OBJECTIVE: The aim of this study was to differentiate septic from non-septic arthritis by measuring lactate concentration with (1)H magnetic resonance spectroscopy (HMRS) and by estimating total protein content with the assessment of T (2) values. MATERIALS AND METHODS: In 30 patients with acute arthritis, synovial fluid was aspirated. Lactate concentrations were analyzed with single voxel HMRS at 1.5 T. T (2) relaxation times were mapped with a multi-spin echo sequence. All samples underwent microbiological testing and routine laboratory analysis to quantify lactate concentration and total protein content. Values obtained in septic and non-septic arthritis were compared with a Mann-Whitney U test. RESULTS: Synovial fluid from patients with septic arthritis (n = 10) had higher concentrations of lactate (11.4 +/- 4.0 mmol/L) and higher total protein content (51.8 +/- 10.7 g/L) than fluid obtained in non-septic arthritis (n = 20; 5.2 +/- 1.1 mmol/L and 40.4 +/- 6.9 g/L, respectively, p < 0.001 and <0.01, respectively). Measured lactate concentrations and T (2) relaxation times (as an indicator of total protein content) were moderately correlated to laboratory-confirmed lactate concentration (r (2) = 0.71) and total protein content (r (2) = 0.73). Markedly increased lactate concentrations (>6 mmol/L) in combination with low T (2) values (<550 ms) identify septic arthritis with a sensitivity of 70% and a specificity of 89%. CONCLUSION: Spectroscopic measurements of lactate concentration in combination with the estimation of protein content using T (2) may be of value in the differentiation of septic from non-septic arthritis.