Osteoarthritis (OA) prevalence increases as the population ages. Diagnosing osteoarthritis often occurs in the late stages when cartilage degradation is severe, making it difficult to distinguish from other types of arthritis. Accurate differentiation of primary osteoarthritis from other arthritic conditions is crucial for effective treatment planning. A new diagnostic test has been developed that uses a dual-biomarker algorithm to inform osteoarthritis diagnosis. Synovial fluid from patients with confirmed primary osteoarthritis showed elevated levels of cartilage oligomeric matrix protein. However, this biomarker alone could not distinguish primary osteoarthritis from other inflammatory conditions that also cause cartilage deterioration. Therefore, a combinatorial algorithm using cartilage oligomeric matrix protein and Interleukin-8 concentrations was developed to differentiate primary osteoarthritis from inflammatory arthritis. Clinical decision limits for cartilage oligomeric matrix protein concentration and the cartilage oligomeric matrix protein to Interleukin-8 ratio were established and validated using 171 human knee synovial fluid specimens. The osteoarthritis algorithm demonstrated clinical sensitivity and specificity of 87.0% and 88.9%, respectively. This is the first report of a biomarker test that can differentiate primary osteoarthritis from inflammatory arthritis with a high degree of accuracy.