Joint fluid testing

Collection of synovial fluid:

After carefully disinfecting the skin, collect the fluid in a syringe, and then transfer it immediately into at the following tubes:

• A dry tube or sterile pot for bacteriology (testing for crystals and for biochemistry)
• An EDTA tube (purple), essential for cytology (white blood cells, blood count etc.)
• A (grey) fluorinated tube, recommended for blood sugar levels.

Samples must be sent to the laboratory as fast as possible. Store at room temperature if needed (maximum 24 hours).

The test differentiates fluids generated by effusion due a mechanical cause and fluids generated due to inflammation.

The test typically includes:

• The quantitative and qualitative test of the elements in question;
• A protein assay and a glucose assay;
• Test for microcrystals;
• Bacterial culture

Culture:

Aspiration must be performed before any antibiotic therapy is initiated. Be aware of the possibility of a contamination with saprophyte bacteria in the skin. This test is key for diagnosing septic arthritis, often due to common bacteria, sometimes to mycobacteria.

Microcrystals:

Key test for diagnosing microcrystalline arthritis (test using a polarised light microscopy). Conservation and transport at 4°C.