A total of 159 participants (118 female, 41 male) were enrolled in the NIH sponsored Strategies to Predict Osteoarthritis Progression (POP) study, approved and in accordance with the policies of the Duke Institutional Review Board. Participants were recruited primarily through Rheumatology and Orthopaedic clinics and met the American College of Rheumatology criteria for symptomatic OA of at least one knee. In addition, all participants met radiographic criteria for OA with a Kellgren-Lawrence (KL) [13 (link)] score of 1–3 in at least one knee. Exclusion criteria included the following: bilateral knee KL4 scores; exposure to a corticosteroid (either parenteral or oral) within 3 months prior to the study evaluation; knee arthroscopic surgery within 6 months prior to the study evaluation; known history of avascular necrosis, inflammatory arthritis, Paget's disease, joint infection, periarticular fracture, neuropathic arthropathy, reactive arthritis, or gout involving the knee, and current anticoagulation. A total of 186 participants were screened to identify the final 159 participants with radiographic and symptomatic knee OA of at least one knee who were willing to undergo arthrocentesis. Of the total 318 knees available for analysis, 10 knees were excluded from evaluation on the basis of knee replacement for a total of 308 knees included in the final analyses. The final knee sample included 8 participants that had undergone unilateral hip replacement for OA and one subject who had unilateral internal hip fixation secondary to a traumatic fracture. Knee symptoms were ascertained by the NHANES I criterion [14 (link)] of pain, aching or stiffnes on most days of any one month in the last year; for subjects answering yes, symptoms were quantified as mild, moderate, or severe yielding a total score of 0–4 for each knee. Current analgesic medication use was recorded (numbers of participants using): acetaminophen (32), narcotics (9), tramadol (2), non-selective non-steroidals (67), cycloxygenase inhibitors (45), glucosamine and chondroitin sulfate (51), and any of these analgesics (118).