What blood work is recommended for the evaluation of possible osteomyelitis?

Blood Work for Evaluation of Possible Osteomyelitis

The most important blood tests for evaluating possible osteomyelitis include erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and blood cultures, with ESR and CRP being particularly valuable for both diagnosis and monitoring treatment response. 1, 2

Essential Laboratory Tests

• Blood Cultures: Obtain two sets of aerobic and anaerobic blood cultures before starting antibiotics to identify potential hematogenous seeding 2
• Erythrocyte Sedimentation Rate (ESR): A highly elevated ESR (>70 mm/h) increases the likelihood of osteomyelitis with a positive likelihood ratio of 11 1
• C-Reactive Protein (CRP): An elevated CRP (>10-30 mg/L) in combination with clinical suspicion warrants further investigation 3
• Complete Blood Count (CBC): While white blood cell count is often obtained, it has poor sensitivity for osteomyelitis and should not be used alone for diagnosis 3, 4
• Procalcitonin (PCT): May be helpful when clinical examination is diagnostically equivocal 2

Interpretation of Laboratory Values

ESR

• Values >60 mm/h have a sensitivity of 74% and specificity of 56% for osteomyelitis in diabetic foot infections 5
• ESR <30 mm/h suggests low likelihood of osteomyelitis 5
• ESR typically peaks 3-5 days after infection onset and normalizes slowly (approximately 18 days) 4

CRP

• Values >7.9 mg/dL have a sensitivity of 49% and specificity of 80% for osteomyelitis in diabetic foot infections 5
• CRP rises and falls more rapidly than ESR, making it more useful for monitoring treatment response 4
• CRP typically peaks at day 2 of infection and normalizes within a week (mean 6.9 days) 4

Diagnostic Algorithm

1. Initial Assessment:

   • If ESR <30 mm/h and CRP <5 mg/L in a patient with low clinical suspicion, osteomyelitis is unlikely 3, 5
   • If ESR >60 mm/h and CRP >7.9 mg/dL, the likelihood of osteomyelitis is high 5

2. Intermediate Values:

   • For patients with ESR 30-60 mm/h or CRP 5-7.9 mg/dL, consider additional diagnostic tests such as imaging 1, 2

3. Monitoring Treatment:

   • Follow CRP for early response (normalizes within a week) 4
   • Follow ESR for longer-term monitoring (normalizes in approximately 3 weeks) 4

Important Considerations

• Organism-Specific Variations: Laboratory values may be influenced by the infecting organism type 6

  • Staphylococcus aureus and antibiotic-resistant organisms typically cause higher ESR and CRP elevations 6
  • Culture-negative, fungal, and tuberculosis cases may present with lower inflammatory marker levels 6

• False Negatives: Normal inflammatory markers do not rule out osteomyelitis in high-risk patients, especially those with foot ulcers or puncture wounds 1

• Definitive Diagnosis: While blood tests are valuable screening tools, definitive diagnosis often requires bone biopsy for culture and histopathology 1

• Comorbidities: Autoimmune disorders and other inflammatory conditions can elevate ESR and CRP, potentially leading to false positives 5

When to Consider Bone Biopsy

Bone biopsy should be considered when:

• Blood cultures suggest antibiotic-resistant organisms 1
• There is progressive bony deterioration or persistently elevated inflammatory markers despite treatment 1
• The patient fails to respond to empiric antibiotic therapy 1
• Definitive diagnosis is needed to justify surgical intervention 1