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Septic Arthritis, Pediatrics: Workup

Author: Edwards P Schwentker, MD, Professor, Departments of Orthopedics and Rehabilitation and Pediatrics, Pennsylvania State College of Medicine
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Updated: Jan 21, 2009

Workup

Laboratory Studies

• White blood cell (WBC) count
  • The WBC count is usually elevated, but it may be within the normal range early in the clinical course. Infants may also have a normal WBC count.
  • A normal WBC count does not rule out septic arthritis.
• Erythrocyte sedimentation rate (ESR)
  • The ESR is elevated in septic arthritis; it returns to normal levels with resolution of the infection.
  • The initial elevation and the return to normal lag behind the clinical status.
• C-reactive protein (CRP)
  • The CRP level is elevated in septic arthritis; it returns to normal with resolution of the infection.
  • CRP is a more valuable diagnostic tool and a better indicator of response to treatment than ESR because CRP is generally more sensitive and more responsive. Blood cultures are frequently positive for the causative organism in septic arthritis and should be obtained. The joint aspirate may not yield a viable culture.
  • Other cultures and Gram stains may be useful.
  • If gonococcal arthritis is suspected, cultures and Gram stain material should be obtained from the cervix in postpubertal girls, from the vagina of prepubertal girls, and from any urethral discharge in a male.
  • If sexual abuse is suspected—and it should be suspected in any prepubertal child with gonococcal arthritis—obtain additional cultures with samples from the pharynx and rectum.
  • If child abuse is suspected, clinicians have a legal obligation to report the suspicions and to preserve all records and laboratory results as possible future legal evidence.
  • Gram stains of joint aspirates should always be performed because a positive Gram stain is valuable information. The Gram stain is positive in a minority of the cases of septic arthritis; however, a negative result should never be interpreted as evidence that infection is not present.
  • Fluid from joint aspirates from children younger than 3 years should be inoculated directly into blood culture bottles to enhance the isolation of K kingae, a fastidious pathogen increasingly common in this age group.

• Lyme titers should be obtained in regions where this disease is endemic. If a rapid test that can provide results within hours is not available, it may be necessary to initially assume that the diagnosis is septic arthritis and treat accordingly.

Imaging Studies

• Plain radiography
  • Obtain radiographs to rule out other conditions in the differential diagnosis, such as trauma or Legg-Calvé-Perthes disease. Radiographs are not helpful in diagnosing acute septic arthritis.
  • Even if the infection developed from an adjacent metaphyseal osteomyelitis, no initial bony findings are likely to be apparent until 7-10 days after onset. Signs of soft-tissue swelling and edematous infiltration into fatty tissue planes may be observed.
• Radionuclide scanning
  • Radionuclide scanning is generally not helpful, and it is contraindicated if it delays more appropriate diagnostic or treatment measures.
  • Scanning may be helpful in locating or ruling out other sites of involvement, particularly in very sick children and in neonates.
• Ultrasonography
  • Ultrasonography is useful in confirming a joint effusion in a deeply placed joint such as the hip.
  • This modality can also be used to guide joint aspiration.
• Magnetic resonance imaging (MRI)
  • MRI has no role in the initial workup.
  • The use of this expensive and time-consuming modality should be reserved for situations in which simpler measures, such as joint aspiration, fail to provide a diagnosis.
  • In such cases, the diagnosis is probably something other than septic arthritis.
• Computed tomography (CT)
  • A CT scan is indicated in patients with a suspected psoas abscess; in such patients, the clinical signs are suggestive of septic arthritis of the hip but the joint aspiration culture is negative.

Diagnostic Procedures

• Joint aspiration
  • Joint aspiration is the single most important diagnostic procedure.
  • Aspirate the joint with a large-bore needle before administering antibiotics.
  • Peripheral joints are readily tapped in a clinic or office setting.
  • Aspirate deep joints, such as the hip, under image-intensifier control or ultrasound guidance and with the patient appropriately sedated or anesthetized.
  • Of vital importance is ensuring that the joint was penetrated before the aspiration results are declared negative. If no fluid is obtained, perform an arthrogram by injecting contrast material through the needle.
  • A positive joint aspirate typically yields opaque yellow or white-gray pus.
  • The WBC count is usually in excess of 50,000 per milliliter, with more than 80% neutrophils.
  • A positive Gram stain is diagnostic for infection, but false-negative results are common. The failure to visualize organisms on the Gram stain does not rule out infection.
  • If gonococcal arthritis is suspected, obtain the culture in a manner approved by the clinical laboratory for this organism.
  • N gonorrhoeae is a difficult organism to grow, and special handling and special culture media are required to maximize recovery.

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