Do I still need to do a blood culture if the White Blood Cell (WBC) count is normal?

Blood Cultures in the Setting of Normal WBC Count

Blood cultures are generally not recommended when the WBC count is normal, unless there are specific clinical indicators of serious infection such as fever, sepsis, or specific high-risk conditions. 1

Diagnostic Value of WBC Count

• An elevated WBC count (>14,000 cells/mm³) has a likelihood ratio of 3.7 for bacterial infection and warrants careful assessment 2
• However, a normal WBC count does not rule out serious bacterial infection:
  • Up to 33% of patients with bacteremia present with normal body temperature and 52% have a normal WBC count 3
  • 21% of patients with bacteremic pneumococcal pneumonia present with a normal WBC count 4

When to Perform Blood Cultures Despite Normal WBC

Blood cultures should still be obtained in the following scenarios despite normal WBC:

1. Suspected sepsis with clinical manifestations such as:

   • Fever, shaking chills, hypotension, or delirium 1
   • New or worsening organ dysfunction 2

2. Specific conditions with high risk of bacteremia:

   • Suspected endocarditis 1
   • Meningitis 5
   • Complicated pyelonephritis 1
   • Healthcare-associated pneumonia 5
   • Suspected urosepsis, especially with indwelling catheters 1

3. Immunocompromised patients who may not mount a normal inflammatory response 2

When to Avoid Blood Cultures

Blood cultures should be avoided in the following scenarios:

• Cellulitis without systemic symptoms 5
• Simple pyelonephritis 5
• Community-acquired pneumonia in stable patients 5, 6
• Long-term care facility residents with low suspicion for bacteremia 1

Blood Culture Collection Technique

If blood cultures are indicated:

• Collect before starting antibiotics 5
• Obtain at least two sets from separate peripheral sites 5
• Each set should include one aerobic and one anaerobic bottle 1
• Collect adequate volume (10 ml per bottle for adults) 1

Important Considerations

• False positives are harmful: They lead to unnecessary antibiotics, increased length of stay, and higher costs 5
• Low yield in certain settings: Blood cultures in community-acquired pneumonia have positive rates of only 3.7% and rarely alter management 6
• Check for bandemia: Bandemia (>5% bands) is present in ~80% of bacteremic patients with normal temperature or WBC count and may be a useful clue for occult bacteremia 3
• Clinical assessment is paramount: Clinical presentation and severity should guide decision-making beyond just laboratory values 1

Special Populations

In long-term care facilities:

• Blood cultures have low yield and rarely influence therapy 1
• They are not recommended for most residents with suspected bacteremia 1
• Consider blood cultures only if the facility has quick access to laboratory facilities, adequate physician coverage, and capacity to administer parenteral antibiotics 1

Remember that clinical judgment should take precedence over any single laboratory value when deciding whether to obtain blood cultures.