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Arthritis, Septic: Differential Diagnoses & Workup

Author: Richard J Scarfone, MD, Associate Professor, Department of Pediatrics, University of Pennsylvania School of Medicine; Attending Physician and Director of Emergency Preparedness, Division of Emergency Medicine, The Children's Hospital of Philadelphia
Contributor Information and Disclosures

Updated: Apr 29, 2009

Differential Diagnoses

Juvenile Rheumatoid Arthritis
Kawasaki Disease
Lyme Disease
Rheumatic Fever
Serum Sickness
Transient Synovitis

Other Problems to Be Considered

The differential diagnosis of a painful monoarthritis is rather extensive, and includes the following:

• In contrast to children with septic arthritis (SA), children with transient synovitis appear well and are usually afebrile with just a mild limp.⁶ The American College of Radiology has established guidelines for the assessment of a limping child.⁷
• In adolescents, a slipped capital femoral epiphysis may manifest as a painful hip, thigh, or knee. Most patients are afebrile and the onset of pain may be preceded by minor trauma.
• Legg-Calve-Perthes disease, which is most common in boys, afflicts children aged 4-8 years. In contrast to septic arthritis, the pain is subacute with a more indolent onset, and these children do not have fever. 
• One study demonstrated that children with septic arthritis were less likely to have knee involvement, a history of a tick bite, or a fever than children with Lyme disease.⁵ Additionally, median values of inflammatory markers were higher among those with septic arthritis; however, a large overlap was noted between the groups. 
• Aside from gonococcal arthritis or septic arthritis in the neonate, polyarthritis is not typically caused by bacteria within the joints. The differential for polyarthritis in children is broad and includes Lyme disease, acute rheumatic fever, serum sickness, Kawasaki disease, systemic lupus erythematosus, and Henoch-Schönlein purpura.

Workup

Laboratory Studies

Diagnosis of septic arthritis (SA) is established by a combination of clinical findings and results of synovial fluid analysis.

• When septic arthritis is suspected, synovial fluid should be obtained for CBC count, 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.
• The synovial fluid glucose concentration averages 30% of that in the serum, a finding unique to septic arthritis.
• 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/μ L.
• Although often elevated, a peripheral WBC count within the reference range does not rule out septic arthritis.
• The C-reactive protein (CRP) is a more sensitive marker for septic arthritis than is the peripheral WBC count. In one study, a CRP of more than 2 mg/dL was found to be a strong independent risk factor for septic arthritis of the hip among children presenting with hip pain.²

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.
  • Radiography may be most helpful in screening for etiologies other than septic arthritis 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, sensitive, 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 to have septic arthritis of the hip, 40 had normal ultrasonography findings and no septic arthritis.
  • Ultrasonography has several advantages over CT in this setting, including eliminating radiation exposure and guiding the aspiration of deep joints such as the hip.
• Scintigraphy: This has a limited role in most cases but may be helpful if multifocal disease is suspected in neonates. It also assists with the detection of an associated osteomyelitis.

Procedures

• Clinicians should have a low threshold for performing an 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 the viscous synovial fluid is necessary.

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