What is the likely diagnosis for a 7-year-old man with acute onset of knee pain, swelling, erythema, warmth, and limited range of motion, along with systemic signs of infection such as fever and tachycardia, and a medical history of type 2 diabetes, gout, and eczema?

Septic Arthritis of the Right Knee

Blood cultures are the most likely diagnostic test to confirm the diagnosis in this patient with septic arthritis of the right knee.

Clinical Presentation and Assessment

This 7-year-old patient presents with classic signs of septic arthritis:

• Three-day progressive right knee pain and swelling
• Inability to bend the knee
• Systemic symptoms: fever (38.3°C), tachycardia (pulse 105)
• Local signs: swollen, erythematous, warm knee with painful and limited range of motion
• Relevant medical history: type 2 diabetes (risk factor for infection)

These findings strongly suggest septic arthritis, which is an orthopedic emergency requiring prompt diagnosis and treatment to prevent joint destruction and significant morbidity 1, 2.

Diagnostic Approach

When evaluating a patient with suspected septic arthritis, several diagnostic tests are relevant:

1. Blood cultures - Most appropriate initial test as they:

   • Can identify the causative pathogen in bacteremic septic arthritis
   • Are essential before starting antibiotics
   • Help guide definitive antibiotic therapy
   • Have high yield in patients with systemic signs (fever, tachycardia) 2, 3

2. Synovial fluid analysis - While critical for diagnosis, this would be performed after blood cultures are obtained:

   • Requires arthrocentesis (joint aspiration)
   • Provides definitive diagnosis but is more invasive
   • Should follow blood culture collection 3

3. Serum CRP - Useful but less specific:

   • Elevated in infection but also in other inflammatory conditions
   • Sensitivity of 73-91% for prosthetic knee infection 1
   • Supportive test but not the most definitive

4. Uric acid - Not the primary diagnostic test:

   • Would help diagnose gout (in differential diagnosis)
   • Patient has history of gout, but clinical presentation strongly favors infection
   • Less relevant given the acute presentation with fever and systemic signs

5. ESR - Nonspecific inflammatory marker:

   • Elevated in infections but also many other conditions
   • Less specific than CRP for acute infection 1

Rationale for Blood Cultures

Blood cultures are the most appropriate initial diagnostic test because:

1. The patient has systemic signs (fever, tachycardia) suggesting bacteremia
2. Isolation of the causative agent is essential before selecting definitive antibiotic therapy 3
3. Staphylococcus aureus is the most common pathogen in septic arthritis, and blood cultures have good yield for this organism 2, 4
4. The patient's diabetes increases risk for bacteremic spread of infection 1, 4

Management Considerations

After obtaining blood cultures:

• Synovial fluid aspiration should be performed for definitive diagnosis
• Empiric antibiotics should be started immediately after cultures are obtained
• Joint drainage (arthroscopic or open) is typically required
• Vancomycin is appropriate empiric coverage for suspected MRSA until culture results return 3

Common Pitfalls to Avoid

1. Delaying blood cultures by starting antibiotics first
2. Mistaking septic arthritis for gout flare (they can coexist)
3. Relying solely on inflammatory markers without obtaining cultures
4. Underestimating the urgency of diagnosis and treatment in septic arthritis
5. Failing to recognize that diabetes is a significant risk factor for septic arthritis 4

Remember that septic arthritis is considered an orthopedic emergency, and prompt diagnosis with blood cultures followed by appropriate antibiotic therapy and joint drainage is essential to prevent permanent joint damage 5.