Appearance of Infected Synovial Fluid
Infected synovial fluid typically appears turbid to purulent, with the degree of turbidity grossly correlating with the amount of inflammation present, though visual appearance alone cannot reliably distinguish infection from other inflammatory conditions. 1
Gross Visual Characteristics
Turbidity is the hallmark feature - infected joints usually produce the most turbid to purulent-appearing fluids, though this is not absolute and exceptions are common 1
The fluid ranges from cloudy to frankly purulent in appearance, contrasting with totally transparent fluid seen in non-inflammatory conditions like osteoarthritis 1
Visual inspection provides only a quick bedside orientation and cannot be relied upon for definitive diagnosis, as highly inflammatory non-infectious conditions can produce similarly turbid fluid 1
Critical Laboratory Analysis Required
While the question asks about appearance, it's essential to understand that visual assessment must be immediately followed by laboratory analysis, as gross appearance is unreliable for diagnosis:
White Blood Cell Count
The AAOS recommends synovial fluid testing including leukocyte count and neutrophil percentage as part of the diagnostic workup 2
Infected fluid typically shows elevated WBC counts, but there is substantial overlap with inflammatory conditions - 50% of culture-proven joint infections had synovial fluid leukocyte counts below 28,000/mm³ 3
The optimal cut-off for prosthetic joint infection is approximately 1590 white cells/µl with 65% neutrophilia for both total knee and hip replacements 4
The traditional 50,000 WBC/mm³ cutoff has only 61% sensitivity and misses 39% of culture-positive infections, making it inadequate for ruling out septic arthritis 5
Additional Testing
Gram stain, aerobic and anaerobic cultures, and cell count with differential are essential components of synovial fluid evaluation 2
Alpha-defensin testing demonstrates 97% sensitivity and 96% specificity for prosthetic joint infection, and when combined with CRP, achieves 100% specificity 2, 6
Clinical Pitfalls
Never rely on visual appearance alone - even clear-appearing fluid can harbor infection, particularly in immunocompromised patients 3
Patients with malignancies, steroid use, or intravenous drug abuse may have culture-positive infections with synovial fluid WBC counts averaging below 50,000 cells/mm³ 3
Gram stain is negative in 55% of culture-proven infections, so negative staining does not exclude infection 5
Prior antibiotic use can cause false-negative aspirations; at least 2 weeks off antibiotics is recommended before aspiration when clinically feasible 2, 6