Glossary
Synovial fluid analysis
Quick answer
Synovial fluid analysis is laboratory testing of the fluid drawn from the knee. It counts white cells, looks for crystals such as those of gout and pseudogout, and grows bacteria if infection is present. It helps tell infection, gout, inflammatory arthritis and wear-related arthritis apart, explains Doha knee surgeon Dr. Pablo Gelber at Aman Hospital.
What is tested
- Appearance: clear, cloudy, bloody or thick.
- White cell count: low in osteoarthritis, higher in inflammatory arthritis and gout, and often very high in infection.
- Crystals: needle-shaped urate crystals mean gout; rhomboid calcium pyrophosphate crystals mean pseudogout.
- Gram stain and culture: look for bacteria. Culture results can take a few days.
Why it matters for patients
A hot, swollen knee can look the same whether the cause is gout or infection, but the treatment is very different. Septic arthritis needs urgent antibiotics and often a washout of the joint, while gout is usually treated with anti-inflammatory medicines.
The analysis also helps after knee replacement, when a painful, swollen joint may be due to a low-grade infection. The patient experience is simply the aspiration itself; the testing happens in the laboratory.
Limits
Results are interpreted with blood tests, examination and imaging. Antibiotics taken beforehand can make cultures falsely negative, so tell your doctor about any recent antibiotic use. Gout and infection can occasionally coexist.
Frequently asked questions
How long do synovial fluid results take?
Cell counts and crystals are often available within hours. Cultures that grow bacteria may take a few days.
If crystals are found, can infection be ruled out?
Not completely. Gout and infection occasionally occur together, so culture results are still checked.
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