Gross synovial fluid analysis in the differential diagnosis of joint effusion

To develop an objective and easy to complete standardised questionnaire for documentation of synovial fluid (SF) gross appearance and use it in the assessment of patients presenting to the rheumatology service with a joint effusion.

A standardised questionnaire to record the gross appearance of SF was developed. Interobserver error in recorded observations and direct gross analysis of synovial fluid between four observers was calculated in a pilot study. In a prospective study over 8 months, SF gross analysis and cell count were documented in all patients presenting with a joint effusion. Fusch Rosenthal manual counting chamber was used for calculating SF cell counts.

There was good interobserver agreement on direct gross analysis and between questionnaire assessors (mean kappa 0.889). 80 SF samples were collected. Gross analysis was performed in all samples and cell count in 72. Of the specimens thought to be inflammatory on gross analysis, 31% were found to be non-inflammatory based on cell count; however, 12 of these patients had an established inflammatory arthritis. Gross analysis had a sensitivity of 94% and specificity of 58% when used to determine whether SF is inflammatory or non-inflammatory. The positive and negative predictive values were 0.69 and 0.91 respectively.

SF cell count did not add any information when SF gross analysis suggested a non-inflammatory process. Gross analysis was better than cell count to determine a potentially septic joint fluid. Further work needs to be done on the value of SF cell counts if gross analysis suggests the fluid to be inflammatory.