O2091: Ticked Off: Challenges in Managing Culture-Negative Septic Arthritis in a Lyme-Endemic Area

Background: In a child with painful knee or hip effusion, elevated synovial fluid (SF) markers are characteristic of septic arthritis (SA). These patients are commonly treated with surgical debridement and antibiotics. However, a significant number of these cases will never grow a causative organism on culture. Management of culture-negative septic arthritis (CNSA) is further complicated by the possibility of Lyme arthritis (LA) in some regions, which may present with similar elevations in SF markers but negative cultures. The purpose of this study was to detail the incidence of CNSA at a tertiary care children’s hospital in a Lyme-endemic area and describe the treatments rendered.

Methods: Patients who underwent hip or knee aspiration for suspected SA in our Emergency Department from 2019-2022 were identified. Patients >18 years were excluded. Demographics, SF analysis, microbiology results (e.g., PCR, ELISA, Western Blot), and treatments were recorded. Patients were considered positive for SA if the SF WBC count was >50,000 cells/mm³, the SF PMN proportion was >90%, or SF yielded a positive gram stain. Those with positive aspiration or wound cultures were termed culture-positive SA (CPSA), while those with a positive SF marker but negative cultures were termed CNSA. Lyme arthritis (LA) was defined by positive SF Lyme PCR or ELISA with Western Blot confirmation. CNSA cases were grouped by microbiology results, and treatments were summarized.

Results: Of the 137 patients that underwent aspiration, 84 tested positive for SA by SF criteria (Figure). Of these, only 19/84 (22.6%) had a positive culture (CPSA). CNSA therefore constituted 77.4% of SA cases in this series. The CNSA group routinely underwent adjunctive microbiology testing, with 98% undergoing Lyme testing and 86% undergoing Kingella PCR. Among patients with CNSA, LA represented the most common final diagnosis (34/65,
52%). Despite exhaustive testing, 22 CNSA patients (34% of CNSA, 26.2% of all SA cases) never had a pathogen identified (Subgroup 3 in Figure). This subgroup, which was larger than the CPSA group, was treated inconsistently: only 12 underwent surgical debridement and 10 were treated with antibiotics alone.

Conclusion: In cases of suspected SA of the hip or knee in a child with elevated SF markers, negative cultures are more common than positive ones. Many patients with CNSA were later diagnosed with Lyme arthritis or Kingella. Treatment for CNSA is inconsistent and improvement in treatment guidelines is needed to better care for this population.