What is the most likely diagnosis for a patient presenting with fever, joint pain, and a tender inflamed joint?

Septic Arthritis

The most likely diagnosis for a patient presenting with fever, joint pain, and a tender inflamed joint is septic arthritis, which must be presumed and urgently evaluated because bacterial proliferation can cause irreversible cartilage damage within hours to days. 1, 2

Clinical Presentation

The classic presentation includes:

• Acute monoarticular joint pain with erythema, warmth, swelling, and pain on palpation and movement 1, 3
• Fever is present in many patients, though the classic triad of fever, pain, and diminished mobility occurs in only approximately 50% of cases 1, 2
• Constitutional symptoms such as chills and rigors may be present but are poorly sensitive 4

Diagnostic Criteria

Meeting the following criteria approaches 100% likelihood of septic arthritis: 1

• Fever >101.3°F (38.5°C)
• Erythrocyte sedimentation rate (ESR) ≥40 mm/hour
• White blood cell count ≥12,000 cells/mm³
• Inability to bear weight
• C-reactive protein >2.0 mg/dL

Immediate Diagnostic Approach

Joint Aspiration (Gold Standard)

Joint aspiration is the definitive diagnostic procedure and should not be delayed to obtain advanced imaging studies. 5, 1

• Synovial fluid white blood cell count ≥50,000 cells/mm³ is highly suggestive of septic arthritis 1, 4
• Gram stain has sensitivity of 0.76 and specificity of 0.96 for distinguishing septic arthritis from other causes 1
• Synovial fluid culture is positive in approximately 80% of non-gonococcal cases 1
• Crystal analysis must be performed even if infection is suspected, as crystal arthropathies (gout, pseudogout) can coexist with septic arthritis in 73% of reported cases 1

Imaging Algorithm

Start with plain radiographs to exclude fractures, tumors, and other bony pathology, though they have low sensitivity for early septic arthritis 1

Ultrasound should be used for:

• Detecting joint effusions (can detect as small as 1 mL) 1
• Guiding aspiration, particularly for hip joints 1
• Note: False-negative results possible if performed within 24 hours of symptom onset 1

MRI with contrast is indicated when: 1

• Clinical suspicion remains high despite negative aspiration
• Concern for concurrent osteomyelitis (present in >50% of pediatric cases)
• Need to assess for soft tissue abscess
• Sensitivity: 82-100%, Specificity: 75-96%

Most Common Pathogens

Staphylococcus aureus is the most common causative organism across all age groups, with methicillin-resistant S. aureus (MRSA) increasingly prevalent 1, 2, 6

Age-specific considerations: 1, 2

• Neonates: Group B streptococcus predominates
• Children <4 years: Kingella kingae is common
• Patients with sickle cell disease: Salmonella species must be considered

Critical Differential Diagnoses

The following conditions can mimic septic arthritis and must be excluded: 5, 1

• Crystal arthropathies (gout, pseudogout) - differentiated by crystal analysis, though can coexist with infection
• Reactive arthritis - may present with conjunctivitis, urethritis, or back pain suggestive of sacroiliitis
• Inflammatory arthritis flares (rheumatoid arthritis) - typically symmetric polyarthritis with positive autoantibodies
• Soft tissue rheumatic disorders (rotator cuff tendinitis)
• Degenerative joint disease/osteoarthritis

Inflammatory markers (ESR, CRP) are usually very elevated in septic arthritis and help differentiate from other rheumatic syndromes 5

Immediate Management

After obtaining synovial fluid cultures, empiric antibiotic therapy must be initiated immediately: 1, 6

• IV vancomycin 15 mg/kg every 6 hours for MRSA coverage in adults 1
• Gram stain results should guide initial antibiotic selection 4
• Surgical drainage is mandatory in all cases, as needle aspiration alone fails in 46% of cases 1

Oral antibiotics are not inferior to IV therapy and can be initiated after 2-4 days if the patient is clinically improving, afebrile, and tolerating oral intake 1, 6

Total antibiotic duration: 1

• 3-4 weeks for uncomplicated bacterial arthritis
• Recent evidence suggests 2 weeks may be adequate after surgical drainage in select cases
• Longer treatment required for concomitant osteomyelitis or prosthetic joint infections

Critical Pitfalls to Avoid

• Negative synovial fluid culture does not exclude infection - approximately 20% of non-gonococcal cases have negative cultures 2
• Do not delay joint aspiration for imaging - irreversible cartilage damage can occur within hours 1, 2
• Normal radiographs do not exclude septic arthritis - they are often normal in early infection (<14 days) 1
• Consider concurrent osteomyelitis, especially in pediatric patients where it occurs in >50% of cases 1
• Monitor CRP and ESR to assess treatment response 1