When should a knee effusion be tapped?

When to Tap a Knee Effusion

Aspirate a knee effusion immediately when septic arthritis is suspected, before initiating antibiotics, as this is an orthopedic emergency requiring definitive diagnosis to prevent irreversible joint destruction. 1

Absolute Indications for Immediate Aspiration

Suspected Septic Arthritis

• Any patient presenting with acute monoarticular knee pain, swelling, fever, and systemic symptoms requires immediate arthrocentesis before antibiotic administration, as bacterial proliferation can cause permanent cartilage damage within hours to days 1
• Synovial fluid analysis is the criterion standard and definitive diagnostic test for septic arthritis 1
• Even in patients with known gout, septic arthritis and crystal arthropathy can coexist—the only way to exclude infection is through synovial fluid analysis 1

Suspected Crystal Arthropathy

• Aspirate when gout or pseudogout is suspected, as crystal analysis of synovial fluid is the gold standard for diagnosis 2
• Serum uric acid levels are unreliable during acute attacks and cannot diagnose gout 1
• Crystal analysis using polarizing microscopy provides definitive diagnosis 2

Prosthetic Joint Concerns

• Aspirate when pain, instability, or loosening occurs after total knee arthroplasty, particularly when ESR or CRP is elevated or clinical suspicion for infection is high 2
• Joint aspiration demonstrates 100% sensitivity, specificity, and accuracy for diagnosing infection in prosthetic joints with these symptoms 3, 2
• Image-guided aspiration (ultrasound or fluoroscopy) is rated "usually appropriate" (9/9) by the American College of Radiology for suspected periprosthetic infection 3, 4

Strong Indications for Aspiration

Diagnostic Uncertainty

• Aspirate any joint effusion of unknown origin when clinical judgment indicates diagnostic testing is necessary 2
• Aspiration aids in earlier establishment of diagnosis in non-traumatic effusions 5
• Unexplained joint effusion warrants arthrocentesis to determine etiology 6

Symptomatic Large Effusions

• Aspirate large, tense effusions for immediate symptomatic relief, though recognize this provides only temporary improvement lasting approximately one week due to early re-accumulation 5
• Local corticosteroid injection after aspiration can provide significant relief in acute exacerbations of knee osteoarthritis with significant effusions 6

Acute Trauma with Effusion

• Consider aspiration in post-traumatic effusions when internal derangement or occult fracture is suspected, as lipohemarthrosis on cross-table lateral radiograph suggests intra-articular fracture 3
• Ultrasound detection of lipohemarthrosis has 94% specificity for acute intra-articular fractures 3
• Aspiration shows temporary improvement especially in post-traumatic effusions, though benefit is limited to the first week 5

Technical Optimization

Timing Considerations

• Discontinue antibiotics for at least 2 weeks before aspiration when clinically feasible, as false-negative cultures occur with recent antibiotic use 3, 2
• Some sources suggest up to one month may be necessary for cultures to become positive after antibiotic cessation 3
• Obtain blood cultures before aspiration if fever or acute symptom onset is present 2

Image Guidance

• Use ultrasound or fluoroscopy guidance to ensure proper needle placement and reduce complications, particularly in difficult cases 3, 1
• Image-guided aspiration is preferred over blind technique 1

Repeat Aspiration Strategy

• Perform weekly repeat aspirations if initial aspiration is negative and clinical suspicion for infection remains high 3
• In 16% of patients, more than three aspirations were necessary to obtain positive culture 3, 2
• Early aspiration leads to significant reduction in treatment duration and better outcomes 3, 2

Essential Synovial Fluid Analyses to Order

• White blood cell count with differential 1
• Gram stain (though sensitivity and specificity are relatively poor) 3, 2
• Aerobic and anaerobic bacterial cultures 3, 2
• Crystal analysis using polarizing microscopy 1, 2
• Alpha-defensin testing for prosthetic joint infection (97% sensitivity, 96% specificity alone; 97% sensitivity with 100% specificity when combined with synovial CRP) 2

Critical Pitfalls to Avoid

• A "dry tap" does not exclude infection or the presence of joint fluid—if initial aspiration is unsuccessful, perform ultrasound-guided repeat aspiration 3, 4, 2
• Never delay aspiration to obtain imaging when septic arthritis is suspected, as this is a medical emergency 1
• Do not rely on serum markers alone—CRP and procalcitonin are elevated in septic arthritis but cannot distinguish between infection, gout, or other inflammatory conditions 1
• Radiographs are inadequate for distinguishing infection from loosening in prosthetic joints—aspiration is superior 2
• Negative cultures do not rule out infection, especially if antibiotics were given prior to aspiration, but Gram stain and cell count still provide critical diagnostic information 1

Contraindications

• Bacteremia (relative contraindication) 6
• Overlying soft tissue infection at the aspiration site 6
• Inaccessible joints 6
• Coagulopathy (relative contraindication requiring correction when possible)