How do you interpret the results of a complete blood count?

Complete Blood Count (CBC) Interpretation

A complete blood count (CBC) is interpreted by systematically evaluating each component against established reference ranges while considering the clinical context, with abnormalities in red blood cells, white blood cells, and platelets potentially indicating specific disease processes that require further investigation. 1

Components of a CBC and Their Interpretation

Red Blood Cell Parameters

• Hemoglobin (Hb) and Hematocrit (Hct)

  • Low values indicate anemia
  • High values suggest polycythemia
  • In iron deficiency anemia: serum ferritin <30 μg/L without inflammation or <100 μg/L with inflammation 1

• Red Cell Indices

  • Mean Corpuscular Volume (MCV): Classifies anemia as microcytic, normocytic, or macrocytic
  • Red Cell Distribution Width (RDW): Elevated in iron deficiency anemia 1
  • Mean Corpuscular Hemoglobin (MCH): May be normal or low in anemia of chronic disease 1

White Blood Cell Parameters

• Total White Blood Cell Count

  • Elevated in infections, inflammation, leukemia
  • Decreased in viral infections, bone marrow suppression, or autoimmune disorders
  • Regular monitoring recommended in HIV patients to evaluate disease progression 2

• Differential Count

  • Neutrophils: Elevated in bacterial infections
  • Lymphocytes: Elevated in viral infections, decreased in HIV
  • Monocytes: Elevated in chronic infections
  • Eosinophils: Elevated in allergic reactions, parasitic infections
  • Basophils: Elevated in myeloproliferative disorders

Platelet Parameters

• Platelet Count

  • Low count (thrombocytopenia): <100 × 10^9/L 1
  • Very low count: <50 × 10^9/L requires closer monitoring 1
  • Critical low count: <25 × 10^9/L requires immediate hematology consultation 1
  • High count (thrombocytosis): May indicate inflammation, infection, or myeloproliferative disorders

• Mean Platelet Volume (MPV)

  • Increased in 22q11.2 deletion syndrome and other conditions 1

Systematic Approach to CBC Interpretation

1. Evaluate Red Blood Cell Parameters

   • Check hemoglobin/hematocrit against age and sex-specific reference ranges
   • If abnormal, examine MCV to classify anemia type:
     • Microcytic (MCV <80 fL): Consider iron deficiency, thalassemia
     • Normocytic (MCV 80-100 fL): Consider anemia of chronic disease, acute blood loss
     • Macrocytic (MCV >100 fL): Consider vitamin B12/folate deficiency, medications, alcohol abuse

2. Assess White Blood Cell Count and Differential

   • Evaluate total WBC count against reference range
   • Examine differential counts to identify specific cell line abnormalities
   • Consider clinical context (e.g., infection, inflammation, malignancy)

3. Review Platelet Count

   • Check for thrombocytopenia or thrombocytosis
   • In thrombocytopenia, grade severity:
     • Grade 1: <100 × 10^9/L - Monitor closely
     • Grade 2: <75 × 10^9/L - Consider holding medications that affect platelets
     • Grade 3: <50 × 10^9/L - Close monitoring required
     • Grade 4: <25 × 10^9/L - Requires hematology consultation 1

4. Consider Reticulocyte Count (if available)

   • Low/normal reticulocytes with anemia: Suggests impaired production
   • Elevated reticulocytes with anemia: Suggests hemolysis or blood loss 1

Special Considerations

Disease-Specific Patterns

• Iron Deficiency Anemia

  • Low hemoglobin, low MCV, high RDW, low ferritin
  • Serum ferritin <30 μg/L without inflammation 1

• Anemia of Chronic Disease

  • Normal or low MCV, normal or low MCH
  • Serum ferritin may be normal or elevated due to inflammation 1

• Immune Thrombocytopenia

  • Isolated low platelet count
  • Requires testing for HIV, hepatitis B/C, H. pylori 1

• HIV-Related Abnormalities

  • Regular CBC monitoring recommended at diagnosis, before starting antiretroviral therapy, and during follow-up 2
  • Severity of cytopenias correlates with CD4 count and viral load 2

Additional Testing Based on CBC Abnormalities

• For Anemia

  • Iron studies (ferritin, transferrin saturation)
  • Vitamin B12 and folate levels
  • Reticulocyte count
  • Hemolysis workup if indicated (haptoglobin, LDH, bilirubin) 1

• For Thrombocytopenia

  • Peripheral blood smear
  • Consider testing for HIV, hepatitis B/C, H. pylori in suspected immune thrombocytopenia 1
  • Direct antiglobulin test to rule out Evans syndrome 1

• For Leukocyte Abnormalities

  • Peripheral blood smear
  • Consider bone marrow evaluation if multiple cell lines affected 1

Clinical Applications

CBC abnormalities can serve as valuable indicators for various conditions:

• Cardiovascular disease risk assessment 3
• Metabolic disease prediction 4
• Monitoring response to medications
• Infection and inflammation surveillance
• Cancer screening and monitoring

Remember that CBC values represent a patient-specific "setpoint" that remains relatively stable over time, making significant deviations from a patient's baseline potentially more meaningful than deviations from population reference ranges 5.