Association between imaging findings and microbiological findings for image‑guided biopsies for spine infections.

BACKGROUND: In the evaluation of spondylodiscitis, a number of factors are thought to contribute to the positive‑predictive‑value of spine biopsy including biopsy technique and equipment, number of sample obtained, timing of antibiotics, imaging findings and lab values. The purpose of this study was to examine which technical, magnetic resonance imaging (MRI), laboratory and clinical findings are most frequently associated with positive cultures or histopathology among patients receiving spine biopsies with a focus on the association of imaging findings and positive cultures/histopathology.

METHODS: Following International Review Board approval, we retrospectively reviewed a consecutive series of spine biopsies performed at our institution over a 28-month period in patients who received spine biopsies for radiographically and clinically suspicious spondylodiscitis. All patients underwent MRI prior to biopsy. Patient charts were reviewed for the following data: erythrocyte sedimentation rate (ESR), C‑reactive protein (CRP), and white blood cell level at the time of biopsy, prior back surgeries, timing of last dose of antibiotics prior to biopsy, and length of antibiotic therapy prior to biopsy. We also reviewed procedure notes from the biopsy and collected the following data: technique (transpedicular versus parapedicular), equipment used, type of imaging guidance (CT versus fluoroscopy) and number of samples obtained. MRI studies were evaluated by two radiologists and rated on an ordinal 1-4 scale based on increasing suspicion for spondylodiscitis (1 lowest, 4 highest). All categorical variables were compared using chi‑squared tests. All continuous variables were compared using Student's t‑tests.

RESULTS: Seventy‑seven patients are included in this study. Overall, 62% (48/77) were positive for infection on pathological and/or microbiological studies. There was a significant association between radiological index of suspicion and biopsy positivity as 0% (0/3) with index of 1, 31% (4/13) with an index of 2, 59% (10/17) with an index of 3 and 83% (30/36) with an index of 4 had positive pathology (P=0.001). Biopsy approach, type of imaging guidance, CBC, and number of passes were not associated with biopsy positivity. Elevated CRP was associated with biopsy positivity (P=0.002) while elevated ESR was not (P=0.12). On multivariate analysis adjusting for the MRI scale, ESR and CRP, increasing degree of suspicion on MRI was independently associated with biopsy positivity (P<0.01) while CRP and ESR were not independently associated with biopsy positivity.

CONCLUSIONS: In this study of 77 patients receiving spinal biopsy for diagnosis of spondylodiscitis, a high index of suspicion based on MRI imaging was strongly associated with positive biopsy samples on culture and/or histopathology. In addition, imaging findings were independently associated with positive biopsy findings while elevated CRP and ESR were not. These findings suggest that MRI findings should be used to guide practitioners in their pursuit of a causative organism among patients with suspected discitis‑osteomyelitis.