Septic arthritis

1. History

• Onset and timing: Acute onset (hours to days) of atraumatic joint pain, swelling, and decreased range of motion; rapid progression is typical [1][5]
• Symptom characterization: Joint pain (sensitivity 85%), joint swelling (sensitivity 78%), fever (sensitivity 57%); sweats (27%) and rigors (19%) are less common [6]
• Key HPI questions:
  • Which joint(s)? Monoarticular in ~80%; knee most common, followed by hip, shoulder, ankle, elbow, wrist [1]
  • Any recent joint surgery, arthroscopy, injection, or arthrocentesis? (LR 6.9 for recent joint surgery) [6]
  • Any skin wounds, cellulitis, or ulcers overlying the joint? [7]
  • IV drug use? Sexual history (gonococcal arthritis in young adults)? [1-2]
  • Recent bacteremia, UTI, or GI infection as a potential hematogenous source? [1]
  • Tick exposure or travel to Lyme-endemic areas? [1]
• Important negatives: Absence of trauma, absence of crystal arthritis history, no recent gout flare

2. Alarm Features

• Sepsis or septic shock (tachycardia, hypotension, altered mental status) — do not delay antibiotics for arthrocentesis [8]
• Hip or shoulder involvement — higher rates of surgical intervention (~50%) and higher mortality [1][9]
• Oligoarticular (multiple joint) involvement — associated with higher mortality and more systemic illness [1]
• Confusion on admission — independent risk factor for mortality [10]
• Bacteremia with positive blood cultures — consider echocardiography for endocarditis, especially with S. aureus, oral streptococci, or enterococci [11-12]
• Grossly purulent synovial fluid — immediate surgical drainage should be considered [8]
• Immunocompromised patients — may present subtly with lower WBC counts and atypical organisms [1][13]

3. Medications

• Relevant contributors:
  • Immunosuppressants (corticosteroids, DMARDs, anti-TNF agents — incidence 4.2/1,000 patient-years with anti-TNF vs. 1.8 with nonbiologic DMARDs) [1]
  • Recent intra-articular corticosteroid injection (risk ~4 per 10,000 injections) [7]
• Empiric treatment (adults):
  • Gram-positive cocci on Gram stain → vancomycin or daptomycin ± cephalosporin/carbapenem/fluoroquinolone; alternative: oral clindamycin + fluoroquinolone [1]
  • Gram-negative cocci → ceftriaxone (consider gonococcal) [1]
  • Gram-negative rods → ceftazidime, cefepime, piperacillin-tazobactam, or carbapenem [1]
  • Negative Gram stain with high suspicion → vancomycin + ceftazidime or aminoglycoside [1]
• Oral antibiotics are not inferior to IV when started within one week of surgery/arthrocentesis [1]
• Duration: 2–6 weeks depending on organism and joint; MRSA requires drainage + 3–4 weeks total [1]
• Contraindicated: Do not use intra-articular corticosteroids in suspected septic arthritis; adjunctive systemic corticosteroids are not recommended [8]

4. Diet

• No specific dietary triggers or recommendations unique to septic arthritis
• Hydration is important in the setting of sepsis and antibiotic therapy
• Adequate nutrition supports immune function and wound healing during prolonged treatment courses

5. Review of Systems

• Constitutional: Fever, chills, rigors, night sweats, malaise, weight loss
• Musculoskeletal: Pain with any joint movement, inability to bear weight, joint stiffness
• Skin: Overlying erythema, warmth, wounds, cellulitis, ulcers, rashes (disseminated gonococcal infection presents with dermatitis-arthritis syndrome)
• GU: Urethral discharge, dysuria (gonococcal source); urinary symptoms (gram-negative bacteremia source)
• Cardiac: New murmur, dyspnea (endocarditis screening) [12]
• Neurologic: Focal deficits (septic emboli if endocarditis present) [14]

6. Collateral History and Family History

• Collateral: Confirm timeline of symptom onset, recent procedures, medication compliance (immunosuppressants), IV drug use, sexual contacts
• Family history: Generally not a major contributor; however, hereditary conditions predisposing to gout (a key mimic) or rheumatoid arthritis may be relevant
• Social context: IV drug use, alcohol abuse, homelessness/low socioeconomic status, nursing home residence (MRSA risk) [7]

