How is septic arthritis diagnosed?

Diagnosis of Septic Arthritis

Septic arthritis is diagnosed definitively by joint aspiration with synovial fluid analysis showing white blood cell count ≥50,000 cells/mm³, positive Gram stain, and/or positive culture, combined with clinical signs of acute monoarticular joint pain, swelling, and fever. 1

Clinical Presentation and Risk Stratification

The classic triad of fever, pain, and diminished mobility occurs in only approximately 50% of cases, making diagnosis challenging. 1 Key clinical features include:

• Acute monoarticular joint pain with swelling and erythema 1, 2
• Fever >101.3°F (though 77% of patients may be afebrile) 1, 3
• Inability to bear weight 1
• Constitutional symptoms (chills, rigors) are poorly sensitive 4

High-risk patient characteristics that should heighten suspicion include: 2, 5

• Age >80 years
• Diabetes mellitus
• Rheumatoid arthritis
• Recent joint surgery or intra-articular injection
• Prosthetic joint
• Skin infection
• Immunosuppressive medication use
• Intravenous drug use

Diagnostic Algorithm

Step 1: Initial Laboratory Evaluation

Before joint aspiration, obtain blood cultures because they are frequently positive in bacteremic septic arthritis. 6 Serum markers include:

• Erythrocyte sedimentation rate ≥40 mm/hour 1
• C-reactive protein >2.0 mg/dL 1
• White blood cell count ≥12,000 cells/mm³ 1

Meeting all five criteria (fever, ESR elevation, CRP elevation, WBC elevation, and inability to bear weight) approaches 100% likelihood of septic arthritis. 1 However, 64% of septic arthritis patients have normal serum WBC counts, limiting its utility. 3

Step 2: Imaging Strategy

The imaging algorithm should start with plain radiographs, though they have low sensitivity and are typically normal in early infection (<14 days). 1, 7 Radiographs serve to:

• Exclude fractures and tumors 1, 7
• Detect joint effusion, effacement of fat planes, or gas in soft tissues 1
• Provide baseline assessment 6

For hip joints, ultrasound is the initial imaging modality of choice to detect effusions and guide aspiration, with only a 5% false-negative rate if symptoms are present >1 day. 1, 7 Absence of hip joint effusion on ultrasound virtually excludes septic arthritis. 7

MRI with contrast should be ordered when: 1, 7

• Clinical suspicion remains high despite negative aspiration
• Concern exists for concurrent osteomyelitis (present in >50% of pediatric cases and up to 30% of adult cases) 1, 6
• Need to assess for soft tissue abscess or pyomyositis 1
• Patient fails to improve after 48-72 hours of appropriate antibiotics 7

MRI has 82-100% sensitivity and 75-96% specificity for diagnosing septic arthritis, and a negative MRI effectively excludes the diagnosis. 1, 7 However, MRI should not delay definitive treatment when septic arthritis is strongly suspected clinically. 1

Step 3: Definitive Diagnosis—Joint Aspiration

Joint aspiration is the definitive diagnostic procedure and must be performed promptly without waiting for advanced imaging. 1, 6

For knee joints, aspiration can be performed at bedside; for hip joints, ultrasound or fluoroscopic guidance is mandatory to ensure proper needle placement and reduce complications. 1, 6

Synovial Fluid Analysis—Three Essential Tests:

1. Cell count with differential: 1, 6

   • WBC ≥50,000 cells/mm³ strongly suggests septic arthritis 1, 4
   • In adults, a cutoff of 64,000 cells/mm³ yields 90% specificity with 40% sensitivity 3
   • Neutrophil predominance is expected 6
   • For prosthetic joints, the threshold may be as low as 1,100 WBC/mm³ with >64% neutrophils 4

2. Gram stain and culture: 1, 6

   • Gram stain has 76% sensitivity and 96% specificity 1, 6
   • Culture is positive in approximately 80% of non-gonococcal cases 1, 6
   • Staphylococcus aureus is the most common pathogen (>66% of cases) 6, 2

3. Crystal analysis under polarized microscopy: 1, 6

   • Must be performed even if infection is suspected, as crystals do not exclude sepsis 1, 6
   • Septic arthritis and gout coexist in approximately 70-73% of reported dual-pathology cases 1, 6

Critical Pitfalls to Avoid

A negative synovial fluid culture does not exclude infection. 1, 6 If clinical suspicion remains high:

• Consider percutaneous image-guided bone biopsy to evaluate for concurrent osteomyelitis 1, 6
• Patients who received antibiotics before aspiration may have false-negative cultures; ideally, patients should be off antibiotics for ≥2 weeks before aspiration 6
• Consider synovial fluid alpha-defensin, leukocyte esterase, or PCR testing if cultures remain persistently negative 6

A "dry tap" at aspiration does not exclude infection and requires further diagnostic workup. 6

**Ultrasound performed <24 hours after symptom onset may yield false-negative results**; schedule after >24 hours when possible. 1, 7

Do not assume transient synovitis in any febrile child without first excluding septic arthritis. 7 In pediatrics, the Kocher criteria (fever, non-weight-bearing, ESR ≥40, WBC ≥12,000) are validated, with ≥2 criteria indicating need for aspiration. 7 However, Kocher criteria have limited utility in adults. 3

Special Diagnostic Considerations

For suspected gonococcal arthritis, consider history of recent sexual exposure and extra-articular features (dermatitis, tenosynovitis). 2, 4

For suspected Lyme arthritis, look for exposure to endemic areas, possible erythema migrans rash, and positive Lyme serology; the knee is most commonly affected. 1

In immunocompromised or neutropenic patients, maintain high suspicion for atypical organisms including fungi and consider broader empiric coverage. 6

Concurrent osteomyelitis occurs in 58% of pediatric elbow septic arthritis cases and should be evaluated with MRI if septic arthritis is confirmed. 1