Synovial fluid aspiration of the knee

What it is

Knee joint aspiration (arthrocentesis) provides synovial fluid for analysis - the single most useful investigation for distinguishing infectious, crystal-induced, and inflammatory causes of an acutely swollen joint, and is also therapeutic for symptomatic large effusions
The knee is the most commonly aspirated joint given its size and accessibility; the procedure is routine in rheumatology and emergency medicine practice for any acutely swollen, undiagnosed joint

Aspirated fluid is sent for: cell count and differential (distinguishing non-inflammatory, inflammatory, and septic ranges), Gram stain and culture (infection), and polarised light microscopy (crystal identification - urate in gout, calcium pyrophosphate in pseudogout)

Fluid type	WCC (per microL)	Example
Non-inflammatory	<2000	Osteoarthritis
Inflammatory	2000-50,000	RA, gout, pseudogout
Septic	>50,000 (often much higher), typically >90% neutrophils	Septic arthritis

Any acutely hot, swollen joint should be aspirated before starting antibiotics if septic arthritis is a realistic differential, since antibiotics reduce culture sensitivity - aspiration should not be delayed while awaiting other results if septic arthritis is a genuine concern
Crystal arthropathy and septic arthritis can coexist - a positive crystal finding does not fully exclude concurrent infection, particularly in a patient with systemic signs of sepsis, and both possibilities should be considered together rather than crystal-positivity being used to stop the infection workup
Aspiration is a relatively low-risk procedure, but should be avoided/approached cautiously over overlying cellulitis (infection introduction risk) and used with appropriate caution in significant coagulopathy

Septic arthritis (a joint emergency requiring urgent drainage/antibiotics), gout, pseudogout, large symptomatic effusions (therapeutic aspiration for relief)

Crystal identification on microscopy does not exclude coexisting infection - both crystal arthropathy and septic arthritis should be actively considered together in an acutely inflamed joint with systemic features, rather than treating a positive crystal finding as ruling out infection

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