Acute Monoarthritis and Synovial Fluid Analysis
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Direct answer
Acute monoarthritis is inflammation of a single joint, and synovial fluid analysis separates septic, crystal-induced and inflammatory causes by cell count, crystal examination and culture. A white cell count often above 50,000 per microlitre, with neutrophil predominance, is typical of septic arthritis, though it does not exclude infection in every case. Needle-shaped, negatively birefringent crystals indicate gout, while rhomboid, positively birefringent crystals indicate pseudogout.
What you must remember
- Normal fluid: clear and viscous, with a white cell count below about 200 per microlitre and few neutrophils.
- Non-inflammatory fluid: a count below about 2000 per microlitre, seen in osteoarthritis and mechanical or traumatic effusions.
- Inflammatory fluid: a count of roughly 2000 to 50,000 per microlitre, seen in gout, pseudogout and rheumatoid arthritis.
- Septic fluid: a count often above 50,000 per microlitre with more than 90 percent neutrophils, a low glucose and a high protein, diagnosed by Gram stain and culture.
- Gout crystals: monosodium urate crystals are needle-shaped and negatively birefringent, appearing yellow when aligned parallel to the compensator axis.
- Pseudogout crystals: calcium pyrophosphate crystals are rhomboid and weakly positively birefringent, typical of chondrocalcinosis.
- Commonest organism: Staphylococcus aureus causes most septic arthritis, while Neisseria gonorrhoeae is considered in sexually active young adults.
- Crystals and infection: crystals and infection can coexist, so a crystal-positive fluid must still be sent for Gram stain and culture.
Four categories of synovial fluid
Synovial fluid is the test that turns a red, swollen joint into a diagnosis. The table places the four categories side by side.
| Fluid type | White cells per microlitre | Key finding | Typical causes |
|---|---|---|---|
| Normal | Below about 200 | Clear, viscous, no crystals | Healthy joint |
| Non-inflammatory | Below about 2000 | No crystals, no organisms | Osteoarthritis, trauma |
| Inflammatory | About 2000 to 50,000 | Urate or CPPD crystals | Gout, pseudogout, rheumatoid arthritis |
| Septic | Often above 50,000 | Above 90 percent neutrophils, organisms on Gram stain | Staphylococcus aureus, Neisseria gonorrhoeae |
Cell count alone is not enough, because crystals and infection can overlap in the same joint. The combination of count, crystal examination and culture is what separates the categories in practice. Joint aspiration is therefore the first investigation in any hot, swollen single joint. Septic arthritis must be excluded before a hot joint is attributed to gout or pseudogout, since the consequences of missing infection are far more serious.
Where the exam tries to catch you
Monoarthritis questions usually test crystal birefringence, the limits of cell counts and the timing of aspiration.
- Trap: confusing the birefringence of urate and CPPD crystals Correct: urate is needle-shaped and negatively birefringent, while CPPD is rhomboid and positively birefringent.
- Trap: assuming a crystal-positive fluid excludes infection Correct: both can coexist, so Gram stain and culture must still be sent.
- Trap: assuming a modest white cell count excludes septic arthritis Correct: counts can be lower in neutropenic or partially treated patients, so clinical suspicion and culture still decide.
- Trap: starting antibiotics before aspirating the joint Correct: aspiration should come first where feasible, because culture yield falls once antibiotics are given.
Frequently asked questions
Which crystal is seen in synovial fluid in acute gout?
Monosodium urate crystals are seen in acute gout, appearing needle-shaped and negatively birefringent under polarised light. Their identification distinguishes gout from pseudogout, whose calcium pyrophosphate crystals are rhomboid and positively birefringent.
What synovial fluid white cell count suggests septic arthritis?
A white cell count often above 50,000 per microlitre, with more than 90 percent neutrophils, suggests septic arthritis. Counts can overlap with inflammatory disease, so Gram stain and culture are needed for certainty.
Which organism commonly causes septic monoarthritis in adults?
Staphylococcus aureus is the commonest cause of septic monoarthritis in adults. Neisseria gonorrhoeae is considered in sexually active young adults, especially when the presentation includes skin lesions or migratory features.
Why should synovial fluid be aspirated before antibiotics are started?
Aspirating synovial fluid before antibiotics gives the best chance of a positive culture and a valid Gram stain. The count, crystal examination and culture together then guide the diagnosis and the duration of therapy.