E2314. Change in Bone CT Attenuation and C-Reactive Protein Are Predictors of Bone Biopsy Culture Positivity for Vertebral Discitis/Osteomyelitis
  1. Harrison Lee; University of Pennsylvania
  2. Bryan Pukenas; University of Pennsylvania
  3. Ronnie Sebro; University of Pennsylvania
Osteomyelitis is often challenging to diagnose, with definitive diagnosis requiring bone biopsy with culture. However, the yield of bone culture positivity is low, ranging from 21-43% in prior literature. Past studies have aimed to identify predictors of bone culture positivity with the goal of improving biopsy yield, however these investigations have only been performed for non-vertebral osteomyelitis patients. Given pathophysiologic differences in vertebral discitis/osteomyelitis (VDO), the objective of this study was to find the best clinical and imaging predictors for bone culture positivity specific to VDO.

Materials and Methods:
A retrospective review was performed for 46 patients with clinically confirmed osteomyelitis (determined by placement on a 6-week course of antibiotics per IDSA guidelines) and who underwent CT-guided bone biopsy as part of their work-up between January 1, 2009 and June 1, 2019. Clinical and imaging markers were recorded for each patient, including white blood cell (WBC) count, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), blood culture growth, aspiration of any fluid during the biopsy procedure, and mean CT attenuation of the lesion. Given that CT attenuation can vary by patient age and by the bone involved, an effort was also made to standardize this metric via a “delta CT attenuation,” defined as the difference in CT attenuation between the lesion and a site of unaffected, normal bone (serving as a within-subject control). Bone culture growth was recorded as the outcome of interest. Unpaired two-sample t-tests with unequal variances were used to determine significant differences across clinical variables between patients with positive and negative bone cultures, and the predictive performance of each variable was assessed using receiver-operator characteristic (ROC) curve analysis.

For one patient, the biopsy specimen was not sent for culture. Of the remaining 45 cultures, 16 were positive (35.6%). Blood cultures were positive in only 3 patients, none of which overlapped with bone culture positivity. The most significant predictors of bone culture positivity were CRP x100% fold above normal reference range (p = 0.011, AUC 86.6%) and delta CT attenuation (p = <0.001, AUC 83.8%). Optimized predictive thresholds were calculated to be CRP 4-fold above normal (90.9% sensitivity, 73.7% specificity) and delta CT attenuation of >25.9 HU (93.8% sensitivity, 75.0% specificity). A multivariable logistic regression model combining the two predictors was: logit(Probability of bone culture positivity) = -3.10075 + 0.02588*(Delta CT attenuation) + 0.24101*(CRP x100% fold above normal reference range)

This study shows that for VDO, CRP x100% fold above normal reference range and delta CT attenuation can be used as strong and reliable predictors for bone biopsy culture positivity. These clinical factors are easily acquirable and can be used to inform whether a lesion suspicious for osteomyelitis may be worth biopsying, and if so, which site would be most promising to select for sampling.