This is a prospective, observational study of patients presenting to the emergency departments at 9 EMERGEncy ID NET sites. The objectives of the proposed study are to:

1. Describe the range and proportion of infectious agents in synovial fluid as detected by standard C&S and investigational PCR testing, i.e., Biofire® Film Array® Bone and Joint Infection (BJI) Panel,
2. Describe the epidemiology of patients receiving diagnostic arthrocentesis and those diagnosed with septic arthritis in the emergency department (ED),
3. Determine the prevalence of septic arthritis in US ED patients presenting with an atraumatic painful swollen joint, and
4. Determine the clinical (history and physical examination) and laboratory characteristics of septic arthritis.
5. Determine the performance characteristics and concordance of synovial fluid PCR compared to culture, and project the impact of point-of-care PCR on patient management.

Study coordinators screen the ED log for adult patients presenting with joint pain and whose treating physician ordered an arthrocentesis. After confirming eligibility, study coordinators approach the patient to explain the study, and present the written consent form. If the patient agrees to participate and consent, the study coordinator completes an enrollment data collection using patient and treating physician interview to gather responses. After enrollment, the study coordinator will ensure that approximately 0.3-1.0 mL of leftover synovial fluid is saved and stored in a freezer for shipment to a central laboratory (Truman Medical Center hospital laboratory, Kansas City, MO) for testing. Approximately 30 days after enrollment, study coordinators complete an electronic medical record (EMR) review.

Inclusion Criteria:

1. Age 18 years and older;
2. diagnostic arthrocentesis in the emergency department (ED) or during hospitalization if admitted from the ED;
3. arthrocentesis and joint culture ordered; and
4. provide written consent in English or Spanish

Exclusion Criteria:

1. Unable to consent or no legal authorized representative is available; or
2. prisoner or parolee

This is a prospective, multicenter, observational study being conducted at 9 EMERGEncy ID NET sites, Olive View-UCLA Medical Center, Los Angeles, CA; Temple University Hospital, Philadelphia, PA; University of Mississippi Medical Center, Jackson, MS; Valleywise Medical Center, Phoenix, AZ; Truman Medical Center/U. Missouri Kansas City, Kansas City, MO; Hennepin Medical Center, Minneapolis, MN, University of New Mexico Health Sciences Center, Albuquerque, NM; University of Iowa Hospital, Iowa City, IA; and Johns Hopkins Medical Institute, Baltimore, MD. These hospitals service a mostly urban and underserved population.

Patients presenting to the above hospital emergency departments will be approached by study team members who meet the above eligibility criteria. The target enrollment for the study is 500 participants.