What labs should be ordered on an olecranon bursa aspirate to rule out infection?

Laboratory Tests for Olecranon Bursa Aspirate to Rule Out Infection

When evaluating an olecranon bursa aspirate to rule out infection, the most essential laboratory tests include cell count with differential, Gram stain, and aerobic and anaerobic cultures. These tests provide the most critical information for diagnosis and management of septic bursitis.

Core Laboratory Tests

Essential Tests:

• Cell count with differential

  • White blood cell (WBC) count >3,000 cells/mm³ suggests infection 1
  • Differential analysis showing neutrophil predominance (>50%) suggests bacterial infection 1
  • Note that some septic bursitis cases may have WBC counts as low as 3,000 cells/mm³ 2, 3

• Gram stain

  • Immediate visualization of bacterial morphology 1
  • Can guide initial empiric therapy while awaiting culture results
  • Presence of gram-positive cocci (especially in clusters) suggests Staphylococcus aureus, the most common pathogen

• Aerobic and anaerobic bacterial cultures

  • Gold standard for definitive diagnosis 1
  • Should be collected before antibiotic administration whenever possible
  • Consider sending fluid in anaerobic blood culture bottle to improve yield 1

Additional Tests Based on Clinical Suspicion

• Glucose and protein levels

  • Low glucose and elevated protein suggest infection 1
  • Particularly useful when comparing to serum values

• Crystal analysis

  • To rule out crystalline arthropathy (gout, pseudogout) as alternative diagnosis
  • Important in patients with history of gout or unexplained recurrent bursitis

• Special cultures/stains when indicated:

  • Fungal stain and culture if immunocompromised or chronic/recurrent bursitis 3, 4
  • Acid-fast bacilli stain and mycobacterial culture if tuberculosis is suspected 1

Interpretation of Results

• Cell count interpretation:

  • WBC >3,000 cells/mm³ with >50% neutrophils strongly suggests infection
  • Lower counts don't completely rule out infection, especially in immunocompromised patients 3

• Culture results:

  • Staphylococcus aureus is the most common pathogen (80% of cases)
  • Streptococcus species account for approximately 5-10%
  • Gram-negative organisms are less common but may be seen in immunocompromised hosts

Common Pitfalls to Avoid

1. Failing to collect samples before antibiotic administration

   • Antibiotics can cause false-negative cultures
   • If patient has received antibiotics, consider this when interpreting negative cultures

2. Inadequate sample collection

   • Ensure sufficient fluid is collected for all requested tests
   • Prioritize culture if sample volume is limited

3. Misinterpreting results in immunocompromised patients

   • These patients may have atypical presentations with lower cell counts
   • Consider fungal pathogens in immunocompromised patients 3, 4

4. Overlooking unusual pathogens

   • Consider fungal or mycobacterial infections in chronic cases or those unresponsive to standard therapy
   • Recent research suggests that empirical treatment without aspiration may be effective for uncomplicated cases, but laboratory analysis is still recommended for initial diagnosis 5

By following this systematic approach to laboratory analysis of olecranon bursa aspirate, clinicians can effectively rule out infection and guide appropriate treatment decisions.