Knee aspiration is a procedure to remove fluid from the knee joint using a needle and syringe. It is performed to diagnose the cause of a swollen knee (infection, crystals, blood) or to relieve a painful tense effusion.
The skin is cleaned with antiseptic. The knee is positioned in slight flexion. A needle is inserted into the joint — commonly through a superolateral approach (above and to the outer side of the kneecap). Fluid is aspirated and sent for analysis: microscopy for crystals (gout, pseudogout), culture and sensitivity (infection), cell count, and appearance. If therapeutic, corticosteroid may be injected after aspiration. The procedure takes five to ten minutes.
Aspiration is performed urgently if septic arthritis is suspected (hot, red, swollen knee with fever), for crystal analysis in suspected gout or pseudogout, to relieve a tense, painful effusion after injury or surgery, and diagnostically for unexplained chronic effusion.
Blood tests and imaging may support a diagnosis but cannot definitively identify infection or crystals. When septic arthritis is suspected, aspiration is mandatory.
No fasting required. Can be performed at the bedside or in clinic. Stop anticoagulants if safe to do so (discuss with prescribing team).
Definitive fluid analysis for diagnosis. Therapeutic relief of a tense effusion. If corticosteroid is injected, additional pain relief for inflammatory conditions.
Brief pain during needle insertion.
Minor bruising.
Fluid may reaccumulate.
Introduction of infection. Very rare with sterile technique (approximately 1 in 10,000–50,000).
Bleeding into the joint, particularly in anticoagulated patients.
Temporary pain increase after injection.
Local anaesthetic spray or infiltration. No sedation needed.
No recovery needed. Rest for 24 hours after injection.
Fluid results within 24–48 hours. Urgent results if sepsis suspected. Follow-up based on diagnosis.
There is brief discomfort as the needle passes through the skin and into the joint. Local anaesthetic spray or injection at the skin site reduces this. Most patients find it tolerable.
If septic arthritis is suspected — a hot, red, swollen joint with fever or elevated inflammatory markers — aspiration must be performed urgently before starting antibiotics, to identify the organism.