What does an abnormal white blood cell count indicate?

Abnormal White Blood Cell Count Interpretation

An abnormal white blood cell (WBC) count is a strong indicator of bacterial infection, particularly when elevated (>14,000 cells/mm³) and accompanied by a left shift (increased immature neutrophils), which together provide compelling evidence for active infection requiring prompt evaluation and potential antibiotic therapy. 1

Types of Abnormal WBC Counts

Elevated WBC Count (Leukocytosis)

• Strong indicator of bacterial infection when WBC >14,000 cells/mm³ (likelihood ratio 3.7) 1
• Even stronger evidence when accompanied by:
  • Band counts >1,500 cells/mm³ (likelihood ratio 14.5)
  • Neutrophils >90% (likelihood ratio 7.5)
  • Band neutrophils >16% (likelihood ratio 4.7) 1
• Other potential causes:
  • Inflammatory conditions (e.g., inflammatory bowel disease) 1
  • Cardiovascular conditions (e.g., pericarditis with inflammatory effusions) 1
  • Medication side effects 2
  • Parasitic infections (with eosinophilia) 1

Low WBC Count (Leukopenia)

• May indicate:
  • Viral infections
  • Bone marrow suppression (e.g., from medications or cancer)
  • Autoimmune disorders
  • Overwhelming bacterial infection 3

Clinical Significance

Diagnostic Value

• WBC count is part of the initial hemostasis tests (along with PT and PTT) when evaluating for bleeding disorders 4
• Abnormal WBC counts can predict risk of:
  • Cardiovascular disease
  • Type 2 diabetes
  • Metabolic syndrome 5
  • Long-term mortality in elderly patients 6

Prognostic Value

• WBC count >8,000/mm³ correlates with increased risk of complications in certain infections 1
• In elderly patients, each 10^9/L increase in WBC count is associated with:
  • 16% increased mortality risk in men
  • 28% increased mortality risk in women 6

Evaluation Algorithm for Abnormal WBC Count

1. Assess for clinical signs of infection:

   • Vital signs (temperature, heart rate, blood pressure, respiratory rate)
   • Fever (>100°F/37.8°C) or increase of 2°F (1.1°C) over baseline
   • Localized symptoms of infection 1

2. Order additional tests based on clinical suspicion:

   • Complete blood count with differential to assess for left shift
   • C-reactive protein (CRP)
   • Blood cultures if sepsis, endocarditis, or complicated urinary tract infection is suspected 1
   • Site-specific cultures based on symptoms

3. Consider empiric antibiotic therapy if:

   • WBC >14,000 cells/mm³ with left shift
   • Signs of systemic illness
   • Immunocompromised status 1

Common Pitfalls and Caveats

• Overreliance on WBC count alone: Clinical signs and symptoms should guide diagnosis, not laboratory values in isolation 1
• Ignoring the differential: The pattern of cell types (neutrophils, lymphocytes, etc.) provides more diagnostic information than the total count 1
• Failure to consider non-infectious causes: Medications, stress, and inflammatory conditions can all affect WBC count 3
• Underdiagnosis in elderly patients: Elderly may not present with typical signs of infection despite abnormal WBC counts 1
• Cost-effectiveness concerns: CBC is one of the most frequently ordered tests but may not always be necessary or cost-effective 7

Special Considerations

• Immunocompromised patients: May not mount a normal WBC response to infection 1
• Elderly patients: WBC count has significant prognostic value for long-term survival 6
• Patients on medications affecting bone marrow: May have altered WBC counts not related to infection 2

Remember that while an abnormal WBC count is a valuable diagnostic clue, it must be interpreted in the context of the patient's clinical presentation and other laboratory findings.