Musculoskeletal Imaging Original Research

Musculoskeletal Imaging Original Research Musculoskeletal Imaging Original Research Connie Y. Chang 1 F. Joseph Simeone 1 Sandra B. Nelson 2 Atul K. Taneja 3 Ambrose J. Huang 1 Chang CY, Simeone FJ, Nelson SB, Taneja AK, Huang AJ Keywords: CT-guided biopsy, culture, diskitisosteomyelitis, spine DOI:10.2214/AJR.14.13545 Received July 29, 2014; accepted after revision December 16, 2014. 1 Department of Radiology, Division of Musculoskeletal Imaging and Intervention, Massachusetts General Hospital, 55 Fruit St, Yawkey 6E, Boston, MA 02114. Address correspondence to C. Y. Chang (cychang@mgh.harvard.edu). 2 Department of Medicine, Infectious Disease Unit, Massachusetts General Hospital, Boston, MA. 3 Imaging Department, Musculoskeletal Radiology Division, Diagnostic Center, Hospital Israelita Albert Einstein, Hospital do Coração and Teleimagem, São Paulo, Brazil. AJR 2015; 205:123 129 0361 803X/15/2051 123 American Roentgen Ray Society Is Biopsying the Paravertebral Soft Tissue as Effective as Biopsying the Disk or Vertebral Endplate? 10-Year Retrospective Review of CT-Guided Biopsy of Diskitis-Osteomyelitis OBJECTIVE. The purpose of this study was to determine whether there is a difference in biopsying bone (endplate), disk, or paravertebral soft tissue to culture the pathogenic organism causing diskitis-osteomyelitis. MATERIALS AND METHODS. A retrospective review was conducted of 111 spinal biopsies performed between 2002 and 2011. Pathologic examination was used as the reference standard for detecting diskitis-osteomyelitis. Microbiologic yield, sensitivity, and specificity were calculated. The yields for different groups were compared by use of Fisher exact test. The analysis was repeated with biopsy samples from patients not being treated with antibiotics at the time of biopsy. RESULTS. A total of 122 biopsy specimens were obtained from 111 spinal biopsy procedures on 102 patients. Overall, 27 (22%) biopsies were performed on the endplate-disk, 61 (50%) on the disk only, and 34 (28%) on paravertebral soft tissue only. The microbiologic yield was 36% for all biopsies, 19% for endplate-disk biopsies, 39% for disk-only biopsies, and 44% for soft-tissue biopsies. The sensitivity and specificity of the microbiologic results for all specimens were 57% and 89%; endplate-disk, 38% and 86%; disk only, 57% and 89%; and paravertebral soft tissue, 68% and 92%. There was no statistically significant difference between the yields of the endplate-disk, disk-only, and paravertebral soft-tissue biopsies. CONCLUSION. Paravertebral soft-tissue changes, when present, may be considered a viable target for biopsy in cases of diskitis-osteomyelitis, even in the absence of a paravertebral abscess. T he diagnosis of diskitis-osteomyelitis can usually be made by use of a combination of history, clinical examination, vital signs, complete blood cell count, erythrocyte sedimentation rate, C-reactive protein level, and imaging findings [1 4]. Isolating an organism is important for targeting antibiotic therapy but remains a challenge [5 7]. In approximately 58% of cases (range, 30 78%), blood culture results are positive, leaving approximately 42% of cases without a target organism [5, 8, 9]. Percutaneous image-guided needle biopsy is performed to help isolate an organism when an open surgical procedure is not otherwise indicated [10]. The yield of biopsies is similar to that of blood culture, that is, 36 65% [11, 12], but as high as 80% after a second percutaneous biopsy. Percutaneous biopsy of the spine is preferred to open biopsy because of the shorter procedure and hospitalization times, lower complication rate and patient morbidity, and lower cost [13, 14]. However, needle biopsy, especially of the endplate and disk, presents additional challenges. If a posterolateral or paravertebral approach not perpendicular to the convex surface of the disk is used, it is possible for the needle to slip off the surface of the spine and injure adjacent important structures, such as the lung and major vessels [15 17]. A transpedicular approach modified with cranial angulation of the needle tip so that it enters the disk carries less risk of slipping off the bone, but as in the transpedicular approach to the vertebral body, the spinal cord and nerve roots may be injured if the needle traverses the medial wall of the pedicle [17]. In the lumbar spine, the disk can usually be entered by traversal of the inferior aspect of the neural foramen, but caution must be exercised not to injure the exiting nerve root. Soft-tissue changes, such as edema, stranding, and enhancement, are often seen around the spine, but it is unclear whether they represent reactive changes or true in- AJR:205, July 2015 123

