Question 1

Why is aspirating a hot, swollen joint so important?

Accepted Answer

The key reason is to exclude septic arthritis, a joint infection that can destroy a joint within days if it is missed. Aspiration provides fluid to look for infection and crystals, and it can also relieve pain from a tense effusion. In a hot, swollen joint, septic arthritis must be urgently considered and excluded.

Question 2

What is synovial fluid sent for?

Accepted Answer

Synovial fluid is sent for a cell count and differential, including the white cell count, a Gram stain to look for bacteria, culture and sensitivities to identify an organism and guide antibiotics, and examination for crystals under polarised light microscopy where crystal arthritis is possible. Where it can be done promptly, the fluid is sent before antibiotics so the culture is not affected, but antibiotics are not delayed in an unwell patient.

Question 3

How are gout and pseudogout crystals told apart?

Accepted Answer

They are distinguished under polarised light microscopy. Monosodium urate crystals, seen in gout, are long, needle-shaped and negatively birefringent. Calcium pyrophosphate crystals, seen in pseudogout, are shorter, rhomboid and weakly positively birefringent.

Question 4

Can crystals or a negative Gram stain rule out infection?

Accepted Answer

No. Crystals do not exclude infection, because gout or pseudogout and septic arthritis can occur together in the same joint. A negative Gram stain does not exclude septic arthritis either, because it misses most infections, so a suspected septic joint is treated as an emergency regardless.

Joint and Bursa Aspiration

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