What is cbc
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complete blood count CBC components normal values interpretation
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Here is a thorough breakdown of the Complete Blood Count (CBC):
Complete Blood Count (CBC)
A CBC is one of the most commonly ordered laboratory test panels in medicine. It provides relative numbers and calculations obtained from the cells (erythrocytes and leukocytes) and formed elements (thrombocytes) in a blood sample. Analysis is typically performed by automated blood cell counters using flow cytometry principles, which identify different cell types based on size and electrical resistance, counting approximately 10,000 cells per category for high accuracy.
- Histology: A Text and Atlas with Correlated Cell and Molecular Biology, p. 728
Components of a CBC
1. White Blood Cell (WBC / Leukocyte) Count
- Normal range: ~4,500 - 11,000 cells/µL (varies by lab)
- Elevated (leukocytosis): suggests infection, inflammation, burns, fractures, strenuous exercise, pregnancy, or labor
- Hyperleukocytosis (>100 × 10⁹/L): commonly indicates leukemia
- Decreased (leukopenia): associated with radiation/chemotherapy, autoimmune disease, aplastic anemia, certain drugs (antipsychotics, antiepileptics, immunosuppressants), HIV/AIDS
2. WBC Differential
Reports the percentage and absolute count of each white cell type:
| Cell Type | Role |
|---|---|
| Neutrophils | First-line defense against bacterial infection |
| Lymphocytes | Adaptive immunity (T cells, B cells) |
| Monocytes | Phagocytosis, antigen presentation |
| Eosinophils | Parasitic infections, allergic reactions |
| Basophils | Allergic and inflammatory responses |
| Band cells | Immature neutrophils - elevated in acute infection ("left shift") |
3. Red Blood Cell (RBC / Erythrocyte) Count
- Elevated (polycythemia): primary (e.g., polycythemia vera, congenital) or secondary (chronic hypoxia, high altitude, EPO-secreting tumor)
- Decreased (anemia): blood loss, iron or B12 deficiency, chronic disease, pregnancy, genetic disorders (e.g., sickle cell anemia)
4. Hemoglobin (Hgb)
- Reflects the erythrocyte's ability to carry oxygen
- Normal values:
- Men: 13.5 - 17.5 g/dL
- Women: 12 - 15.5 g/dL
- Along with hematocrit, it is the primary test for anemia or polycythemia
5. Hematocrit (HCT / PCV)
- The percentage of total blood volume made up of red blood cells
- Calculated as: HCT = RBC × MCV / 10
6. Red Cell Indices
Automatically calculated from Hgb, HCT, and RBC; help identify the cause of anemia:
| Index | Meaning |
|---|---|
| MCV (Mean Corpuscular Volume) | Size of RBCs - microcytic, normocytic, or macrocytic |
| MCH (Mean Corpuscular Hemoglobin) | Average amount of Hgb per RBC |
| MCHC (Mean Corpuscular Hgb Concentration) | % Hgb concentration in an average RBC |
| RDW (Red Cell Distribution Width) | Variation in RBC size - helpful in anemia workup |
7. Platelet (Thrombocyte) Count
- Normal: 150,000 - 400,000/µL
- Elevated (thrombocythemia): bone marrow disorders, inflammation, post-splenectomy
- Decreased (thrombocytopenia): leukemia, infections, B12 deficiency, autoimmune disease, DIC, certain medications
- MPV (Mean Platelet Volume) - average platelet size, also reported
Why Is a CBC Ordered?
- Routine health screening
- Suspected infection or inflammation
- Diagnosis and monitoring of anemia
- Detection of blood cancers (leukemia, lymphoma)
- Monitoring the effects of chemotherapy or radiation
- Evaluation of bleeding disorders
- Assessing immune function (including immunodeficiencies - per Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1175)
How Is It Done?
A small blood sample is drawn (usually from an arm vein) and placed in a tube with an anticoagulant. The sample is run through an automated cell counter. In certain situations (e.g., suspected blood cancer, abnormal cell morphology), a peripheral blood smear is reviewed manually under a microscope to examine cell shape, size, and abnormalities.
Sources:
- Histology: A Text and Atlas with Correlated Cell and Molecular Biology, pp. 728-730
- Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1175
- StatPearls - Normal and Abnormal CBC
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