fection from which an organism can be cultured. The paravertebral soft tissues may be a more accessible biopsy target, especially in the lumbar spine. In addition, soft-tissue biopsy does not carry the risk of needle slippage off a surface. Because less force is applied, soft-tissue biopsy can be performed with greater control, theoretically reducing the risk of injury to the adjacent soft-tissue structures. In our experience, soft-tissue biopsy also does not typically require more than local anesthesia for pain control, even in patients with infection, and therefore the procedures are shorter overall. The purpose of this study was to compare the microbiologic yield, sensitivity, and specificity of biopsies of a endplate with vertebral disk, disk only, and paravertebral soft tissue for diskitis-osteomyelitis with pathologic examination as the reference standard. Materials and Methods Institutional review board approval was obtained, and this study was HIPAA compliant. Because of the retrospective study design, subject informed consent was not required. From the searchable database of all radiologic examinations performed at our institution, we identified 155 biopsies of 141 patients referred to our division between January 1, 2002, and December 31, 2011, for CT-guided biopsy for the purpose of diagnosing diskitis-osteomyelitis. Thirty-four biopsies of 29 patients were excluded because they had no pathologic results (examination performed either at biopsy or at surgery soon after the biopsy) or because what was called a biopsy was a direct sample of a paravertebral abscess. Another 10 biopsies of 10 patients were excluded because the pathologic specimen was nondiagnostic. Twenty-two patients had nondiagnostic needle biopsy pathologic results but underwent surgical biopsy within 10 days. These patients were included, and the surgical pathologic examination was used as the reference standard. After the exclusions, 111 CT-guided biopsies remained that were performed on 102 patients. In all 111 cases, at least one of the obtained specimens was submitted for surgical pathologic analysis. In 100 cases, an additional specimen was obtained for microbiologic examination. In the other 11 cases, an additional two specimens were obtained for microbiologic examination and were evaluated as separate specimens. Thus, an overall 122 biopsy specimens were submitted for microbiologic analysis. Pathologic and microbiologic results were reviewed and recorded. Pathologic examination was used as the reference standard for diagnosing diskitis-osteomyelitis [12]. CT-guided biopsy was performed by 1 of 12 staff musculoskeletal radiologists with 1 33 years of experience performing CT-guided biopsy. All biopsies were performed with coaxial technique. Endplate bone biopsies were performed with Ackerman biopsy needles (14 gauge) (Cook Medical) or with a combination of a bone penetration system (14 gauge) (Bonopty, Apriomed) to access the endplate and a biopsy needle (16 gauge) (Ostycut, Bard Biopsy Systems) to perform the biopsy. Disk and paravertebral soft-tissue biopsies were performed with a biopsy needle (14, 16, or 18 gauge) (Temno, CareFusion). All specimens were placed in nonbacteriostatic saline solution in a sterile vial and transported to the microbiology and pathology laboratories within 4 hours. All pathologic and cytologic results were reviewed by expert board-certified pathologists and cytologists. Corresponding needle pathologic results were available for 122 specimens, 22 of which also had surgical pathologic results. All microbiologic samples were sent for routine aerobic and anaerobic cultures with a 7-day incubation period in enriched media. Two musculoskeletal radiologists with 2 and 3 years of experience performing CT-guided spinal biopsy reviewed the images from each case at a PACS workstation (Impax, Agfa Healthcare) and determined in consensus whether the biopsy