I have a hot, swollen, painful joint that developed without any obvious injury—what is the appropriate evaluation and management?

Evaluation and Management of Hot, Swollen, Painful Joint Without Injury

A hot, swollen, painful joint without trauma is septic arthritis until proven otherwise—immediate joint aspiration with synovial fluid analysis is mandatory to rule out this medical emergency, as bacterial proliferation can cause irreversible cartilage damage within hours to days. 1, 2, 3

Immediate Diagnostic Approach

Step 1: Urgent Joint Aspiration

• Perform arthrocentesis immediately—this is both diagnostic and therapeutic and should not be delayed for imaging. 1, 2
• Joint aspiration is the definitive diagnostic procedure, with synovial fluid white blood cell count ≥50,000 cells/mm³ suggestive of septic arthritis 1
• Obtain synovial fluid for: Gram stain, culture (positive in ~80% of non-gonococcal cases), cell count with differential, and crystal analysis 1, 2
• Do not start antibiotics before obtaining synovial fluid unless the patient is septic or unstable, as this reduces culture yield 2
• Use ultrasound guidance for hip joints, as bedside aspiration is unsafe—ultrasound can detect effusions as small as 1 mL with only 5% false-negative rate 1, 2
• Knee joints can be aspirated at bedside without imaging guidance 1

Step 2: Obtain Plain Radiographs

• Order radiographs of the affected joint immediately, even though they have limited sensitivity in early disease 1, 2
• Radiographs exclude fractures, tumors, and other bony pathology, and may show joint effusion, soft tissue swelling, or gas in soft tissues 1, 2
• Normal radiographs do not exclude septic arthritis—radiographs are often completely normal in early acute infection (<14 days) 1, 2

Step 3: Laboratory Evaluation

• Draw blood cultures, complete blood count, ESR, and CRP before starting antibiotics 1
• Meeting all five criteria approaches 100% likelihood of septic arthritis: fever >101.3°F, ESR ≥40 mm/hour, WBC ≥12,000 cells/mm³, inability to bear weight, and CRP >2.0 mg/dL 1
• However, the classic triad of fever, pain, and diminished mobility occurs in only ~50% of cases—absence of fever does not exclude septic arthritis 1, 4

Empiric Treatment

Immediate Antibiotic Therapy

• Start IV vancomycin 15 mg/kg every 6 hours immediately after obtaining synovial fluid—do not wait for culture results 1, 2
• Vancomycin provides first-line MRSA coverage in adults; alternatives include linezolid, daptomycin, or teicoplanin 1
• Staphylococcus aureus is the most common pathogen overall across all ages 1

Surgical Drainage

• Immediate surgical drainage combined with appropriate antibiotic therapy is mandatory, as bacterial proliferation causes irreversible cartilage damage within hours to days 1, 4, 3
• Surgical drainage is required when symptoms persist >7 days, patient has severe sepsis, or hip/shoulder involvement 1
• Repeated needle aspiration alone fails in 46% of cases 1

Advanced Imaging (When Indicated)

MRI with Contrast

• Order MRI when clinical suspicion remains high despite negative aspiration, concern exists for concurrent osteomyelitis, or need to assess soft tissue abscess 1, 2
• MRI has 82-100% sensitivity and 75-96% specificity for diagnosing septic arthritis and osteomyelitis 1
• A negative MRI effectively excludes septic arthritis 1
• Key MRI findings include joint effusion, synovial thickening/enhancement, bone marrow edema, and decreased femoral head enhancement on early post-contrast imaging 1
• However, MRI should not delay definitive treatment when septic arthritis is strongly suspected clinically 1

When to Consider Ultrasound

• Ultrasound is highly sensitive for detecting hip joint effusions and excellent for guiding joint aspiration 1, 2
• Ultrasound may yield false-negative results if performed within 24 hours of symptom onset 1
• Absence of hip joint effusion on ultrasound virtually excludes septic arthritis 1

Critical Differential Diagnoses to Exclude

Crystal Arthropathy

• Crystal analysis is essential even if infection is suspected—crystals do not exclude sepsis, and both conditions can coexist 1, 2
• Among reported cases of coexistent sepsis and gout, 73% had positive synovial fluid cultures 1
• Synovial fluid culture has high discriminative value (sensitivity 0.76, specificity 0.96) between septic arthritis and gout 1

Other Mimics

• Gout is the most common final diagnosis (18% of cases), followed by cellulitis (12%), bursitis (9%), and reactive arthritis (8%) 5
• Crystal arthropathies can produce similarly elevated cell counts and decreased viscosity as septic arthritis 1
• In patients with rheumatoid arthritis, septic arthritis is often confused with a disease flare—maintain high suspicion and always aspirate 6, 7

Treatment Duration and Monitoring

Antibiotic Duration

• Total antibiotic duration is 3-4 weeks for uncomplicated bacterial arthritis 1
• Recent evidence suggests 2 weeks may be adequate after surgical drainage in select cases 1
• Longer treatment is required for concomitant osteomyelitis or prosthetic joint infections 1

Transition to Oral Therapy

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

Monitoring Response

• Monitor CRP and ESR to assess treatment response 1
• Monitor vancomycin trough levels to adjust for toxicity 1
• If symptoms worsen or persist, repeat MRI—this results in management changes in 21% of cases 1

Critical Pitfalls to Avoid

• Negative synovial fluid culture does not exclude infection, especially if antibiotics were already started—if clinical suspicion remains high, consider percutaneous bone biopsy to evaluate for concurrent osteomyelitis 1, 2
• Do not assume "just a sprain" or cellulitis based on negative radiographs—joint pain with effusion demands synovial fluid analysis 2
• In patients with rheumatoid arthritis, RA itself and treatment with corticosteroids/DMARDs decrease immune function and lower synovial fluid leukocyte counts, making diagnosis more challenging 6, 7
• Over 50% of pediatric patients with septic arthritis have infection beyond the joint space on MRI, highlighting importance of assessing for concurrent osteomyelitis 1
• Do not use CT as initial imaging—it has decreased sensitivity for bone marrow pathology and inferior soft-tissue contrast compared to MRI 1

Prognosis

• Poor functional outcomes (amputation, arthrodesis, prosthetic surgery, severe functional deterioration) occur in 24-33% of patients 4
• 90-day mortality rate is 7% in patients ≤79 years and 22-69% in patients >79 years 4
• 30% of patients ultimately require surgical treatment despite initial medical management 4