Arthrocentesis is the Most Likely Test to Reveal the Diagnosis
Arthrocentesis with synovial fluid analysis is the definitive diagnostic test and must be performed immediately in this patient with acute monoarticular knee pain, swelling, fever, and systemic symptoms to diagnose septic arthritis—an orthopedic emergency that can cause irreversible cartilage damage within hours to days. 1
Why Arthrocentesis is Essential
Septic arthritis is the primary concern in this elderly diabetic patient presenting with acute monoarticular knee pain, fever (38.3°C), tachycardia, and constitutional symptoms, as bacterial proliferation can rapidly destroy the joint permanently. 1, 2
The American College of Radiology explicitly recommends joint aspiration as the definitive diagnostic procedure for suspected septic arthritis of the knee, which should be performed immediately before initiating antibiotics to maximize organism identification. 1, 3
Synovial fluid analysis is the criterion standard for diagnosing septic arthritis and must include white blood cell count with differential, Gram stain, aerobic and anaerobic cultures, and crystal analysis to differentiate infectious from crystal-induced arthropathy. 1, 3
Why Other Tests Are Inadequate
Serum C-reactive Protein (Option A)
- While CRP is elevated in septic arthritis, it is nonspecific and cannot distinguish between septic arthritis, gout, or other inflammatory conditions, making synovial fluid analysis essential for diagnosis. 1
- CRP is useful for monitoring treatment response but not for establishing the diagnosis. 2
Serum Uric Acid (Option B)
- Serum uric acid levels do not reliably diagnose acute gout, as levels can be normal during an acute attack. 1
- Crystal analysis of synovial fluid is required to definitively diagnose gout, and even if crystals are identified, septic arthritis must still be ruled out since the two conditions can coexist in 73% of reported cases. 2
Procalcitonin (Option C)
- Like CRP, procalcitonin is elevated in septic arthritis but is nonspecific and cannot distinguish between infectious and non-infectious inflammatory conditions. 1
- It does not provide the definitive diagnosis that synovial fluid analysis offers. 1
Blood Cultures (Option E)
- While blood cultures should be obtained if sepsis is present, they are positive in only a minority of septic arthritis cases. 3
- Synovial fluid culture has higher sensitivity (76%) and specificity (96%) for diagnosing septic arthritis compared to blood cultures. 2
- Blood cultures do not exclude septic arthritis if negative and cannot differentiate from other causes of monoarticular arthritis. 2
Critical Clinical Pitfalls
The most dangerous error is failing to perform arthrocentesis when septic arthritis is suspected, as this can lead to permanent joint destruction and mortality. 1
Even in patients with a history of gout (as this patient has), septic arthritis and gout can coexist, and the only way to exclude infection is through synovial fluid analysis. 1, 2
The American College of Physicians emphasizes that if septic arthritis is in the differential, aspiration for Gram stain and culture remains essential, even if gout is suspected. 1
Practical Execution
Knee arthrocentesis can be performed at bedside without imaging guidance, unlike hip aspiration which requires ultrasound. 2, 3
Image-guided aspiration using ultrasound or fluoroscopy is preferred in difficult cases to ensure proper needle placement and reduce complications. 1, 3
Arthrocentesis must be performed prior to antibiotic administration to maximize the likelihood of identifying the causative organism. 1, 3
A negative culture does not rule out infection, especially if antibiotics were given prior to aspiration, but Gram stain and cell count still provide critical diagnostic information. 1, 3