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Microbiology - Arthrocentesis (Knee)

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Joint effusion of unknown cause (diagnostic)


Joint effusion with pain and impaired mobility (therapeutic)

Contraindications (absolute in bold)

Prosthetic joint (orthopaedic team to perform in theatre)

Overlying cellulitis or local infection (admission for IV antibiotics required)

Skin lesion overlying the joint (particularly psoriasis)

Uncontrolled coagulopathy (not including oral anticoagulants at therapeutic doses)

Acute fracture (increased risk of osteomyelitis)



Informed consent

Medical emergency

Consent is not required if the patient lacks capacity or is unable to consent

Brief verbal discussion is recommended if the situation allows


Verbal consent

Less complex non-emergency procedure with low risk of complications

Potential complications



Infection (1:2,500 procedures)


Tissue injury (cartilage, tendon or neurovascular)

Procedural hygiene

Standard precautions

Aseptic non-touch technique

PPE: non-sterile gloves


Any bed space


Procedural clinician



Absorbent pad or ‘bluey’

Drawing up needle

25g and 21g needle (anaesthetic infiltration)

18-21g needle (large needle required for aspiration of viscous fluid)

Syringes: 5ml (anaesthetic), 20ml & 50ml (aspiration)

Haemostat (for grasping needle hub at syringe change)

Sterile specimen container, full blood count tube, blood culture bottles

Gauze or plaster


Supine with knee exposed and flexed 20 degrees (rolled towel placed under knee)

Ascertain if maximal fluid felt laterally or medially (defining side of insertion)

Mark insertion point: 3-4mm inferior to the middle of the patella at the joint line of the knee


10ml lignocaine 1-2%

Blood products prior to an aspiration in uncontrolled coagulopathy (e.g. haemophilia)

Sequence (medial or lateral approach)

Infiltrate skin over the entry site with lignocaine using the 25g needle, wait one minute

Switch to the 21g needle and infiltrate the underlying tissue with lignocaine, wait one minute

Attach the 18g needle to a 20ml syringe and stabilise the joint with the non-dominant hand holding skin taut

Insert needle advancing slowly underneath the patella into the joint, maintaining a slight negative pressure

If bone is contacted, withdraw the needle and re-advance, directing away from the obstruction

Obtain flashback of joint fluid as the capsule is breached, and maintain position

Aspirate the 20ml of fluid from the joint then disconnect the syringe, leaving the needle in place

Attach a second syringe (20ml or 50ml) and continue this procedure until fluid stops draining

Massaging the suprapatellar area towards the needle insertion site may aid aspiration

Withdraw needle once aspiration has ceased, apply pressure with gauze, then a plaster once bleeding ceased

Post-procedure care

Macroscopic examination of synovial fluid:

Streaks of blood: traumatic aspirate

Uniformly bloody fluid: hemarthrosis

Fat droplets or greasy surface: possible fracture

Grossly cloudy: infection, crystal deposition or inflammatory arthritis


Divide the first 20ml (or available sample) of aspirate into three specimen containers:

Plain sterile container: gram stain, culture, PCR, microscopy (monosodium urate, calcium pyrophosphate)

Full blood count tube: cell count and differential

Blood culture bottle: direct inoculation increases the chance of a positive culture


Insertion site, number of attempts and any immediate complications


Ultrasound assessment comparing both knees may aid the diagnosis of an effusion

Relaxing the patient and quadriceps will aid the procedure, muscle tension narrows the joint space

The knee can accommodate up to 70ml, so a large aspiration syringe may be required

Avoid injecting lignocaine into the joint, as it is bactericidal and may affect culture growth

Avoid strong negative pressure on the syringe causing tissue collapse and occlusion of needle

A three-way tap can be used to avoid changing the aspirating syringe


Joint aspiration (arthrocentesis) is the most important initial investigation in monoarthritis. Complications are rare and risks are usually outweighed by the benefits of prompt diagnosis (infection, crystal deposition disease, haemarthrosis). We recommend the parapatellar approach avoiding the suprapatellar bursa (which does not communicate with joint in 10% of people) and inferiorly the patella tendon.

The available studies suggest arthrocentesis in patients on anticoagulation with warfarin or direct oral anticoagulants is safe without altering the anticoagulation regimen. Arthrocentesis to relieve tense haemarthrosis in bleeding disorders such as haemophilia is accepted practice after correction of coagulopathy.

The risk of complications is increased in the following situations:

Prosthetic joint: Requires orthopaedic consult and aspiration in theatre

Infection overlying the joint: There is risk of seeding infection into the joint. Admission for IV antibiotics is required even if the aspirate is negative for infection. Confirm effusion with ultrasound or X-ray, and aspirate only after consultation with the admitting team.

Psoriasis or skin lesion over the joint: Increased risk of skin colonisation with Staphylococcus aureus and seeding of infection. Confirm effusion with ultrasound or X-ray, and aspirate after consultation with the admitting team.

Peer review

This guideline has been reviewed and approved by the following expert groups:

Emergency Care Institute

Please direct feedback for this procedure to


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