7. Risk Factors

8. Differential Diagnosis

• Crystal arthropathies (gout, pseudogout) — most common mimic; crystals on polarized microscopy, but crystals do NOT exclude coexisting septic arthritis [1][15]
• Reactive arthritis — post-infectious, typically sterile joint; history of preceding GI or GU infection
• Rheumatoid arthritis flare — particularly dangerous overlap; RA patients are at high risk for septic arthritis [1]
• Cellulitis / periarticular abscess — overlying soft tissue infection without true intra-articular involvement [5]
• Osteomyelitis — contiguous or hematogenous; MRI differentiates; may coexist with septic arthritis [1]
• Lyme arthritis — endemic areas, knee most common, less acute/toxic appearance, serology + PCR [1][8]
• Gonococcal arthritis — young, sexually active patients; migratory polyarthralgia, tenosynovitis, dermatitis [1-2]
• Transient synovitis — primarily pediatric (hip); Kocher criteria help differentiate [16]
• Avascular necrosis — insidious onset, risk factors (steroids, alcohol, sickle cell)
• Hemarthrosis — trauma or coagulopathy; bloody aspirate
• Malignancy — atypical presentation, chronic course

9. Past Medical History

• Key conditions: Rheumatoid arthritis, osteoarthritis, gout/pseudogout, diabetes, chronic kidney disease, liver cirrhosis, malignancy, HIV [7][17]
• Previous episodes of septic arthritis or joint infections
• Surgical history: Prior joint surgery, arthroplasty, arthroscopy, recent intra-articular injections
• Prosthetic joints — periprosthetic joint infection has a more subtle presentation [1]

10. Physical Exam

• Vital signs: Fever (present in ~57%; often low-grade), tachycardia, hypotension if septic [6]
• Joint exam:
  • Severely painful joint with any passive or active motion — most sensitive finding [18]
  • Obvious effusion, warmth, erythema, swelling
  • Limited range of motion
  • Inability to bear weight (especially hip/knee)
• Skin: Inspect for overlying cellulitis, wounds, ulcers, injection sites, track marks, gonococcal skin lesions (pustules on erythematous base)
• Subtle presentations: Periprosthetic infections, small joint infections, immunosuppressed patients, and atypical organisms (fungal, TB, Lyme) may lack classic inflammatory signs [1]
• Cardiac: Auscultate for new murmurs (endocarditis) [14]
• Spine: Assess for tenderness (vertebral osteomyelitis as concurrent source) [14]

11. Lab Studies

Pearl: If antibiotics were given before arthrocentesis, the optimal synovial WBC cutoff drops to >16,000 cells/μL (sensitivity 82%, specificity 76%) vs. >33,000 in antibiotic-naïve patients. [20]

12. Imaging

• Plain radiography: First-line; establishes baseline, rules out fracture, may show soft tissue swelling or joint space widening; no pathognomonic findings for SA in adults [1]
• Ultrasound: Detects joint effusion (sensitivity 81% overall); guides arthrocentesis for deep joints (hip) and small joints [1][8]
• MRI (with and without contrast): Best for assessing concurrent osteomyelitis, soft tissue abscess, and extent of synovial inflammation; sensitivity 83%, specificity ≥93% for differentiating SA from transient synovitis at the hip [1][8]
• CT: Limited role; uncertain accuracy; radiation exposure; may be used when MRI is unavailable [8]
• When imaging is unnecessary: If clinical suspicion is high and the joint is accessible, proceed directly to arthrocentesis — imaging should not delay diagnosis or treatment [21]

13. Special Tests

• Kocher Criteria (pediatric hip): Fever >101.3°F, ESR ≥40, WBC ≥12,000, inability to bear weight — meeting all 4 criteria approaches 99% probability of SA [16]
• Modified Kocher Criteria: Addition of CRP >2.0 mg/dL improves diagnostic accuracy [16]
• Gächter Classification (arthroscopic staging):
  • Stage I: Opaque fluid, synovial erythema
  • Stage II: Fibrinous deposits, purulent material
  • Stage III: Synovial thickening, compartment formation
  • Stage IV: Aggressive pannus, cartilage/subchondral bone erosion
  • Stages III–IV predict treatment failure with arthroscopy alone and may require more aggressive debridement [22-23]
• Synovial fluid NLR (neutrophil-to-lymphocyte ratio): Threshold of 25 has 78% sensitivity and 81% specificity — more accurate than traditional WBC >50,000 cutoff [24]
• Bedside ultrasound: Point-of-care detection of effusion and guided aspiration

14. ECG

• ECG is not routinely indicated for isolated septic arthritis
• Obtain ECG if:
  • Sepsis or septic shock is present — common ECG abnormalities include sinus tachycardia (39%), atrial fibrillation/flutter (8.8%), and QT prolongation (54.4% in sepsis patients) [25]
  • Concern for endocarditis (S. aureus bacteremia) — new conduction abnormalities may suggest perivalvular abscess [14]
  • Demand ischemia in elderly or comorbid patients with sepsis [26]
• Dangerous patterns: New atrial fibrillation/flutter (OR 2.19 for poor outcomes), QT prolongation, ST changes in the setting of sepsis [25]

