Study Purpose: This research aims to find better ways to quickly detect septic arthritis, a serious joint infection in children. Septic arthritis requires surgery to clean out the infection to prevent damage. The study checks if certain proteins in joint fluid, like alpha-defensin, can accurately diagnose this infection faster than current methods.

Study Details: If a child is suspected of having a joint infection, doctors often remove some joint fluid for testing. This study will include children under 18 who already need this procedure. The fluid will be tested for different markers, such as alpha-defensin, to see if they indicate infection. The study will also include children having unrelated surgeries to compare results. Only one visit is needed for this study, and it's done alongside regular care. Results will be compared with traditional culture tests, which are slower.

• 1 Visit: No extra visits are needed; testing is part of regular care.
• Eligibility: Includes children under 18 needing joint fluid testing.
• Exclusions: Recent major joint injury or infection may disqualify participation.