Laboratory Studies
In most children presenting with limp, extensive laboratory testing is not needed for the diagnosis but is helpful when infectious, oncologic, and rheumatologic causes are considered. Inflammatory markers such as the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are elevated in most acute infectious processes. A complete blood count (CBC) should also be obtained in cases of suspected infection to evaluate the white blood cell count (WBC). Due to the high prevalence of joint infections and postinfectious processes caused by group A streptococcus (GAS), an antistreptolysin titer (ASO), throat culture, and rapid streptococcal antigen swab should be considered. If infection is suspected and a joint effusion is present, cell counts, gram stain, and culture from the synovial fluid should be sent to assist with diagnosis and antibiotic management. If a child appears septic, a blood culture should be obtained as well. With suspected osteomyelitis, cultures of the bone should be taken in the operating room in addition to a peripheral blood culture. Ideally, antibiotics should be withheld until cultures are sent unless the child appears acutely ill.
Diagnostic Imaging
Toxic Synovitis and Septic Arthritis
Clinicians may have difficulty differentiating between toxic synovitis (also known as transient synovitis) and septic arthritis of the hip in pediatric patients. In both toxic synovitis and septic arthritis, the child is unable to bear weight on the affected side, and ultrasound may demonstrate effusion.3,4
Toxic synovitis is thought to be a viral or postviral phenomenon, though the exact agent responsible for initiating the inflammatory cascade is not known. It has a relatively benign course and generally responds over 3 to 10 days to rest and nonsteroidal anti-inflammatory drugs (NSAIDs).5,6
Septic arthritis, however, is a serious infection with the capacity to cause permanent joint damage as well as spread into the blood or bone. Staphylococcal and streptococcal bacteria, including GAS and methicillin-resistant staphylococcus aureus (MRSA) are the most common causes of septic arthritis in the pediatric population.7 However, Kingella kingae, a gram-negative organism, is an increasingly recognized cause of septic arthritis in children younger than 3 years of age. Commonly found in the posterior pharynx, Kingella is very difficult to culture but may be detected by polymerase chain reaction (PCR) assays.8 The spectrum of GAS-related joint infections ranges from a postinfectious type that may be indistinguishable from toxic synovitis (but with evidence of recent GAS pharyngitis) to typical bacterial septic arthritis.9
Multiple clinical prediction rules and algorithms have been developed to assist with the management of these cases. The Kocher criteria, which include history of fever >101.3˚F, nonweight-bearing status, ESR >40 mm/hour, and WBC >12,000 cells/mm3 were found to be independent clinical predictors for the differentiation between transient arthritis and septic arthritis. Children with none of the risk factors had a 0.2% chance of septic arthritis while those with two risk factors had a 40% chance of septic arthritis. Subsequent validation studies among pediatric populations with a lower prevalence of septic arthritis found the positive predictive values to be lower; nevertheless, the Kocher criteria remain useful in guiding management.3,4
If a diagnosis of septic arthritis remains a consideration after the history, physical examination, and plain radiography, laboratory studies should be obtained, including a CBC, ESR, CRP, blood culture, rapid streptococcal assay, throat culture, and ASO titer. An ultrasound should also be performed to assess for the presence of a hip effusion.
Elevated inflammatory markers (ESR >40 mm/hour, CRP >20 mg/L, WBC >12,000 cells/mm3) and an effusion should prompt orthopedic consultation and arthrocentesis for synovial fluid-cell counts, gram stain, and culture. Synovial fluid WBC counts >50,000 cells/mm3 or a positive gram stain or culture is diagnostic of septic arthritis and requires treatment with intravenous (IV) antibiotics and likely operative intervention. The absence of an effusion on ultrasound, synovial fluid WBC counts <50,000 cells/mm3, and inflammatory markers that are not significantly elevated indicate an alternative diagnosis such as toxic synovitis. However, there may be cases in which the diagnosis is not clear due to ambiguous laboratory values from the blood or synovial fluid; in these cases the decision to proceed with IV antibiotics must be made in consultation with an orthopedist.10