White Blood Cell Elevation in Infection
White blood cell (WBC) counts typically begin to elevate within hours of an infection, with a significant left shift (increased immature neutrophils) detectable within 24-48 hours of infection onset. 1
Timeline of WBC Response to Infection
- Initial response: WBC elevation begins within hours of bacterial invasion
- Peak response: A left shift in WBC count becomes prominent within 24-48 hours
- Magnitude: The degree of elevation varies by infection type and severity:
- Bacterial infections typically cause more pronounced elevations (>14,000 cells/mm³)
- Viral infections generally cause milder or no elevations
Diagnostic Value of WBC Changes
The Infectious Diseases Society of America and American Geriatrics Society highlight that specific WBC patterns strongly indicate bacterial infection 1:
- WBC count >14,000 cells/mm³ (likelihood ratio: 3.7)
- Band counts >1,500 cells/mm³ (likelihood ratio: 14.5)
- Neutrophils >90% (likelihood ratio: 7.5)
- Band neutrophils >16% (likelihood ratio: 4.7)
These changes are most valuable when interpreted together rather than in isolation. The combination of left shift plus elevated WBC count provides much stronger evidence for bacterial infection than either finding alone 1.
Clinical Implications
- High specificity: WBC counts >15,000 cells/mm³ have a specificity of 74% for serious bacterial infections, while counts >20,000 cells/mm³ have a specificity of 91% 2
- Limited sensitivity: Even with significant bacterial infections, WBC may not always elevate, with sensitivity of only 56% for counts >15,000 cells/mm³ 2
- Comparative value: C-reactive protein (CRP) generally outperforms WBC count for detecting serious bacterial infections, with CRP >80 mg/L having similar specificity (91%) but higher sensitivity (55% vs 32%) compared to WBC >20,000 cells/mm³ 2
Important Caveats
- False positives: Corticosteroid administration can cause WBC elevation of up to 4.84 × 10⁹/L within 48 hours, particularly with high-dose steroids 3
- False negatives: Normal or low WBC counts do not rule out bacterial infection 4
- Age considerations: Interpretation may differ in elderly patients, where even subtle WBC elevations may have prognostic significance 5
- Clinical context: WBC counts should never be interpreted in isolation but always in the context of clinical signs and symptoms 1
Bottom Line
When evaluating for infection, remember that WBC elevation occurs rapidly (within hours) but becomes most diagnostically useful within 24-48 hours when a left shift develops. While high WBC counts strongly suggest bacterial infection, normal counts don't exclude it, making clinical assessment essential in all cases.