Is a poor string sign consistent with septic arthritis?

Poor String Sign and Septic Arthritis

A poor string sign (low synovial fluid viscosity) is consistent with septic arthritis and should raise clinical suspicion for infection, but it is nonspecific and cannot definitively diagnose or exclude septic arthritis—arthrocentesis with synovial fluid culture, Gram stain, and cell count remains mandatory for diagnosis.

Understanding the String Sign

The "string sign" refers to synovial fluid viscosity testing, where normal synovial fluid forms a string 4-6 cm long when allowed to drip from a syringe. A poor string sign (fluid that doesn't string or forms only short strings <3 cm) indicates:

• Degraded hyaluronic acid from inflammatory processes 1
• Elevated white blood cell count with enzymatic breakdown of normal synovial components 2
• Loss of normal viscosity seen in both infectious and inflammatory arthritis 1

Why Poor String Sign Suggests But Doesn't Confirm Septic Arthritis

Consistent with Infection

• Septic arthritis characteristically produces synovial fluid with markedly decreased viscosity due to bacterial enzymes and inflammatory cells breaking down hyaluronic acid 1
• The presence of purulent material with >50,000 white blood cells/mm³ (typical in septic arthritis) destroys normal fluid viscosity 3, 1
• Bacterial proliferation causes rapid cartilage damage within hours to days, accompanied by inflammatory fluid with poor viscosity 3, 4

Critical Limitation: Nonspecific Finding

• Crystal arthropathies (gout, pseudogout) also produce poor string sign with similarly elevated cell counts and decreased viscosity 5
• Inflammatory arthritis from any cause can degrade synovial fluid viscosity 1
• Septic arthritis and gout can coexist in the same joint—among 30 reported cases of concurrent infection and gout, 73% had positive synovial fluid cultures despite crystal presence 5, 4

Mandatory Diagnostic Approach

Synovial fluid analysis is the criterion standard and must be performed immediately before antibiotics 3, 4:

Essential Synovial Fluid Tests

• Cell count with differential: WBC >50,000 cells/mm³ with >90% PMNs strongly suggests septic arthritis (but can occur with gout) 3, 1, 6
• Gram stain and culture: Sensitivity 0.76, specificity 0.96 for distinguishing septic arthritis from gout 5, 4
• Crystal analysis: Must be performed even if infection suspected, as crystals don't exclude sepsis 5, 4, 6

When to Suspect Concurrent Septic Arthritis in Crystal-Positive Joints

If crystals are identified, septic arthritis is still highly likely when 6:

• Synovial TNC >50,000 cells/mm³ (odds ratio 7.7)
• PMN >90% (odds ratio 2.17)
• Serum CRP >10 mg/dL (odds ratio 3.2)
• Female sex (odds ratio 1.9)

These patients warrant urgent irrigation and debridement plus antibiotics 6

Critical Clinical Pitfalls

Never Rely on String Sign Alone

• The most dangerous error is failing to perform arthrocentesis when septic arthritis is suspected, as this leads to permanent joint destruction and 2-15% mortality 3, 7
• Poor string sign cannot distinguish between septic arthritis, gout, or other inflammatory conditions—culture is essential 3, 1

Image-Guided Aspiration Preferred

• Ultrasound or fluoroscopy guidance ensures proper needle placement and reduces complications, particularly for hip joints 3, 4
• Hip aspiration has 5% false-negative rate if performed <24 hours after symptom onset 5, 4

Negative Culture Doesn't Exclude Infection

• Culture sensitivity drops if antibiotics given before aspiration, but Gram stain and cell count still provide critical information 3, 1
• If culture negative but clinical suspicion remains high, consider percutaneous bone biopsy for concurrent osteomyelitis 4

Bottom Line for Clinical Practice

A poor string sign indicates inflammatory or infectious arthritis but cannot differentiate septic arthritis from crystal arthropathy or other inflammatory conditions. When you observe poor synovial fluid viscosity:

1. Immediately perform complete synovial fluid analysis (cell count, Gram stain, culture, crystals) before antibiotics 3, 4
2. If WBC >50,000 with >90% PMNs and CRP >10 mg/dL, treat as septic arthritis even if crystals present 6
3. Start empiric antibiotics after cultures obtained if clinical suspicion high—vancomycin for gram-positive cocci, ceftriaxone for gram-negative cocci 4, 1
4. Surgical drainage is mandatory in all confirmed cases 4, 2