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For prosthetic joint infections the diagnostic approach is essentially the same although early radiographic imaging is more important than in native joint infection as it may show signs of prosthesis failure or loosening (seen in many late prosthesis infections). Additionally, the synovial fluid white blood cell (WBC) is often lower than in nativejoint infection, with a diagnostic cutoff suggested as greater than 1,700 cells/mm3 or >65% neutrophils (37).
Nonspecific blood tests such as a white blood count, erythrocyte sedimentation rate, or C-reactive protein argue against joint infection if they are normal, but do not specifically suggest septic arthritis if elevated. Other important diagnostic tests include blood cultures (positive in 50–70% of acute bacterial arthritides) (27), but in only 30% or less of gonococcal arthritis cases) (38), wound cultures (although these often correlate poorly with synovial fluid culture results, except when the pathogen is S. aureus), and serologic testing for B. burgdorferi in selected cases with clinical features of Lyme arthritis in endemic areas. If gonococcal arthritis is suspected urethral and cervical specimens should be sent for N. gonorrhoeae culture and nucleic acid amplification tests. Radiographic and scintillographic imaging may yield additional information that will assist in identifying preexisting joint disease or for confirming a diagnosis of native or prosthetic joint infection or its complications (Table 3 on page 33).
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Treatment
Native joint
Prompt joint drainage and antimicrobial therapy are the mainstays of treatment in bacterial, fungal, or mycobacterial joint infection. Drainage can be through closed needle aspiration performed daily, or arthroscopy. The former modality allows direct visual inspection of the joint with concomitant irrigation, lysis of adhesions, and removal of necrotic tissue and purulent material (42). Open surgical drainage is recommended for septic arthritis of the hip and when less invasive methods fail to control infection.
Initial antimicrobial therapy should be withheld until synovial fluid has been obtained and should be based on synovial fluid gram staining (Table 4). In the case of a nondiagnostic gram stain, empiric antimicrobial coverage of likely infecting pathogens is indicated. Therapy should be narrowed based on identification and antimicrobial susceptibility testing of bacteria cultured from synovial fluid, blood, or in some cases from ancillary cultures. For patients with MRSA-related infection who are allergic to or intolerant of vancomycin, linezolid or daptomycin are potential alternatives, although not approved by the U.S. Food and Drug Administration for this indication. Linezolid is a potentially attractive option for treatment as it is available as an oral tablet, but for bone and joint infection treatment experience is limited. For septic arthritis related to animal or human bites ampicillin-sulbactam or amoxicillin-clavulanate (clindamycin plus ciprofloxacin in penicillin-allergic patients) provides activity against Pasteurella multocida and other oral bacteria. Gonococcal arthritis is best treated initially with ceftriaxone or cefotaxime; oral ciprofloxacin or levofloxacin may be substituted in regions without fluoroquinolone resistance as the patient improves (Table 4). Septic arthritis due to Candida sp. should be treated initially with an amphotericin B preparation followed by a prolonged course of fluconazole if susceptibility testing confirms activity against the cultured yeast isolate (43).
Duration of intravenous antimicrobials for bacterial joint infections is usually 2 to 4 weeks, while for gonococcal arthritis 2 weeks is sufficient. Antimicrobial therapy that continues for 2 weeks or longer should have weekly followup and laboratory monitoring for hematologic, renal, and liver toxicity.
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Prosthetic Joint
Treatment of prosthetic joint septic arthritis is complex, and early consultation with an orthopedic surgeon and infectious diseases physician is recommended. Extensive surgical debridement of the afflicted joint and effective, prolonged antimicrobial therapy is necessary in almost all cases. In order to achieve an optimal synovial fluid and tissue culture yield, antimicrobial therapy should be delayed until the time of debridement surgery unless the patient is septic or exhibiting serious systemic complications of infection. Suggestions for early empiric therapy while awaiting culture results are given in Table 4. Final antimicrobial choices should be based on culture results with assistance from an infectious diseases consultant.