15. Assessment

• Clinical summary: Septic arthritis is an acute monoarticular (occasionally oligoarticular) infection requiring emergent diagnosis via arthrocentesis and prompt antibiotic therapy. The most common pathogen is S. aureus across all age groups. [1-2]
• Severity stratification:
  • Low risk: Single small joint, young patient, no comorbidities, rapid presentation
  • High risk: Hip/shoulder involvement, age ≥65, oligoarticular, bacteremia, immunosuppression, MRSA, delayed presentation [1][9]
• Atypical presentations: Immunosuppressed patients, periprosthetic infections, fungal/TB/Lyme arthritis — may lack fever, have lower synovial WBC counts, and follow an indolent course [1]
• Complications: Cartilage destruction, osteomyelitis, joint ankylosis, need for arthroplasty (~9% at 15 years), amputation (0.4% at 1 year), endocarditis (3.7% of SA patients), death [1][4][12]
• Mortality: 90-day mortality ~7% overall; 1-year mortality 11–17%; significantly higher with age ≥80, chronic renal disease (OR 2.80), malignancy (OR 3.40), and heart failure (OR 2.62) [1][3][17]

16. Treatment Plan

Initial stabilization:

• IV access, fluid resuscitation if septic, pain management
• Arthrocentesis before antibiotics when feasible; do not delay antibiotics in septic/toxic patients [8][21]

Empiric antibiotics (adults):

• Vancomycin (MRSA coverage) + 3rd/4th-generation cephalosporin (gram-negative coverage) is a common ED regimen [1][22]
• Narrow based on Gram stain, then culture/sensitivity results
• Oral step-down (e.g., clindamycin + fluoroquinolone) is non-inferior to prolonged IV therapy [1]

Duration:

• Nongonococcal: 2–6 weeks (at least 2 weeks for small joints; commonly 4–6 weeks for large joints) [1]
• MRSA: Drainage + 3–4 weeks total [1]
• Gonococcal: IV ceftriaxone → oral switch after 2 days, ≥7 days total [1]
• Pediatric (uncomplicated): 10–14 days with monitoring of CRP [8][27]

Joint drainage:

• Arthrocentesis (serial aspirations) vs. arthroscopic lavage vs. open arthrotomy [1][23]
• Cohort studies suggest medical management (antibiotics + serial aspirations) is not inferior to surgical management, but ~30% ultimately require surgery [1]
• Hip and shoulder infections more often require surgical drainage (~50%) [1]
• Grossly purulent fluid or Gächter stage III–IV → consider early surgical intervention [8][23]

Echocardiography: Indicated for S. aureus, oral streptococci, S. gallolyticus, or E. faecalis bacteremia, or when endocarditis is clinically suspected [11]

17. Disposition

• Admission criteria:
  • All confirmed or highly suspected septic arthritis cases require admission for IV antibiotics, joint drainage, and monitoring [1][5]
  • Sepsis, hemodynamic instability → ICU
  • Need for surgical intervention (hip/shoulder involvement, failed aspiration, Gächter III–IV)
• Orthopedic surgery consultation: Obtain early for all cases; urgent for hip, shoulder, prosthetic joints, and failed medical management [5][22]
• Infectious disease consultation: Recommended for culture-directed therapy, MRSA, atypical organisms, prosthetic joint infections [21]
• Discharge criteria: Septic arthritis is not a discharge diagnosis from the ED; patients require inpatient management until clinical improvement, culture results, and transition to oral therapy are established

18. Follow Up / Return Precautions

• Follow-up timing:
  • Orthopedics: 1–2 weeks post-discharge for wound/joint reassessment
  • Infectious disease: Within 1–2 weeks for antibiotic management and duration decisions
  • Primary care: 2–4 weeks; monitor CRP trending to normalization [21]
• Symptoms requiring immediate reassessment:
  • Recurrent fever, worsening joint pain/swelling, new joint involvement
  • Signs of sepsis (confusion, tachycardia, hypotension)
  • Wound drainage or erythema at surgical/aspiration site
• Patient counseling:
  • Complete the full antibiotic course; do not stop early even if feeling better
  • Early physical therapy and range-of-motion exercises are important to prevent stiffness and functional loss [11]
  • Poor functional outcomes (amputation, arthrodesis, arthroplasty, severe deterioration) occur in 24–33% of patients, particularly with older age and preexisting joint disease [1]
• Expected recovery: Clinical improvement typically within 48–72 hours of appropriate therapy; CRP should trend downward within the first week; full recovery may take weeks to months depending on severity and joint involved