Differentiating between septic arthritis and other causes of joint inflammation in pediatric patients is challenging and of the utmost importance because septic arthritis requires surgical debridement as part of the treatment regimen. The current gold standard to diagnose septic arthritis in children is a positive synovial fluid culture; however, joint cultures may take several days to return. If a bacterial infection is present, it requires immediate surgical intervention in order to prevent lasting articular cartilage damage. Frequently surgeons must decide whether to surgically debride a joint before culture results are available. There is no single lab test or clinical feature that reliably indicates bacterial infection over other causes of joint inflammation. The alpha-defensin assay has shown high sensitivity and specificity for joint infection in other studies.The purpose of this study is to determine the sensitivity and specificity of several synovial biomarkers for diagnosing pediatric septic arthritis.

The purpose of this study is to determine if alpha-defensin and other proteins present in joint fluid may be able to rapidly diagnose bacterial joint infections. Patients with suspected joint infection typically undergo joint aspiration so that tests can be performed to help diagnose joint infection, including gram stain, cell count, and culture. Patients under 18 years old that are undergoing sampling of their joint fluid due to suspicion of infection or inflammation will be enrolled in this multi-center trial. Joint fluid will also be sampled from normative controls made up of patients who are undergoing an unrelated procedure without inflammation or infection. Joint fluid from patients with suspected inflammation/infection and from normative controls will be analysed for presence of alpha-defensin, leukocyte esterase, neutrophil elastase, synovial C-reactive protein, and synovial lactate. The alpha-defensin assay has shown high sensitivity and specificity for joint infection in other studies. Additionally a Staphylococcus spp antigen panel, Candida spp antigen panel, Enterococcus faecalis assay, BACTAlert culture, cell count plus differential, gram stain, and aerobic, anaerobic, and fungal cultures will be done using synovial fluid. A synovial fluid PCR for Kingella kingae will be performed if the patient is under eight years of age. Blood tests will include cell count and differential, erythrocyte sedimentation rate, C-reactive protein, procalcitonin, and D-dimer, as well as relevant inflammatory or rheumatologic marker tests. Results from these tests will be compared to joint fluid culture which the gold standard for diagnosing bacterial infection. The study includes 1 visit per patient, the standard of care visit in which the patient would be undergoing joint aspiration or arthroscopy. Once data has been collected, the sensitivity and specificity will be determined for these experimental tests both individually and in combination.

Inclusion Criteria- Septic Cases and Inflamed, Non-Septic Comparators:

• Synovial fluid is obtained to assess for infection or inflammatory/rheumatologic disease (all medium and large joints will be included: hip, knee, ankle, shoulder, subtalar, elbow, and wrist joints)
• Patients with recent antibiotic exposure are eligible to participate but will be analyzed separately

Inclusion Criteria- Normative Controls:

• Patients undergoing a procedure unrelated to infection (the procedure may be arthroscopy, or an open or percutaneous bony or soft tissue procedure)

Exclusion Criteria- All Participants:

• Family declines to participate/consent
• Patients with a major joint trauma (such as a documented ligament tear or fracture) within the past 8 weeks are not eligible to have that joint aspirated, but could have another joint aspirated

Exclusion Criteria- Normative Controls:

• A history of recent infection (within the past 3 months)
• Received antibiotics in the past 7 days

Collaborating sites will share de-identified patient data with the primary site, the Hospital for Special Surgery. The Hospital for Special Surgery, will incorporate de-identified aggregated data into a separate de-identified aggregated data sat to be provided to all study sites, but individual participant data will not be shared with all sites.

For patients with inflamed joints, the medical team will be blind to the results of the alpha-defensin assay and other experimental tests for a period of 2 weeks, after which results may be divulged to the clinical team at their request. As such, the results will not influence the physician/surgeons' decision for acute treatment of the patient, as the decision to treat suspected septic arthritis is typically made within 24 hours, and the final diagnosis is made within 2 weeks (pending culture results). As the testing of the leukocyte esterase strips will occur in the operating room, the surgeon will not be blinded to these results. A blinded team member will read the pictures of all of the leukocyte test strip results at once, after they have all been collected. The patient will be made aware of results at the physician/surgeon's discretion. For normative controls, the results of experimental testing will not be divulged to the clinical team or to the patient.