Synovial Fluid Analysis
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Direct answer
Synovial fluid analysis classifies a joint effusion as non-inflammatory, inflammatory or septic, using appearance, viscosity, cell count, crystals and culture. Clear, viscous fluid with a low white cell count suggests degenerative or traumatic causes, while neutrophil-rich counts in the inflammatory range suggest crystal or immune arthritis. A turbid fluid with a very high neutrophil count, often above 50,000 per microlitre, requires urgent exclusion of septic arthritis.
What you must remember
- Normal fluid: clear, pale yellow and viscous, with a white cell count below about 200 cells per microlitre, mostly mononuclear.
- Non-inflammatory fluid: clear and viscous with a count below about 2000 per microlitre, typical of osteoarthritis and mechanical derangement.
- Inflammatory fluid: a count of roughly 2000 to 50,000 with neutrophils predominant and a poor mucin clot, seen in rheumatoid arthritis, gout and spondyloarthropathy.
- Septic fluid: turbid or purulent, often above 50,000 cells per microlitre with neutrophil predominance; Gram stain and culture confirm the organism.
- Monosodium urate: needle-shaped, negatively birefringent crystals in gout.
- Calcium pyrophosphate: rhomboid, weakly positively birefringent crystals in pseudogout.
- Blood and fat: blood-stained fluid suggests trauma, haemophilia or pigmented villonodular synovitis, and fat globules point to an intra-articular fracture.
A laboratory workflow for the aspirated fluid
Aspirated fluid is examined in a fixed order, because each result narrows the differential before the next test is read. Infection is excluded early, since delay in septic arthritis risks the joint.
- Note the appearance: clear, pale yellow fluid suggests a normal or degenerative joint, whereas turbid, purulent or blood-stained fluid calls for a more urgent differential.
- Test the viscosity: a firm mucin clot on the Ropes test points to a non-inflammatory process, while a poor, friable clot indicates inflammation.
- Count the cells with a differential: a count below about 2000 per microlitre suggests osteoarthritis, whereas a very high count with predominant neutrophils raises the possibility of sepsis.
- Examine for crystals: polarised light microscopy identifies urate crystals in gout and pyrophosphate crystals in pseudogout, and both can coexist with infection.
- Send for Gram stain and culture: these confirm the organism in septic arthritis and remain the reference standard, so cultures are taken before antibiotics where practical.
- Check for fat and blood: fat globules suggest an intra-articular fracture, and blood-stained fluid prompts a search for trauma, bleeding disorder or synovial tumour.
Where the exam tries to catch you
Synovial fluid questions often hinge on a single number or a single crystal type.
- Trap: accepting crystals as proof of gout in a hot, swollen joint Correct: crystals can coexist with infection, so Gram stain and culture are still required before the diagnosis is settled.
- Trap: confusing urate and pyrophosphate crystals Correct: urate crystals are needle-shaped and negatively birefringent, whereas pyrophosphate crystals are rhomboid and weakly positively birefringent.
- Trap: assuming that a high white count always means infection Correct: gout and rheumatoid arthritis can produce counts in the inflammatory range, so the full profile must be read together.
Frequently asked questions
What is the normal white cell count in synovial fluid?
Normal synovial fluid has a white cell count below about 200 cells per microlitre, and most of these cells are mononuclear. Counts above this level indicate an effusion that needs classification.
Which test is the reference standard for septic arthritis?
Culture of the synovial fluid is the reference standard for septic arthritis, with Gram stain as a rapid supporting test. A turbid fluid with a high neutrophil count raises suspicion, but only culture confirms the organism.
Which crystals are seen in gout and pseudogout?
Monosodium urate crystals, which are needle-shaped and negatively birefringent, are seen in gout. Calcium pyrophosphate crystals, which are rhomboid and weakly positively birefringent, are seen in pseudogout.
How does synovial fluid differ in inflammatory and non-inflammatory arthritis?
Inflammatory synovial fluid has a white cell count of roughly 2000 to 50,000 per microlitre, with predominant neutrophils and a poor mucin clot. Non-inflammatory fluid is clear and viscous, with a count below about 2000 that is mostly mononuclear.