How do you interpret an abnormal differential White Blood Cell (WBC) count?

Interpretation of Differential White Blood Cell Count

A differential white blood cell count should be interpreted by comparing values to established normal ranges, with abnormal values indicating specific pathological processes such as infection, inflammation, allergic reactions, or malignancy.

Normal Reference Ranges

• Normal differential WBC count ranges for adults 1:

  • Lymphocytes: <15% indicates normal, >15% indicates lymphocytosis
  • Neutrophils: <3% indicates normal, >3% indicates neutrophilia
  • Eosinophils: <1% indicates normal, >1% indicates eosinophilia
  • Mast cells: <0.5% indicates normal, >0.5% indicates mastocytosis

• Total WBC count is considered elevated (leukocytosis) when >14,000 cells/mm³ 1

Interpretation of Abnormal Values

Neutrophilia (Elevated Neutrophils)

• Neutrophil count >3% or total band neutrophil count >1,500 cells/mm³ indicates a high likelihood of bacterial infection 1
• A left shift (band neutrophils >16%) has a likelihood ratio of 4.7 for bacterial infection 1
• Neutrophilia can also be caused by 2:
  • Increased bone marrow production
  • Redistribution among body neutrophil pools
  • Stress response
  • Corticosteroid administration

Lymphocytosis (Elevated Lymphocytes)

• Lymphocyte count >25% suggests granulomatous disease (e.g., sarcoidosis, hypersensitivity pneumonitis) 1
• Lymphocyte count >50% particularly suggests hypersensitivity pneumonitis or cellular NSIP (non-specific interstitial pneumonia) 1
• Other causes include chronic infections and allergic reactions 2

Eosinophilia (Elevated Eosinophils)

• Eosinophil count >1% indicates eosinophilia 1
• Common causes include allergic reactions, parasitic infections, skin diseases, neoplasia, and adrenocortical insufficiency 2

Monocytosis (Elevated Monocytes)

• Associated with stress, infections, hematologic disorders, gastrointestinal disease, tissue necrosis, and hemolysis 2

Clinical Applications

Infection Detection

• The presence of an elevated WBC count (>14,000 cells/mm³) or a left shift (band neutrophils >16% or total band count >1,500 cells/mm³) warrants careful assessment for bacterial infection, even without fever 1
• An elevated total band count (>1,500 cells/mm³) has the highest likelihood ratio (14.5) for detecting documented bacterial infection 1
• Neutrophil percentage >90% has a likelihood ratio of 7.5 for bacterial infection 1

Interstitial Lung Disease Evaluation

• In bronchoalveolar lavage (BAL) fluid analysis, differential cell counts help identify specific types of interstitial lung disease 1:
  • Lymphocytosis suggests sarcoidosis, hypersensitivity pneumonitis, or drug reactions
  • Neutrophilia may indicate infection
  • Eosinophilia suggests eosinophilic lung disease

Mortality Risk Assessment

• Elevated WBC counts, particularly neutrophils, monocytes, and neutrophil-to-lymphocyte ratio, are associated with increased all-cause mortality risk 3

Important Considerations

• Manual differential counts are preferred over automated counts when assessing for band forms and other immature cells 1
• The neutrophil count is more informative than the neutrophil percentage for detecting neutrophilia and monitoring recovery 4
• Biological variation is much greater than statistical sampling variation in differential WBC counts 4
• In overwhelming sepsis, leukopenia may follow an initial leukocytosis 5
• Consider factors that may affect interpretation, including age, smoking status, and presence of chronic conditions 3

Common Pitfalls to Avoid

• Relying solely on total WBC count without differential analysis can miss important diagnostic clues 4
• Failing to consider the clinical context when interpreting results 1
• Not accounting for the timing of the blood draw in relation to the disease process (e.g., early sepsis vs. late sepsis) 5
• Overlooking the possibility of contamination in BAL fluid samples, which can affect differential counts 1