Synovial fluid examination

Optimally 5 mL synovial fluid in sterile container; 5 mL in anticoagulant (lithium heparin or EDTA tube) if cell count required.

Synovial fluid cannot be fixed and should be transported to the laboratory immediately. 

Even with refrigeration the specimen should reach the laboratory within 24 h.

Macroscopic examination. 

Microscopic examination: wet film including polarised light for the identification of crystals; Gram stain and stains for Mycobacterium spp if appropriate. 

Glucose, Protein, and Complement components C3 and C4, if appropriate. 

Bacterial culture if indicated.

Normal synovial fluid is clear, straw-coloured, and being mucoid has a high viscosity, with a strong mucin clot after the addition of acetic acid. 

In normal synovial fluid the cell count should be < 200 x 10⁶/L.

Investigation of suspected inflammatory conditions or infective arthritis.

Red cells may indicate recent haemarthrosis or a traumatic aspiration. 

Bacterial arthritis is associated with turbid or purulent synovial fluid, white cell counts > 50,000 x 10⁶L (mainly neutrophils), low glucose (compared to concurrent plasma levels) and high protein.

These findings can also be present in other inflammatory joint conditions, eg, rheumatoid arthritis, rheumatic fever, and crystal arthropathy. 

Cell counts > 100,000 x 10⁶ /L suggest septic arthritis. Not all cases of bacterial infection are associated with an elevated white cell count. 

In viral arthritis the synovial fluid white cell count is increased with predominantly mononuclear cells, although rubella has been associated with predominance of neutrophils.  

Sodium urate and calcium pyrophosphate crystals are diagnostic of gout and pseudogout, respectively. 

A fibrin clot is present in inflammatory or septic fluid and the mucin clot is friable. 

Complement components C3 and C4 are low in rheumatoid arthritis.