When to Order a CBC for Fever
A CBC should be ordered after 3 days of persistent unexplained fever, especially when evaluating for possible serious bacterial infection or in patients with risk factors for complications.
Evaluation of Fever Based on Duration
Initial Presentation (Days 1-3)
- For most patients with fever without localizing symptoms, a CBC is not necessary in the first 3 days unless there are concerning symptoms or risk factors 1
- In the initial evaluation of fever, clinical assessment is more important than laboratory tests, as CBC results may not reliably distinguish between viral and bacterial infections in the early stages 2
- The white blood cell count lacks specificity and sensitivity in distinguishing between bacterial and viral infections, particularly early in the course of illness 2, 3
After 3 Days of Fever (Days 3-5)
- For persistent fever lasting 3 days or more without an identified source, a CBC should be ordered as part of the diagnostic reassessment 1
- By day 3 of persistent fever, diagnostic reassessment including CBC is recommended to evaluate for occult bacterial infection 1
- In patients with fever lasting more than 3 days, CBC results become more reliable indicators of potential serious bacterial infection 4
After 5 Days of Fever
- For fever persisting 5 days or more, a CBC is essential for diagnosis of conditions like Kawasaki disease, which requires fever ≥5 days plus laboratory findings including anemia, elevated WBC count, and thrombocytosis after day 7 1
- In cases of fever lasting 5 days or more without an identified source, a CBC is necessary to evaluate for incomplete Kawasaki disease, particularly in infants and children 1
Special Populations Requiring Earlier CBC Testing
High-Risk Patients
- Neutropenic patients with fever should have a CBC ordered immediately at fever onset, not waiting for 3 days 1, 5
- Immunocompromised patients should have a CBC drawn at the first sign of fever 1, 5
- Infants under 6 months with fever require earlier CBC evaluation due to higher risk of serious bacterial infection and less reliable clinical signs 1
Risk Factors for Occult Bacterial Infection
- Patients with any of the following risk factors should have a CBC ordered earlier (within 24 hours of fever onset): age ≥50 years, diabetes mellitus, or appearing clinically ill 6
- The predictive value of CBC for serious bacterial infection increases when fever has been present for >12 hours 4
Interpretation of CBC Results
- A white blood cell count ≥15,000/mm³ increases the likelihood of bacterial infection, particularly when fever has been present for >12 hours 6, 4
- An absolute neutrophil count ≥10,000/mm³ or band count ≥500/mm³ may indicate bacterial infection, though these are more reliable after 12 hours of fever 4, 3
- C-reactive protein (CRP) performs better than WBC and ANC in predicting serious bacterial infection, especially after 12 hours of fever 4
Common Pitfalls to Avoid
- Ordering CBC too early (before 3 days) in otherwise healthy patients with fever may lead to unnecessary testing and false reassurance 2, 3
- Failing to order CBC after 3-5 days of persistent fever may delay diagnosis of serious conditions like Kawasaki disease 1
- Relying solely on CBC results without considering clinical context can lead to misdiagnosis, as bacterial markers are more predictive after 12 hours of fever 4
- Not ordering a CBC for high-risk patients (neutropenic, immunocompromised, infants <6 months) at the first sign of fever can lead to delayed treatment of potentially serious infections 1, 5