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Here is a thorough overview of the Complete Blood Count (CBC) based on authoritative medical textbooks.
Complete Blood Count (CBC)
The CBC measures the three major cellular components of blood: red blood cells (RBCs), white blood cells (WBCs), and platelets. It uses automated electronic cell counters that count, size, and characterize cells, providing a five-part WBC differential and indices like MCV and RDW.
CBC Components and Normal Reference Ranges
Red Blood Cell Parameters
| Test | Description | Normal Range (Male) | Normal Range (Female) |
|---|---|---|---|
| Hemoglobin (Hb) | O2-carrying capacity of blood | 13.5-17.5 g/dL | 12.0-16.0 g/dL |
| Hematocrit (Hct) | % of whole blood volume occupied by RBCs | 39-49% | 35-45% |
| RBC count | Erythrocyte count | 4.3-5.7 × 10⁶ cells/µL | 3.8-5.1 × 10⁶ cells/µL |
| MCV (Mean Corpuscular Volume) | Average RBC size | 80-100 fL | 80-100 fL |
| MCH (Mean Corpuscular Hgb) | Hb per RBC (Hb ÷ RBC) | 27-33 pg | 27-33 pg |
| MCHC (Mean Corpuscular Hgb Concentration) | Hb concentration per volume of RBCs | 33-36 g/dL | 33-36 g/dL |
| RDW (Red Cell Distribution Width) | Variation in RBC size (anisocytosis) | 11.5-14.5% | 11.5-14.5% |
| Reticulocyte count | Immature RBCs; reflects bone marrow activity | 0.5-1.5% of RBCs |
White Blood Cell Parameters
| Test | Normal Range |
|---|---|
| Total WBC count | 4,500-11,000 cells/mm³ |
| Neutrophils (segs + bands) | 50-70% (absolute: 1,800-7,000/µL) |
| Lymphocytes | 20-40% |
| Monocytes | 2-8% |
| Eosinophils | 1-4% |
| Basophils | 0.5-1% |
Platelet Parameters
| Test | Normal Range |
|---|---|
| Platelet count | 150,000-400,000/mm³ |
| MPV (Mean Platelet Volume) | 7.5-12.5 fL |
Classifying Anemia by MCV
The first step when anemia is found on a CBC is to classify it by MCV:
- Microcytic (MCV < 80 fL): Think iron deficiency, thalassemia, anemia of chronic disease. Check ferritin - if < 30 ng/mL, confirms iron deficiency.
- Normocytic (MCV 80-100 fL): Think acute blood loss, hemolysis, renal insufficiency, early mixed deficiency. In early anemia, MCV may change before Hb/Hct.
- Macrocytic (MCV > 100 fL): Think B12/folate deficiency, alcohol, hypothyroidism, medications (hydroxyurea, methotrexate).
WBC Differential - Key Abnormalities
| Finding | Definition | Common Causes |
|---|---|---|
| Leukocytosis | WBC > 10,000/mm³ | Infection, inflammation, leukemia, steroids, stress |
| Leukopenia | WBC < 4,500/mm³ | Viral infection, drugs, autoimmune, B12/folate deficiency |
| Neutrophilia | ANC > 7,000/µL | Bacterial infection, steroids, stress, burns, MI |
| Neutropenia | ANC < 1,800/µL (severe: <500) | Drug reaction, viral infection, aplastic anemia |
| Lymphocytosis | Lymphs > 4,000/µL (adult) | Viral infections (EBV, CMV), CLL, pertussis |
| Lymphopenia | Lymphs < 1,500/µL (adult) | HIV, steroids, radiation, immunosuppressants |
| Eosinophilia | Eosinophils > 500/µL | Parasites, atopy/asthma, drug reactions, vasculitis |
| Monocytosis | Monocytes > 800/µL | Chronic infection (TB), inflammatory disease, malignancy |
Platelet Abnormalities
- Thrombocytopenia (platelets < 150,000/mm³): Causes include decreased production (B12/folate deficiency, aplastic anemia, chemotherapy, leukemia, alcohol, viral infections including HIV) or increased destruction (immune thrombocytopenia [ITP], DIC, HUS, heparin-induced [HIT], HELLP syndrome, sepsis, splenic sequestration).
- Thrombocytosis (platelets > 400,000/mm³): Reactive (iron deficiency, infection, inflammation, post-splenectomy) or primary (essential thrombocythemia).
Peripheral Blood Smear
Manual smear review is performed when automated analyzers flag abnormalities. Indications include:
- Suspected hemolysis
- RBC inclusions (Howell-Jolly bodies, Heinz bodies, malaria)
- Myelodysplasia or megaloblastic changes
- Unexplained thrombocytopenia/thrombocytosis
- Immature or abnormal WBCs (blasts)
Clinical Use of CBC
The CBC is ordered for:
- Screening for anemia, infection, or clotting disorders
- Monitoring bone marrow response to treatment (e.g., chemotherapy)
- Evaluating unexplained fatigue, fever, bruising, or bleeding
- Surveillance in chronic disease (CKD, HIV, autoimmune diseases)
- Pre-operative assessment
The absolute cell counts (not just percentages) are most clinically meaningful - calculate the absolute neutrophil count (ANC) = Total WBC × (% neutrophils + % bands).
Sources: Textbook of Family Medicine, 9e (eTable 14-4); Henry's Clinical Diagnosis and Management by Laboratory Methods; Junqueira's Basic Histology, 17e
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All 6 sections are fully present across 2 pages. The PDF is complete and ready.
Your CBC Quick Reference Guide is ready:
CBC Quick Reference
PDF Document · PDF
What's inside (2 pages, A4):
| Section | Content |
|---|---|
| 1. RBC Parameters | Hb, Hct, RBC count, MCV, MCH, MCHC, RDW, Reticulocytes with male/female/SI ranges |
| 2. WBC Differential | All 5 cell types with % ranges, absolute counts, and clinical roles |
| 3. Anemia by MCV | Micro/Normo/Macrocytic classification, causes, and first-line tests |
| 4. WBC Abnormalities | Leukocytosis, leukopenia, neutrophilia/penia, lymphocytosis/penia, eosinophilia, monocytosis, basophilia |
| 5. Platelet Interpretation | Severity grading from normal to severe thrombocytopenia + thrombocytosis, bleeding thresholds |
| 6. Clinical Pearls | ANC formula, acute hemorrhage caveat, left shift, pancytopenia, reactive vs. leukemic leukocytosis, smear indications, RDW significance, PV clues |
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