How to interpret CBC report
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How to Interpret a CBC (Complete Blood Count) Report
A CBC is one of the most commonly ordered laboratory tests. It measures the three main cellular components of blood - red blood cells (RBCs), white blood cells (WBCs), and platelets - along with a set of calculated indices. Here is a systematic breakdown of every component.
1. Red Blood Cell (RBC) Parameters
Hemoglobin (Hb)
Measures the oxygen-carrying capacity of blood in g/dL.
| Sex | Normal Range |
|---|---|
| Male | 13.5 - 17.5 g/dL |
| Female | 12.0 - 16.0 g/dL |
- Low (anemia): blood loss, iron/B12/folate deficiency, chronic disease, hemolysis, bone marrow disorders
- High (polycythemia): polycythemia vera, chronic hypoxia, high altitude, EPO-secreting tumor
Hematocrit (Hct / PCV)
The percentage of blood volume occupied by RBCs. Roughly 3x the Hb value.
| Sex | Normal Range |
|---|---|
| Male | 39 - 49% |
| Female | 35 - 45% |
Note: In acute blood loss, Hb and Hct may be normal for the first 12-24 hours while plasma volume equilibrates.
RBC Count
Direct count of erythrocytes per microliter.
| Sex | Normal Range |
|---|---|
| Male | 4.3 - 5.7 × 10⁶ cells/μL |
| Female | 3.8 - 5.1 × 10⁶ cells/μL |
2. RBC Indices - The Key to Classifying Anemia
These calculated values tell you what kind of anemia is present.
MCV (Mean Corpuscular Volume) - 80 to 100 fL
Average size of a red blood cell. This is the first step in classifying anemia.
| MCV | Classification | Common Causes |
|---|---|---|
| < 80 fL | Microcytic | Iron deficiency, thalassemia, chronic disease |
| 80 - 100 fL | Normocytic | Acute blood loss, hemolysis, renal failure, early mixed deficiency |
| > 100 fL | Macrocytic | B12/folate deficiency, alcohol, liver disease, medications (methotrexate, hydroxyurea) |
In early anemia, MCV may change before Hb and Hct - making it a sensitive early marker.
MCH (Mean Corpuscular Hemoglobin) - 27 to 33 pg
Average amount of Hb per RBC. Parallels MCV - low in microcytic, high in macrocytic anemia.
MCHC (Mean Corpuscular Hemoglobin Concentration) - 32 to 36 g/dL
Hb concentration per unit volume of RBCs.
- Low MCHC = hypochromic cells (iron deficiency)
- High MCHC = spherocytosis (consider hereditary spherocytosis)
RDW (Red Cell Distribution Width) - 11.5 to 14.5%
Measures variation in RBC size (anisocytosis). A high RDW alongside low MCV strongly suggests iron deficiency anemia (vs. thalassemia trait, which usually has normal RDW).
3. White Blood Cell (WBC) Count and Differential
Total WBC: 4,500 - 11,000 cells/μL (4.5 - 11.0 × 10⁹/L)
- Leukocytosis (> 11,000): infection, inflammation, burns, trauma, stress, pregnancy, corticosteroids, leukemia
- Hyperleukocytosis (> 100 × 10⁹/L): strongly suggests leukemia
- Leukopenia (< 4,500): chemotherapy, radiation, HIV/AIDS, autoimmune disease, aplastic anemia, certain drugs (antipsychotics, antiepileptics, immunosuppressants)
WBC Differential - Normal Percentages
| Cell Type | Normal % | Absolute Count | Role |
|---|---|---|---|
| Neutrophils | 50 - 70% | 1.8 - 7.7 × 10⁹/L | First responders to bacterial infection |
| Lymphocytes | 20 - 40% | 1.0 - 4.8 × 10⁹/L | Adaptive immunity (T and B cells) |
| Monocytes | 2 - 8% | 0.2 - 0.8 × 10⁹/L | Phagocytosis, antigen presentation |
| Eosinophils | 1 - 4% | 0.0 - 0.45 × 10⁹/L | Parasitic infection, allergies |
| Basophils | 0.5 - 1% | 0.0 - 0.2 × 10⁹/L | Hypersensitivity reactions |
| Bands (immature neutrophils) | 0 - 5% | - | "Left shift" = active bacterial infection |
Key patterns to recognize:
- Neutrophilia + left shift (bands): bacterial infection, sepsis
- Lymphocytosis: viral infections (EBV, CMV), CLL
- Absolute lymphocytes < 1,500/mm³: primary T-cell immunodeficiency
- Eosinophilia: parasites, atopy, drug reactions, Omenn syndrome
- Low absolute neutrophils (ANC < 1,500): neutropenia - raises infection risk; ANC < 500 = severe risk
4. Platelet Count
Normal: 150,000 - 400,000/μL (150 - 400 × 10⁹/L)
- Thrombocytosis (> 400,000): bone marrow proliferative disorders, inflammation, post-splenectomy, iron deficiency
- Thrombocytopenia (< 150,000): decreased production (leukemia, aplastic anemia, B12 deficiency, infections) or increased destruction (ITP, DIC, HUS/TTP, drug-induced)
- < 50,000: significant bleeding risk
- < 20,000: spontaneous bleeding risk
Mean Platelet Volume (MPV) - 7.5 to 12.5 fL
Larger platelets (high MPV) suggest active bone marrow turnover (as seen in ITP or after destruction). Small platelets are seen in Wiskott-Aldrich syndrome.
5. When to Order a Peripheral Blood Smear
Automated analyzers are highly accurate, but a manual smear review is indicated when:
- Hemolysis is suspected
- RBC inclusions (Howell-Jolly bodies, ring sideroblasts) are possible
- Myelodysplasia or megaloblastic changes are being evaluated
- Immature or abnormal cells (blasts) are flagged by the analyzer
- Thrombocytosis or thrombocytopenia needs characterization
- Large vacuoles in neutrophils (Chediak-Higashi) or small platelets (Wiskott-Aldrich) are suspected
Quick Summary Table
| Parameter | Low | Normal | High |
|---|---|---|---|
| Hb | Anemia | M: 13.5-17.5, F: 12-16 g/dL | Polycythemia |
| Hct | Anemia | M: 39-49%, F: 35-45% | Polycythemia |
| MCV | Microcytic (<80) | 80-100 fL | Macrocytic (>100) |
| WBC | Infection risk / drug effect | 4.5-11.0 × 10⁹/L | Infection / leukemia |
| Neutrophils | Neutropenia / immunodeficiency | 50-70% | Bacterial infection |
| Lymphocytes | T-cell immunodeficiency | 20-40% | Viral infection / CLL |
| Platelets | Bleeding risk | 150-400 × 10³/μL | Thrombosis risk |
Sources: Histology: A Text and Atlas with Correlated Cell and Molecular Biology (Pawlina), p. 728-730; Textbook of Family Medicine 9e, eTable 14-4, p. 227; Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1175
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