 × 10⁹/l (4.5/nl); 1 million/µl equals 10¹²/l; 150,000/µl equals 150 × 10⁹/l. Hemoglobin: g/dl × 10 = g/l. Hematocrit: 40% = 0.40 l/l.
↓ What a low value means
Low red blood cells, hemoglobin or hematocrit can point to anemia. Anemia can be linked to blood loss, a lack of iron, folate or vitamin B12, chronic conditions such as kidney disease, or reduced blood cell production in the bone marrow, among other causes. A low white cell count (leukopenia) can be linked to viral infections, autoimmune diseases, medicines, liver or spleen disease and bone marrow disorders; a low platelet count can be linked to an enlarged spleen, pregnancy or bone marrow disorders. Discuss any deviation with your doctor.
↑ What a high value means
High red blood cells, hemoglobin or hematocrit can be linked to dehydration, smoking, living at high altitude, testosterone use, heart or lung disease with long-term low oxygen, or polycythemia vera. A high white cell count (leukocytosis) often occurs with infections, and also with severe physical or emotional stress, corticosteroids, tissue damage, autoimmune diseases and allergies, and leukemia. A high platelet count can be linked to bleeding, iron deficiency or bone marrow disorders. A single value outside the range is not a finding on its own; discuss it with your doctor.
When to test
A CBC is often part of a routine checkup. It is also ordered for symptoms such as fatigue, fever, signs of infection, bruising or unintended weight loss, and to monitor conditions or treatments that can change blood cell counts. Usually no preparation is needed; fasting may be required if other values are measured from the same sample.
Frequently asked questions
What is the difference between a CBC and a CBC with differential? +
A basic CBC counts red and white blood cells and platelets and measures hemoglobin, hematocrit and the red cell indices. A CBC with differential adds the count of each of the five types of white blood cells. Your doctor decides which version is ordered.
Do I need to fast for a CBC? +
Usually not for the CBC itself. If other values such as blood glucose or blood lipids are measured from the same sample, you may be asked to fast for several hours. Your practice or lab will tell you what applies.
Does a value outside the reference range mean something is wrong? +
Not necessarily. Fluid intake, physical activity, diet, menstruation, medicines and altitude can shift a CBC. It is only one piece of the picture; your doctor interprets it together with your history, symptoms and other values.
Why do lab reports use different units? +
Some labs report cell counts per microliter, others in SI units per liter, for example 4.5 × 10⁹/l instead of 4,500/µl. The measured value is the same, only the notation changes. For trends it helps to always compare the same unit.
Complete blood count (CBC)
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.