Jennifer M. Newquist, DVM; Gary M. Baxter, VMD, MS, DACVS Journal of the American Veterinary Medical Association August 15, 2009 235 4 415-419 Objective—To determine whether high plasma fibrinogen concentration (≥ 900 mg/dL) is a valid indicator of physeal or epiphyseal osteomyelitis in foals. Design—Retrospective case series. Animals—17 foals with physeal or epiphyseal osteomyelitis with or without septic arthritis, 17 foals with septic arthritis alone, 20 foals with non–Rhodococcus equi pneumonia, and 22 healthy foals. Procedures—Medical records were reviewed for information regarding signalment and total WBC count, segmented neutrophil count, and plasma fibrinogen concentration measured when foals were initially evaluated at the hospital. Whether the foals survived to discharge from the hospital was also determined. Results—Foals with physeal or epiphyseal osteomyelitis had higher plasma fibrinogen concentrations than did foals in the other 3 groups. Sixteen of 17 affected foals had values ≥ 900 mg/dL. The positive predictive and negative predictive values for plasma fibrinogen concentrations between 900 and 1,500 mg/dL as an indicator of osteomyelitis were 84.2% and 98.2%, respectively. Fibrinogen concentrations of 200 to 400 mg/dL and 500 to 800 mg/dL were associated with foals not having physeal osteomyelitis. Having septic arthritis alone or non–R equi pneumonia was significantly associated with a plasma fibrinogen concentration between 500 and 800 mg/dL; however, the positive predictive value of this range as an indicator of those diseases was low. Foals with osteomyelitis had greater total WBC and segmented neutrophil counts than did foals with septic arthritis alone. Conclusions and Clinical Relevance—A plasma fibrinogen concentration of ≥ 900 mg/dL may be useful as an indicator of physeal or epiphyseal osteomyelitis in foals.