Objective: To determine the lactic acid and lactate dehydrogenase levels in synovial fluid and differentiate between inflammatory and non-inflammatory arthritis. Study Design: Cross-sectional study. Place and Duration of Study: Department of Rheumatology, Liaquat National Hospital, Karachi, from Feb to May 2019. Methodology: All patients of age >18 years, of either gender, who presented with knee ... [Show full abstract] joint effusion were enrolled in the study. Synovial fluid aspiration for the analysis of lactate and lactate dehydrogenase (LDH) was done for all patients. Results: Seventy-seven patients were enrolled, of which 75 were included in the analysis. Two patients were excluded as one had lymphoma and the other had recent joint trauma. 31 (41.3 %) patients had non-inflammatory, or osteoarthritis, and 44 (58.7 %) had inflammatory arthritis. The mean value of synovial LDH in inflammatory and non-inflammatory arthritis was 737.38 ± 102.76 mmol/L and 265.5 ± 17.43 mmol/L, respectively, (p<0.001). The mean value of synovial lactate in inflammatory arthritis (32.16 ± 2.84 mmol/L) was higher than the mean value of synovial lactate in non-inflammatory arthritis (19.81 ± 1.08 mmol/L) (p<0.001). There mean plasma LDH in inflammatory arthritis and non-inflammatory arthritis was 495.77 ± 41.67 mg/dl and 437.90 ± 30.99 mg/dl, respectively (p>0.05). The plasma lactate in inflammatory arthritis and noninflammatory arthritis was 12.84 ± 0.59 mg/dl and 12.97 ± 0.78 mg/dl, respectively (p>0.05). Conclusion: Synovial fluid lactic acid and synovial LDH can serve as rapid diagnostic and cost-effective tests to differentiate between non-inflammatory and inflammatory arthritis.