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Pediatric Septic Arthritis Workup

  • Author: Richard J Scarfone, MD; Chief Editor: Russell W Steele, MD  more...
Updated: Jun 09, 2016

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.

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. 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 SA of the hip among children presenting with hip pain.[4]

Deanehan reported that septic arthritis is unlikely in children from Lyme disease–endemic areas who have knee monoarthritis. The study identified 673 patients with knee monoarthritis; 19 (3%) had septic arthritis, 341 (51%) had Lyme arthritis, and 313 (46%) had other inflammatory arthritis. The following predictors of knee septic arthritis were identified: peripheral blood absolute neutrophil count ≥10 × 103 cells per mm3 and an erythrocyte sedimentation rate ≥40 mm/hr.[10, 11]

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.[12]



Imaging Studies


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 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 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.

Contributor Information and Disclosures

Richard J Scarfone, MD Associate Professor, Department of Pediatrics, Perelman School of Medicine at the University of Pennsylvania; Attending Physician, Division of Emergency Medicine and Medical Director of Emergency Preparedness, The Children's Hospital of Philadelphia

Richard J Scarfone, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Pediatrics

Disclosure: Nothing to disclose.

Specialty Editor Board

Mary L Windle, PharmD Adjunct Associate Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Nothing to disclose.

Joseph Domachowske, MD Professor of Pediatrics, Microbiology and Immunology, Department of Pediatrics, Division of Infectious Diseases, State University of New York Upstate Medical University

Joseph Domachowske, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Pediatrics, American Society for Microbiology, Infectious Diseases Society of America, Pediatric Infectious Diseases Society, Phi Beta Kappa

Disclosure: Received research grant from: Pfizer;GlaxoSmithKline;AstraZeneca;Merck;American Academy of Pediatrics<br/>Received income in an amount equal to or greater than $250 from: Sanofi Pasteur;Astra Zeneca;Novartis<br/>Consulting fees for: Sanofi Pasteur; Novartis; Merck; Astra Zeneca.

Chief Editor

Russell W Steele, MD Clinical Professor, Tulane University School of Medicine; Staff Physician, Ochsner Clinic Foundation

Russell W Steele, MD is a member of the following medical societies: American Academy of Pediatrics, American Association of Immunologists, American Pediatric Society, American Society for Microbiology, Infectious Diseases Society of America, Louisiana State Medical Society, Pediatric Infectious Diseases Society, Society for Pediatric Research, Southern Medical Association

Disclosure: Nothing to disclose.

Additional Contributors

Itzhak Brook, MD, MSc Professor, Department of Pediatrics, Georgetown University School of Medicine

Itzhak Brook, MD, MSc is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians-American Society of Internal Medicine, American Medical Association, American Society for Microbiology, Association of Military Surgeons of the US, Infectious Diseases Society of America, International Immunocompromised Host Society, International Society for Infectious Diseases, Medical Society of the District of Columbia, New York Academy of Sciences, Pediatric Infectious Diseases Society, Society for Experimental Biology and Medicine, Society for Pediatric Research, Southern Medical Association, Society for Ear, Nose and Throat Advances in Children, American Federation for Clinical Research, Surgical Infection Society, Armed Forces Infectious Diseases Society

Disclosure: Nothing to disclose.

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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.
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