What is the diagnostic approach for suspected septic arthritis?

Diagnostic Approach for Septic Arthritis

Septic arthritis requires immediate joint aspiration with synovial fluid analysis as the definitive diagnostic procedure, combined with imaging (ultrasound first for hip joints, plain radiographs for others) to guide aspiration and assess for complications. 1, 2

Clinical Presentation and Initial Assessment

• Suspect septic arthritis in any patient with acute monoarticular joint pain, swelling, and warmth, though the classic triad of fever, pain, and diminished mobility occurs in only 50% of cases 2
• Constitutional symptoms like fever and chills have poor sensitivity and should not be relied upon to rule out infection 3
• Key clinical predictors include: fever >101.3°F, ESR ≥40 mm/hour, WBC ≥12,000 cells/mm³, inability to bear weight, and CRP >2.0 mg/dL—meeting all criteria approaches 100% likelihood of septic arthritis 2

Imaging Algorithm

Initial Imaging Selection

• For hip joints: Start with plain radiographs followed immediately by ultrasound 4
• For knee and other peripheral joints: Begin with plain radiographs, then proceed to ultrasound if effusion suspected 5, 4
• Plain radiographs have low sensitivity (often normal in <14 days of infection) but help exclude fractures, tumors, and other bony pathology 1, 2

Ultrasound Findings

• Ultrasound is highly sensitive for detecting joint effusions (can detect as small as 1 mL) with only 5% false-negative rate in patients with symptoms <1 day 5
• Absence of hip joint effusion on ultrasound virtually excludes septic arthritis 5
• Use ultrasound to guide hip aspiration—this is mandatory for safe needle placement 5, 2
• Critical pitfall: False-negative ultrasound results occur if performed within 24 hours of symptom onset 2

MRI Indications

• Order MRI when: clinical suspicion remains high despite negative aspiration, concern for concurrent osteomyelitis (occurs in 30-58% of cases), or need to assess soft tissue abscess/pyomyositis 1, 2
• MRI has 82-100% sensitivity and 75-96% specificity for diagnosing septic arthritis and osteomyelitis 5
• Decreased femoral head enhancement on early post-contrast imaging distinguishes septic arthritis from transient synovitis 4
• Consider preoperative MRI if ≥3 high-risk criteria present: CRP >13.8 mg/L, absolute neutrophil count >8,600 cells/mm³, platelet count <314,000 cells/mm³, symptoms >3 days, age >3.6 years 4

Imaging Modalities to Avoid

• CT has limited utility—decreased sensitivity for bone marrow pathology and inferior soft-tissue contrast compared to MRI; reserve for when MRI is contraindicated 5, 4
• Bone scintigraphy has poor spatial resolution and specificity; only consider for suspected multifocal infections 4

Definitive Diagnosis: Joint Aspiration

Aspiration Technique

• Perform US-guided aspiration for hip joints—this is highly accurate and reduces complications 5, 1
• Knee joints can be aspirated at bedside without imaging guidance 2
• Obtain aspiration before starting antibiotics whenever possible, but never delay antibiotics to obtain imaging 1

Synovial Fluid Analysis Thresholds

• Synovial fluid WBC ≥50,000 cells/mm³ is suggestive of septic arthritis in patients not on antibiotics 1, 3
• Critical adjustment for antibiotic exposure: If patient received antibiotics within 2 weeks before aspiration, use lower cutoff of >16,000 cells/mm³ (sensitivity 82%, specificity 76%) 6
• Without prior antibiotics, optimal cutoff is >33,000 cells/mm³ (sensitivity 96%, specificity 95%) 6
• Neutrophil percentage >90% supports diagnosis in antibiotic-exposed patients; >83% in non-exposed patients 6
• For prosthetic joints, use much lower threshold: as low as 1,100 WBC/mm³ with neutrophils >64% 3

Synovial Fluid Culture and Gram Stain

• Synovial fluid culture is positive in approximately 80% of non-gonococcal septic arthritis cases 1, 2
• Gram stain has 76% sensitivity and 96% specificity for distinguishing septic arthritis from crystal arthropathy 2
• Always perform crystal analysis even when infection suspected—gout and septic arthritis coexist in 73% of reported cases with positive cultures 2

Rapid Bedside Tests

• Leukocyte esterase (LE) strip test positive alone: 95% sensitivity, 82% specificity, 99% NPV 7
• Combined positive LE with negative glucose strip test: 100% specificity, 85% sensitivity, 100% PPV—can rule in septic arthritis definitively 7

Laboratory Studies

• Obtain blood cultures before starting antibiotics—positive in many cases and guides therapy 1
• Elevated CRP and ESR support diagnosis and are useful for monitoring treatment response 1, 2
• Peripheral WBC count has limited diagnostic value but contributes to clinical prediction rules 2

Management of Negative Cultures

• If joint aspirate culture is negative but clinical suspicion remains high, proceed to percutaneous image-guided bone biopsy to evaluate for concurrent osteomyelitis 1, 2
• Negative culture does not exclude infection—consider prior antibiotic exposure, fastidious organisms (Kingella kingae in children <4 years), or gonococcal arthritis 2

Special Populations

• Neonates: Group B streptococcus predominates 2
• Children <4 years: Consider Kingella kingae 1, 2
• Sickle cell disease: Consider Salmonella species 1
• All ages: Staphylococcus aureus is most common overall pathogen 2, 3

Critical Pitfalls to Avoid

• Never delay antibiotics to obtain imaging—start vancomycin immediately after joint aspiration and blood cultures 1
• Do not rely on normal radiographs to exclude septic arthritis—they may be completely normal in early infection 2, 4
• Do not skip crystal analysis—crystals do not exclude sepsis, and both conditions frequently coexist 2
• Remember the 5% false-negative rate for ultrasound in patients with symptoms <1 day 5
• Adjust synovial fluid WBC thresholds downward if patient received antibiotics within 2 weeks 6