From the Guidelines
The most appropriate diagnostic study for this 18-month-old boy with fever, upper respiratory symptoms, and a warm, swollen right knee with decreased range of motion is arthrocentesis of the knee (option E). This is based on the latest guidelines from the American College of Radiology, which emphasize the importance of prompt diagnosis and treatment of septic arthritis in children 1. The clinical presentation of this patient, including fever, recent respiratory infection, and an acutely inflamed joint, strongly suggests a bacterial joint infection, which requires immediate diagnosis and treatment. Arthrocentesis is the first-line diagnostic approach in this case because it allows for direct sampling of synovial fluid to identify the causative organism in what appears to be septic arthritis. This procedure provides crucial information about cell count, gram stain, culture, and sensitivity that guides appropriate antibiotic therapy. Other options like X-ray, MRI, or nuclear scan may provide structural information but cannot identify the specific pathogen, while rheumatoid factor testing is not relevant for acute infectious processes in this age group. According to the ACR Appropriateness Criteria, arthrocentesis is the recommended initial imaging study for suspected septic arthritis or soft tissue infection, especially when the initial radiographs are normal or show findings suggestive of joint effusion or soft tissue swelling 1. Prompt arthrocentesis followed by appropriate antibiotic therapy is essential to prevent joint destruction and other complications. The recent study by Shet et al. also highlights the importance of prompt diagnosis and treatment of septic arthritis in children, and recommends arthrocentesis as the first-line diagnostic approach 1. In summary, arthrocentesis of the knee (option E) is the most appropriate diagnostic study for this patient, as it allows for prompt diagnosis and treatment of septic arthritis, and is recommended by the latest guidelines and studies 1.
From the Research
Diagnostic Study Options
The following options are available for diagnostic study:
- Assay for serum rheumatoid factor
- X-ray of the knee
- Nuclear scan of the bone and joint of the knee
- MRI of the knee
- Arthrocentesis of the knee
Most Appropriate Diagnostic Study
Based on the provided evidence, the most appropriate diagnostic study for the 18-month-old boy with fever, upper respiratory tract symptoms, and a warm and swollen right knee is arthrocentesis of the knee 2. This procedure involves inserting a needle into the knee joint to aspirate synovial fluid, which can be tested to determine the nature of the knee effusion. Arthrocentesis can aid in diagnosing septic arthritis, crystal-induced arthritis, and non-inflammatory arthritis, and can help guide treatment.
Rationale for Arthrocentesis
The rationale for choosing arthrocentesis is supported by studies that demonstrate its effectiveness in diagnosing septic arthritis and other conditions. For example, a study published in 2020 found that a synovial cell count greater than 50,000/mm3 is not always a reliable indicator of septic arthritis, especially in the presence of crystalline arthropathy 3. Another study published in 2021 found that the optimal synovial leukocyte count cutoff for diagnosing septic arthritis varies depending on whether the patient has received antibiotics prior to arthrocentesis 4. A study published in 2020 also found that antibiotics can affect the accuracy of synovial fluid leukocyte cell count in patients with septic arthritis 5.
Key Points
Key points to consider when performing arthrocentesis include:
- The procedure should be performed with caution in patients with suspected cellulitis over the knee joint to avoid the potential risk of causing iatrogenic septic arthritis 2.
- The supine position may be more effective than the sitting position for obtaining complete drainage of the knee joint 6.
- Arthrocentesis can be used to diagnose a range of conditions, including septic arthritis, crystal-induced arthritis, and non-inflammatory arthritis 2, 3, 4.