Review Question - QID 218594

Differentiating between transient synovitis and septic arthritis is a critical skill in the orthopaedic surgeon's armamentarium. A CRP < 20 mg/L has been found to be an effective laboratory value to help rule out the diagnosis of septic arthritis (Answer 1).

Workup of the limping child demands accurate and timely workup and, if indicated, treatment. While pathologies such as transient synovitis are often self-limiting, missing the diagnosis of septic arthritis can lead to substantial increases in patient morbidity and mortality. The Kocher criteria have been developed as a clinical tool through which providers can predict the likelihood of a pediatric patient having septic arthritis of the hip. These criteria have been validated, and the relative specificity and sensitivity of each value in ruling in/out septic arthritis have been studied. Furthermore, the modified Kocher criteria includes CRP in its framework, as this lab value is routinely ordered upon patient presentation to the emergency department. While a value greater than 20 mg/L constitutes a positive state in the application of the Kocher criteria, a value less than 20 mg/L most strongly suggests an alternative diagnosis.

In their seminal paper, Kocher et al. performed a retrospective review of all children who presented at a major tertiary-care children's hospital over a 17-year period. They identified four independent multivariate clinical predictors to aid in differentiating septic arthritis from transient synovitis: history of fever > 38.5° C, ESR > 40 mm/hr, serum WBC > 12,000 and a refusal to bear weight. They went on to analyze all 16 possible combinations of these four variables and found that the predictive probability was < 0.2% for zero predictors, 3.0% for one predictor, 40.0% for two predictors, 91.3% for three predictors and 99.6% for all four predictors.

Caird et al. expanded upon the work of Kocher et al. and added CRP to the original four variables. They identified 53 children who underwent a hip aspiration to rule in/out septic arthritis. They found that not only was elevated CRP an independent risk factor for the presence of septic arthritis, but elevated CRP values portended worse clinical outcomes. The authors concluded that a CRP > 20 mg/L (2.0 mg/dl) was both highly sensitive and specific in the diagnosis of septic arthritis and that a CRP value under this threshold was associated with alternative diagnoses (i.e. transient synovitis).

Livingston et al. investigated which pre-operative patient characteristics were associated with worse clinical outcomes following irrigation and debridement for septic arthritis. Specifically, the authors sought to determine which, if any, variables would be predictive of the need for more than one surgical irrigation and debridement. The authors studied 25 patients over a 21-year period at a single high-volume pediatric tertiary referral center. The authors found that preoperative elevated CRP, left shift, positive cultures and higher fever were predictors for the need for multiple surgical interventions.

Schlung et al. performed a retrospective review investigating the diagnostic accuracy of femoral aspiration in identifying the causative infectious agent. Their study included 83 patients, with 31 having undergone an aspiration at the time of hip I&D. The authors found that while aspiration did not improve microorganism detection, it did aid in the diagnosis of osteomyelitis, especially in patients with false negative MRI findings for osteomyelitis. As a result, the authors recommend routine aspiration at the time of septic hip I&D.

Murphy et al. performed a retrospective review of pediatric patients who underwent an arthrotomy for septic arthritis of the hip. Their study included 139 hips with an average patient age of 6.1 years. The authors found that 41% of patients in this cohort underwent more than one surgical procedure, and the risk factors for return to the operating room included elevated initial ESR and CRP, infection with MRSA and the presence of osteomyelitis.

Illustration A is a table outlining the variables present within the original and modified Kocher criteria, along with the aforementioned predictive values.

Incorrect Answers
Answers 2,4,5: these are all positive Kocher Criteria
Answer 3: elevated Leptin has been associated with an increased incidence of slipped cap femoral epiphysis (SCFE). There is no known correlation between Leptin and the presence/absence of septic arthritis or transient synovitis.