Management of Knee Effusion: Cell Analysis of Fluid Knee Aspirate
The initial step in managing a patient with knee effusion is to perform a diagnostic arthrocentesis (joint aspiration) with synovial fluid analysis that includes cell count, differential leukocyte count, and culture for aerobic and anaerobic organisms. 1
Diagnostic Approach to Knee Effusion
Initial Assessment
- Plain radiographs of the knee should be obtained prior to joint aspiration to assess for underlying structural abnormalities 1
- Joint aspiration (arthrocentesis) is rated as the most appropriate initial procedure (rating 9/9) for evaluating knee effusion, especially when infection is a concern 1
- Aspiration should be performed after reviewing the radiographs of the knee 1
Synovial Fluid Analysis
- Complete synovial fluid analysis should include: 1
- Total cell count and differential leukocyte count
- Culture for both aerobic and anaerobic organisms
- Crystal analysis if clinically indicated (to rule out gout or pseudogout)
Timing Considerations
- If infection is suspected and the patient is medically stable, withholding antimicrobial therapy for at least 2 weeks prior to collecting synovial fluid increases the likelihood of recovering an organism 1
- False-negative aspirations may occur in patients who have had preaspiration antibiotic treatment 1
Clinical Significance of Cell Analysis
Diagnostic Value
- Cell count and differential are crucial for distinguishing between infectious and non-infectious causes of knee effusion 1
- Synovial white blood cell count and percentage of polymorphonuclear leukocytes help differentiate between septic arthritis and other causes of effusion 1
- Joint aspiration has been found to be extremely useful in diagnosing joint infection with reported sensitivity, specificity, and accuracy of up to 100% in some studies 1
Interpretation Challenges
- There are discrepancies in the literature regarding optimal cutoff levels for determining the WBC count and percentage of polymorphonuclear leukocytes that best distinguish infected from non-infected fluid 1
- The absence of fluid (a "dry tap") at the time of aspiration does not rule out infection 1
Additional Diagnostic Considerations
Laboratory Tests
- C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) should be performed in all patients with suspected joint infection 1
- The combination of an abnormal ESR and CRP provides the best combination of sensitivity and specificity for detecting infection 1
Blood Cultures
- Blood cultures should be obtained if fever is present, there is an acute onset of symptoms, or if the patient has a condition that would make bloodstream infection more likely 1
Common Pitfalls and Caveats
- Relying solely on imaging without fluid analysis can lead to missed diagnoses, as radiographic findings may not distinguish between infection, loosening, or other pathologies 1
- In endemic areas, Lyme arthritis should be considered in the differential diagnosis of knee effusion, especially in children 2
- Multiple aspirations may be necessary in some cases, as early aspiration can lead to a significant reduction in the duration of treatment and better outcomes 1
- Imaging studies such as bone scans, leukocyte scans, MRI, CT, and PET scans should not be routinely used to diagnose prosthetic joint infection 1
By following this systematic approach to knee effusion with appropriate cell analysis of the aspirated fluid, clinicians can efficiently diagnose the underlying cause and implement appropriate management strategies to improve patient outcomes.