The main objective of this study is to assess the ability of Fiber-optic Evanescent Wave Spectroscopy (FEWS) to discriminate synovial fluids samples of septic arthritis patients from other causes of joint effusion by analyzing ex vivo joint fluid samples collected during routine clinical care.

Inclusion Criteria:

• patients benefiting from joint fluid puncture for diagnostic purposes in the rheumatology hospitals participating in the study
• patients over 18 year old,
• patients who received information about the protocol and have not expressed opposition to participate.

Exclusion Criteria:

• patients opposing the study,
• patients aged below 18 year old
• Protected major (safeguard justice , trusteeship and guardianship) and persons deprived of liberty

patients benefiting from joint fluid puncture for diagnostic purposes in the rheumatology hospitals participating in the study