Approach Considerations
Diagnosis of septic arthritis (SA) is established by a combination of clinical findings and results of synovial fluid analysis. Clinicians should have a low threshold for performing arthrocentesis, especially for children with a painful monoarthritis, significantly limited range of motion, and no plausible noninfectious explanation.
Emergency department physicians are usually adept at performing arthrocentesis of most joints. The knee joint, for example, can usually be entered fairly easily using either a medial or superolateral approach. However, aspiration of fluid from the hip typically requires the involvement of a radiologist and an orthopedic surgeon.
General practices that help to ensure the safety and success of arthrocentesis include liberal use of sedatives and analgesics, joint immobilization, sterile technique, and local anesthesia. Use of a needle large enough (18-20 gauge) to aspirate what may be viscous synovial fluid is also necessary.
Laboratory Studies
When septic arthritis (SA) is suspected, synovial fluid should be obtained for a complete blood count (CBC), glucose, Gram stain, and culture. Synovial culture has poor sensitivity (60-70%), and the data that establish the typical characteristics of synovial fluid in septic arthritis were collected in the era of widespread H influenzae type B infection. How these characteristics will change in the current era remains to be seen. Similarly, although blood cultures should be obtained, they have relatively low yields.
A synovial fluid WBC count of more than 50,000/mL suggests SA, especially if the count exceeds 100,000/mL or if a predominance of polymorphonuclear cells is observed. Still, counts are often lower, and high counts may be associated with other conditions.
A retrospective study by Dart et al that included 238 patients with septic arthritis, Lyme arthritis, and other arthritis reported that synovial fluid WBC count is not helpful in differentiating septic from Lyme arthritis and that rapid Lyme diagnostics would be useful. [16] [17, 18] A separate report found no differences in the synovial fluid white blood cell or neutrophil counts among children with Lyme or septic arthritis of the knee. [19]
The synovial fluid glucose concentration averages 30% of that in the serum, a finding unique to SA. The erythrocyte sedimentation rate is typically elevated, but in one series, fewer than one half of children with septic arthritis had peripheral WBC counts above 15,000 cells/μL.
Although often elevated, a peripheral WBC count within the reference range does not rule out septic arthritis. C-reactive protein (CRP) is a more sensitive marker for septic arthritis than is the peripheral WBC count. The CRP level is also a more sensitive diagnostic marker and a better indicator of response to treatment than the erythrocyte sedimentation rate. [20] In one study, a CRP level of more than 2 mg/dL was found to be a strong independent risk factor for SA of the hip among children presenting with hip pain. [5]
A study evaluated the clinical utility of polymerase chain reaction (PCR) as a supplemental diagnostic tool in the evaluation and the treatment of children with septic arthritis. The study concluded that PCR provides supplemental information for diagnostic confirmation through an increased rate of detection of bacteria. [21]
Imaging Studies
Radiography
Although plain radiography may reveal an effusion as widening of the joint space with displacement of fat planes, it is insensitive in the diagnosis of septic arthritis (SA). Radiography may be most helpful in screening for etiologies other than SA as a cause of joint pain. For example, it may reveal bony changes suggestive of osteomyelitis, bony tumors or fractures as the source of swelling, and Legg-Perthes disease or a slipped capital femoral epiphysis, which are diagnostic considerations in a child with an irritable hip.
Ultrasonography
Ultrasonography is a simple and relatively inexpensive technique for detecting a hip effusion. This test has a greater sensitivity than plain radiography and is becoming the modality of choice to reveal hip effusions. Ultrasonography is also used to guide the aspiration needle if an effusion is detected.
In a study of 96 children suspected of having septic arthritis of the hip, 40 had normal ultrasonographic findings and no septic arthritis.
Ultrasonography has several advantages over computed tomography (CT) scanning in this setting, including eliminating radiation exposure and guiding the aspiration of deep joints, such as the hip.
Scintigraphy
Scintigraphy has a limited role in most cases of SA, but it may be helpful if multifocal disease is suspected in neonates. It also assists with the detection of an associated osteomyelitis.
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Emergency room photograph of an infant with septic arthritis of the left hip. The child holds his hip rigidly in the classic position of flexion, abduction, and external rotation, a position that maximizes capsular volume. The patient is relatively comfortable as long as the hip joint remains immobile in this position.
