🩸 Complete Blood Count (CBC) Basics

WBC · RBC · Hb · Hct · Platelet — Clinical Meaning & Interpretation Guide

A Complete Blood Count (CBC) measures white blood cells (WBC), red blood cells (RBC), hemoglobin (Hb), hematocrit (Hct), and platelets (PLT).
It is one of the most essential laboratory tests used to evaluate infection, inflammation, anemia, bleeding, and bone marrow disorders.

Below is a complete breakdown of each major CBC parameter, including reference ranges, clinical implications, and interpretation tips.

---

1️⃣ WBC — White Blood Cell Count

A marker of infection, inflammation, immune status

🔍 Definition

Number of white blood cells per microliter (µL) of blood.

🔢 Reference Range

4,000–10,000 /µL (may vary by laboratory)

📈 Increased WBC (Leukocytosis)

• Bacterial infections (pneumonia, sepsis)
• Inflammatory conditions (appendicitis, enteritis)
• Steroid therapy
• Leukemia (CML, ALL)
• Smoking, physical stress, exercise
• Pregnancy, postpartum
• Physiologic elevation in newborns

📉 Decreased WBC (Leukopenia)

• Viral infections (influenza, RSV)
• Bone marrow suppression (chemotherapy, radiation)
• End-stage severe infection (consumptive leukopenia)
• Aplastic anemia
• Autoimmune diseases or immunosuppressive therapy

⚠ Interpretation Tips

• Do not interpret WBC alone — differential count is essential.
• Temporary increases occur with stress, smoking, exercise.
• Chemotherapy nadir typically occurs 7–14 days after treatment.

---

2️⃣ RBC — Red Blood Cell Count

Reflects oxygen-carrying capacity of blood

🔍 Definition

Number of red blood cells per microliter (µL) of blood.

🔢 Reference Range

• Male: 4.5–5.9 ×10⁶/µL
• Female: 4.0–5.2 ×10⁶/µL

📈 Increased RBC (Erythrocytosis)

• Dehydration (relative increase)
• High-altitude living
• Chronic lung disease (COPD)
• Polycythemia vera
• Smoking (CO exposure)

📉 Decreased RBC (Erythropenia)

• All types of anemia (iron deficiency, B12/folate deficiency, anemia of chronic disease)
• Acute or chronic blood loss
• Bone marrow failure
• Hemolytic anemia

⚠ Interpretation Tips

• RBC should be interpreted with Hb, Hct, and MCV together.
• Dehydration may mask underlying anemia.
• Reticulocyte count is essential for assessing marrow response.

---

3️⃣ Hemoglobin (Hb)

The central marker used to diagnose anemia

🔍 Definition

Concentration of hemoglobin in blood (g/dL)

🔢 Reference Range

• Male: 13–17 g/dL
• Female: 12–15 g/dL

📉 Decreased Hb → Anemia

• Iron deficiency anemia
• Vitamin B12 / folate deficiency (megaloblastic anemia)
• Anemia of chronic disease (AoCD)
• Blood loss
• Myelodysplastic syndrome (MDS)
• Hemolytic anemia

📈 Increased Hb

• Polycythemia vera (PV)
• Dehydration
• Conditions causing chronic hypoxia

⚠ Interpretation Tips

• Hb is the most direct measure of anemia.
• Physiologic decrease occurs during pregnancy.
• Dehydration or overhydration may alter results.

---

4️⃣ Hematocrit (Hct)

Percentage of blood volume occupied by RBCs

🔍 Definition

Proportion of RBCs in total blood volume (%)

🔢 Reference Range

• Male: 40–52%
• Female: 36–46%

📉 Decreased Hct

• Anemia
• Acute hemorrhage
• Pregnancy (hemodilution)
• Fluid overload

📈 Increased Hct

• Dehydration
• High altitude, chronic lung disease
• Polycythemia vera

⚠ Interpretation Tips

• Highly affected by hydration status.
• Rough correlation: Hct ≈ Hb × 3
• Increased MCV can artificially elevate Hct.

---

5️⃣ Platelet Count (PLT)

Critical for clot formation and hemostasis

🔍 Definition

Number of platelets per microliter of blood

🔢 Reference Range

150,000–450,000 /µL

📈 Increased Platelets (Thrombocytosis)

• Reactive thrombocytosis (infection, inflammation)
• Post-surgery
• Iron deficiency
• Recovery from bleeding
• Myeloproliferative neoplasms (ET, PV)

📉 Decreased Platelets (Thrombocytopenia)

• ITP (immune thrombocytopenic purpura)
• Bone marrow suppression (chemo, MDS, aplastic anemia)
• DIC / sepsis
• Liver disease, splenomegaly
• Drug-induced (e.g., heparin → HIT)

⚠ Interpretation Tips

• Platelet clumping (EDTA-dependent) can cause falsely low PLT → check peripheral smear.
• Combine with MPV/PDW for better interpretation of platelet disorders.

---

🧪 How CBC Is Measured

Automated Hematology Analyzers

Modern CBC testing relies on automated analyzers using multiple technologies:

✔ Key Measurement Methods

1. Impedance (Electrical Resistance) Method

• Measures size and number of cells
• Used for RBC, WBC, platelet counts

2. Flow Cytometry (Light Scatter)

• Analyzes forward and side scatter
• Essential for WBC differential

3. Spectrophotometry

• Used for Hb measurement (cyanmethemoglobin or SLS methods)

4. Hydrodynamic Focusing

• Aligns cells in single file for improved accuracy

✔ Advantages

• High precision and reproducibility
• Rapid, high-throughput testing
• Automatic differential count

✔ Limitations

• Poor recognition of abnormal cells (blasts, malignant cells) → peripheral smear required
• Affected by sample delay, EDTA-induced artifacts (PLT clumping, MPV changes)

Reticulocyte Count Test: Complete Guide

---

📚 References

• Henry’s Clinical Diagnosis and Management by Laboratory Methods, 24th ed.
• Rodak’s Hematology: Clinical Principles and Applications, 6th ed.
• ICSH Guidelines for CBC and Platelet Indices
• CLSI H26-A2: Enumeration of Cellular Elements of the Blood
• Hoffbrand’s Essential Haematology, 8th ed.
• WHO Laboratory Manual for Hematology, 2023 Update