Gross synovial fluid analysis in the differential diagnosis of joint effusions

Abstract

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

Methods: 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 4 observers was calculated in a pilot study. In a prospective study over 8 months both SF gross analysis and cell count was documented in all patients presenting with a joint effusion. Fusch Rosenthal manual counting chamber was used for calculating SF cell counts. SPSS data base was used for data collection and statistical tests.

Results: There was good interobserver agreement on direct gross analysis and between questionnaire assessors (mean Kappa score of 0.889). 80 SF samples were collected. Gross analysis was performed in all samples and cell count in 72. 31%of the specimens thought to be inflammatory on gross analysis were subsequently 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.

Conclusions: Documentation of gross SF analysis using a simple tick list was found to have good interobserver agreement. SF cell count did not add any information when SF gross analysis suggested a non inflammatory process. Gross analysis was found to be 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.