What is the first investigation for a patient presenting with swollen and warm knees?

Initial Investigation for Swollen, Warm Knees

The first investigation should be plain radiographs (X-rays) of both knees, followed immediately by laboratory testing including ESR, CRP, and blood cultures if septic arthritis is suspected. 1

Rationale for Radiographs First

• Radiographs are the mandatory initial imaging study because they are safe, inexpensive, quickly obtained, and widely available for all suspected musculoskeletal infections including septic arthritis. 1

• Plain X-rays provide essential baseline information by:

  • Excluding fractures, tumors, or degenerative changes as alternative diagnoses 1
  • Detecting joint effusion and soft tissue swelling suggestive of septic arthritis 1
  • Identifying gas in soft tissues or foreign bodies 1
  • Establishing anatomic detail that guides interpretation of subsequent imaging 1

• Early radiographs may be normal in acute infection (especially within 14 days), showing only soft tissue swelling, but they remain essential to exclude other pathology. 1

Immediate Laboratory Workup

Once radiographs are obtained, proceed immediately with:

• ESR and CRP together - The combination achieves 93% sensitivity, 100% specificity, and 97% accuracy when at least 2 of 3 tests are abnormal for septic arthritis. 2, 3

• Blood cultures (aerobic and anaerobic) - Essential given the presence of fever and systemic signs of infection. 2

• Serum interleukin-6 - Provides higher predictive value than most other serologic markers when combined with CRP. 3, 4

Critical Next Step: Joint Aspiration

After initial radiographs and labs, proceed immediately with image-guided joint aspiration for synovial fluid analysis if inflammatory markers are elevated or clinical suspicion remains high. 1, 3, 4

Key Points About Aspiration:

• Withhold antibiotics for at least 2 weeks prior to aspiration if clinically feasible, as preaspiration antibiotic treatment causes false-negative cultures. 3, 4

• However, if there is strong clinical concern for septic arthritis, do not delay empiric antibiotics while awaiting culture results, as withholding antibiotics increases morbidity and mortality. 2

• Synovial fluid should be analyzed for:

  • Total and differential cell counts 3
  • Aerobic and anaerobic bacterial cultures 3
  • Leukocyte esterase, alpha-defensin, and CRP 3
  • Nucleic acid amplification testing 3

• A "dry tap" does not exclude infection - weekly repeat aspirations are recommended if initial aspiration is negative and clinical suspicion remains high. 3

Common Pitfalls to Avoid

• Do not rely on absence of fever, erythema, or warmth to exclude infection - chronic infections frequently present with pain alone without these classic signs. 3, 4

• Peripheral WBC counts are not elevated in most patients with infected joints, making a normal WBC count meaningless for excluding infection. 4

• Be aware that ESR and CRP have higher false-negative rates than previously reported (sensitivity of 0.85 and 0.88 respectively), particularly with low-virulence organisms and culture-negative cases. 5

• Staphylococcus aureus and coagulase-negative Staphylococcus are the most common causative organisms in septic arthritis. 3, 4

When Advanced Imaging Is Needed

If radiographs are normal but suspicion remains high after initial workup:

• MRI is not indicated for initial evaluation but may be considered if radiographs and aspiration are inconclusive, as it has 100% negative predictive value for excluding osteomyelitis. 1

• CT and ultrasound are not routinely used for initial evaluation of suspected septic arthritis. 1

• Image guidance (ultrasound, CT, or fluoroscopy) for aspiration is ideal because it confirms proper needle placement and reduces vascular complications. 1

Clinical Context

Most patients with septic arthritis respond within 72-96 hours of appropriate antibiotic therapy, and persistent symptoms beyond 4 days warrant reassessment for complications or alternative diagnoses. 2