An inflamed joint with synovial fluid WBC > 25K (or >90% PMNs),  should be presumed septic. Temperature > 38.3°C is suggestive of the diagnosis, but nearly half of patients will present without fever. TWithout immediate treatment rapid joint destruction with lasting functional impairment can result. In-hospital mortality can reach 7-15% despite the provision of antibiotics. Rule out in all patients with acute-onset monoarticular arthritis. This is especially important to keep in mind for those with preexisting rheumatic disease who carry an increased risk for septic arthritis due to prior joint damage and immunocompromise.  

Etiology / Risk Factors

History of injection drug use, increasing age, indwelling catheters, osteoarthritis, rheumatoid arthritis (0.5% annual incidence), post-arthroscopy, post-injection of joint (exceedingly rare), prosthetic joints, DM, HIV, immunosuppression, endocarditis.

Differential Diagnosis

• Approach differential as gonococcal vs. non-gonococcal bacterial infection
  • Other infectious causes: Lyme disease (typically monoarthritis of the knee in late, disseminated disease), TB, fungus
  • Noninfectious causes: crystalline (gout, pseudogout), RA, reactive arthritis, psoriatic arthritis (all can have cell counts >50,000 cells/µL with > 90% neutrophils)
  • Infections outside the joint: osteomyelitis, bursitis
  • Intra-articular derangement: torn meniscus, fracture, hemarthrosis 
• Gonococcal: Neisseria gonorrhea; usually young, sexually active patients. Increased risk with late complement deficiency, SLE
  • Two clinical presentations which may overlap: 1) Triad of migratory tenosynovitis, vesicular or pustular dermatitis, and polyarthralgias without purulent arthritis, aka arthritis-dermatitis syndrome, 2) 25-50% develop a mono- or oligo-arthritis with purulent joint effusions typically of knees, wrists, or ankles. Rarely involves axial joints. 80% of patients have a concomitant, asymptomatic genitourinary infection. Joint culture and blood cultures are often negative
• Non-gonococcal: S. aureus >> Streptococcal species (more likely in asplenic patients). Other less common causes include aerobic gram negatives (increased in patients with injection drug use, diabetes, sickle cell disease, or immunocompromise), anaerobic organisms (trauma, immunocompromise, post-GI surgery), mycobacteria (HIV, immunocompromised, travel), fungus (HIV, other immunocompromise, regional exposures), and spirochete (ticks)
  • Presents in knee in > 50% cases; polyarticular in 10-20%. Polyarticular infections more likely to occur in those with SLE, RA or who are immunocompromised by drugs or systemic illness. Patients with IVDU have a predilection for axial joints (sternoclavicular joint). Symptoms: rapid onset (hours to days) joint pain, warmth, and restricted motion

Diagnosis/Management

• Bacterial septic arthritis is an emergency.
• Rule out septic joint with needle aspiration (avoiding any overlying cellulitis). History and exam findings are neither sensitive nor specific enough to affect the pretest probability of disease. On synovial fluid analysis, key data include WBC count and %PMNs (see table below), gram stain, and culture. There is no gold standard for the diagnosis of septic joint; gram stain has a sensitivity of only 29-50% and culture has a sensitivity of only 82%. Therefore, the diagnosis is suggested by considering in aggregate the clinical symptoms (acute monoarthritis), key synovial fluid studies (see below), and the results of gram stain/culture.
• Obtain blood cultures, begin empiric antibiotics, and consult orthopedic surgery early, since timely washout of the joint is often necessary. X-rays can be ordered to evaluate for possible osteomyelitis and joint destruction, but be warned that films are often normal in the early course of these conditions.

Arthrocentesis data in the diagnosis of septic joints

*NOTE: synovial glucose, protein, and LDH are individually neither sensitive nor specific enough to aid in the diagnosis of a septic joint.

• Gonococcal:
  • Obtain a sexual history.   
  • Send cultures of all appropriate sites — pharynx, joint (30% sensitive; do gram stain as well as culture and sensitivity), blood, rectum (treatment differs, always test even if patient denies rectal contact), urethra, cervix (50-80% sensitive). Specifically request gonorrhea culture (Chocolate agar or Thayer-Martin media).
  • Cover empirically with ceftriaxone 1 g IV daily after cultures in sexually active patients without crystals or other known cause of arthritis. CDC recommends minimum 7 days of treatment with switch to orals 24 hours after improvement in symptoms. 
  • If purulent, joint may need drainage. Treat partners. Treat concurrently for chlamydia.
• Non-gonococcal:
  • Send synovial fluid for crystal examination, cell count/differential, gram stain/culture.
  • Blood cultures have 50% sensitivity.
  • Begin empiric antibiotics and consult orthopedics before results of culture because of the rapidly destructive nature of this condition.
  • Antibiotic choice directed by history and initial gram stain, adjust once culture data is available. Cover for staph, strep, and gonococcal arthritis (unless sexual contact can be excluded). Treat for >2 weeks of IV antibiotics followed by 2-6 weeks of oral antibiotics (a shorter course is acceptable only in gonococcal arthritis). 

Fluid analysis

Fluid should be sent for cell count, differential, crystals, and culture. Other tests on synovial fluid are not helpful.

• The synovial fluid leukocyte count seen with septic arthritis, crystal-induced arthritis, or other noninfectious inflammatory causes overlap considerably. As such, one must always consider septic arthritis. The higher the WBC count (>50,000) and the greater the proportion of neutrophils (>90%), the higher the likelihood of septic arthritis.
• Lower cell counts may be observed among immunocompromised patients and in infections due to mycobacteria, some Neisseria, and some gram-positive organisms.

References

Mathews CJ, Weston VC, Jones A, et al. Bacterial Septic Arthritis in Adults. Lancet 2010; 375; 9717: 846-55 

Margaretten M, Kohlwes J, Moore D, et al. Does this adult patient have septic arthritis? JAMA 2007;297;13:1478-1488.

Mathews CJ, Kingsley G, Field M, et al.  Management of septic arthritis: a systematic review. Ann Rheum Dis 2007;66:440-445.

Courtney P, Doherty M.  Joint aspiration and injection and synovial fluid analysis. Best Practice Clin Rheum. 2009; 23; 161-192.Pascual E, Jovani V. Synovial Fluid Analysis. Best Pract Res Clin Rheumatol. 2005; 19:371-386.