Septic arthritis is a highly destructive form of joint disease most often caused by hematogenous spread of organisms from a distant site of infection. Direct penetration of the joint as a result of trauma or surgery and spread from adjacent osteomyelitis may also cause bacterial arthritis. Any joint in the body may be affected.

Infectious arthritis

Bacterial arthritis

Pyogenic arthritis

• Hallmark: Acute onset of monoarticular joint pain, erythema, heat, and immobility
• Limited range of motion of the joint
• Effusion, with varying degrees of erythema and increased warmth around the joint
• Single joint affected in 80% to 90% of cases of nongonococcal arthritis
• Gonococcal dermatitis-arthritis syndrome:
  1. Typical pattern is a migratory polyarthritis or tenosynovitis
  2. Small pustules on the trunk or extremities
• Febrile patient at presentation
• Most commonly affected joints in adult: Knee and hip, but any joint may be involved; in children-hip

• Bacteria spread from another locus of infection.
  1. Highly vascular synovium is invaded by hematogenously spread bacteria.
  2. White blood cell (WBC) enzymes cause necrosis of synovium, cartilage, and bone.
  3. Extensive joint destruction is rapid if infection is not treated with appropriate intravenous (IV) antibiotics and drainage of necrotic material.
• Predisposing factors: Rheumatoid arthritis, prosthetic joints, advanced age, immunodeficiency (HIV, diabetes mellitus [DM], immunosuppressive drugs), gout, sexual activity (gonococcal arthritis), skin infections, cutaneous ulcers (contiguous spread), recent joint surgery, recent intraarticular infection. Risk factors for development of septic arthritis are summarized in Table 1. Fig. E1 illustrates routes by which bacteria can reach the joint.
• The most common nongonococcal organisms are staphylococci (40%), streptococci (28%), and gram-negative bacilli (19%). Less common are mycobacteria (8%), gram-negative cocci (3%), anaerobes (1%), and gram-positive bacilli (1%).
• Staphylococci (S. aureus and coagulase-negative staphylococcal species) account for >50% of prosthetic-hip and prosthetic-knee infections. S. aureus is very common in patients with rheumatoid arthritis.

• Gout
• Pseudogout
• Trauma
• Hemarthrosis
• Rheumatic fever
• Adult or juvenile rheumatoid arthritis
• Spondyloarthropathies such as reactive arthritis (Reiter syndrome)
• Osteomyelitis
• Viral arthritides
• Septic bursitis
• Lyme disease caused by Borrelia burgdorferi

• Joint aspiration, Gram stain, and culture of the synovial fluid. Fig. 2 describes an algorithm for synovial fluid analysis in septic arthritis.
• Immediate arthrocentesis before other studies are undertaken or antibiotics instituted. Synovial fluid should be evaluated at bedside and then sent for lab evaluation.
• Criteria for diagnosis of prosthetic joint infections are summarized in Table 2.

Figure 2 Algorithm for synovial fluid analysis in septic arthritis.

OA, Osteoarthritis; RA, rheumatoid arthritis; WBC, white blood cell count.

From Harris ED et al: Kelley’s textbook of rheumatology, ed 7, Philadelphia, 2005, Saunders.

• Joint fluid analysis:
  1. Synovial fluid leukocyte count is usually elevated >50,000 cells/mm³ with >80% polymorphonuclear cells.
  2. Counts are highly variable, with similar findings in gout, pseudogout, or rheumatoid arthritis. Lower WBC counts can occur in joint replacement, disseminated gonococcal disease, and peripheral leukopenia.
  3. Synovial fluid glucose or protein is not helpful because results are not specific for septic arthritis. The differential diagnosis of synovial fluid abnormalities is described in Section IV.
  4. Polymerase chain reaction (PCR) testing: Useful for detection of uncommon organisms (e.g., Lyme disease).
  5. Crystal analysis: Septic arthritis can coexist with crystal arthropathy; therefore the presence of crystals does not preclude a diagnosis of septic arthritis.
• Blood cultures: Positive in 25% to 50% of patients with septic arthritis.
• Culture of possible extraarticular sources of infection.
• Elevated peripheral WBC count, erythrocyte sedimentation rate (ESR) (nonspecific), C-reactive protein (CRP) (nonspecific). When elevated, ESR and CRP may be useful to monitor therapeutic response.
• If gonococcus is suspected, perform nucleic acid amplification tests (NAATs) on synovial fluid.

• Radiograph of the affected joint (Fig. 3): Useful to rule out osteomyelitis, fractures, chondrocalcinosis, or inflammatory arthritis
• MRI: Findings that suggest an acute intraarticular infection include the combination of bony erosions with marrow edema
• Computed tomography (CT) scan: Useful for early diagnosis of infections of the spine, hips, and sternoclavicular and sacroiliac joints
• Ultrasound: Can be useful for detecting effusions in joints that are more difficult to examine (e.g., hip)

• Affected joints aspirated daily to remove necrotic material and to follow serial WBC counts and cultures
• If no resolution with IV antibiotics and closed drainage: Open debridement and lavage, particularly in nongonococcal infections
• Prevention of contractures:
  1. After acute stage of inflammation, range-of-motion exercises of the affected joint
  2. Physical therapy helpful

• IV antibiotics immediately after joint aspiration and Gram stain of the synovial fluid. Empiric antibiotic therapy (Table 3) is based on organism found on Gram stain of synovial fluid:
  1. Gram-positive cocci: Vancomycin: 15 to 20 mg/kg IV q8 to 12h. Keep trough levels at 15 to 20 mcg/ml. Alternatives include daptomycin and linezolid
  2. Gram-negative cocci: Ceftriaxone: 1 to 2 g IV daily in adults (children: 50-100 mg/kg IV daily). Alternative includes cefotaxime
  3. Gram-negative rods: Ceftriaxone, cefepime: 1 to 2 g IV q8 to 12h in adults (children: 100-150 mg/kg/day divided in q8h dosing), piperacillin-tazobactam: 3.375 g IV to 4.5 g IV q6h. Aztreonam or fluoroquinolones can be used in patients with allergy to penicillin or cephalosporins
  4. Negative Gram stain: Vancomycin plus either cefepime or a carbapenem such as meropenem: 1 g IV q8h in adults (children: 60 mg/kg/day divided in q8h dosing) or ertapenem
• The optimal duration of antibiotic use in septic arthritis has traditionally been 3 to 6 wk usually following surgical drainage. Recent trials (Gjika et al, Ann Rheum Dis Aug 2019, 78: 1114) support a 2-wk course in conjunction with surgery. The duration of antimicrobial therapy in patients with prosthetic joint infections remains unclear however trials have shown that outcomes are better with a 12-wk antibiotic course than with a 6-wk course.¹

Septic Arthritis (Patient Information)