needles for the 111 CT-guided biopsies were placed into the endplate-disk unit (Fig. 1), the disk (Fig. 2), or the paravertebral soft tissues (Fig. 3). The needle positions on the images were also corroborated with the pathologic results. For example, the biopsy was considered to include the endplate only if the biopsy needle was seen traversing the endplate on the CT images and the pathology report also noted the presence of bone. Yield (the number of true-positive findings divided by total number of biopsy specimens and multiplied by 100 to yield a percentage), sensitivity, and specificity were calculated for all patients. The patients antibiotic regimens were obtained from A B Fig. 1 42-year-old man with end-stage liver and renal disease and recently increasing back pain. A and B, Sagittal T1-weighted (A) and STIR (B) MR images of lumbar spine show endplate erosions and marrow edema at L2-3 (arrow). C, Axial unenhanced CT image with patient prone during percutaneous biopsy 4 days after A and B shows bone biopsy needle (arrow) sampling L2 inferior endplate. Pathology report noted presence of fragment of viable trabecular bone and acute and chronic osteomyelitis. Cultures did not grow any organism. C 124 AJR:205, July 2015

the medical record. Yield, sensitivity, and specificity were recalculated for the subgroup of patients not treated with antibiotics at the time of biopsy. Yields from the three biopsy locations and from patients treated with as opposed to without antibiotics were compared by use of Fisher exact test. The dictated radiology reports were reviewed to determine the gauge of the needle and the number of specimens obtained, and the yield for each of these groups was calculated. The Fisher exact test was used to compare the groups. The distribution of needle gauges for the entire population and for the subset of false-negative findings only was compared by use of the chi-square test. JMP software (version 11 Pro, SAS Institute) was used for all statistical analysis. Values of p < 0.05 were considered statistically significant. Results The study group included 102 patients (67 [66%] male patients, 35 [34%] female patients; mean age, 59 ± 17 [SD] years; range, 15 90 years). Of the 122 biopsy specimens from 111 spinal biopsy procedures, 27 (22%) were of the endplate and disk, 61 (50%) of the disk only, and 34 (28%) of the paravertebral soft tissues. The distribution of biopsies in the different parts of the spine was as follows: nine (7%) in the cervical spine, 75 (61%) 31 in the thoracic spine, and 38 (31%) in the lumbar spine. For the purposes of tallying the biopsies, T12-L1 was considered thoracic, and L5-S1 was considered lumbar. Seventy-seven (63%) biopsies had pathologic results positive for diskitis-osteomyelitis. The types of bacteria recovered from the biopsy specimens are recorded in Table 1. There were no known complications for any of the biopsies. The numbers of true-positive, true-negative, false-positive, and false-negative findings for all patients and only for patients not being treated any antibiotics at the time of biopsy are recorded in Table 2. The calculated yields, sensitivities, and specificities are recorded in Table 3. Table 4 shows p values for yields based on biopsy location and antibiotic status. There was no information for 11 patients regarding whether they were being treated with antibiotics at the time of the biopsy. Seventy-four biopsies were performed on patients whose medical record confirmed that the patient was not being treated with antibiotics for this suspected infection or any other infection for at least 1 week before the biopsy. The overall yield from the percutaneous biopsy specimens was 36% (44/122) for all specimens and 41% (30/74) for patients not undergoing antibiotic therapy at the time of TABLE 1: Bacterial and Fungal Organisms Recovered at Biopsy Organism False Positive True Positive Bacteroides species 0 2 Bifidobacterium species 0 1 Candida species 0 2 Coagulase-negative staphylococci 2 7 Corynebacterium species 0 1 Escherichia coli 0 2 Enterococcus species 0 2 Klebsiella species 0 3 Methicillin-resistant Staphylococcus aureus 0 4 Methicillin-sensitive Staphylococcus aureus 0 8 Mycobacterium tuberculosis 0 3 Propionibacterium acnes 3 2 Pseudomonas species 0 1 Streptococcus species 0 6 Total 5 44 TABLE 2: Biopsy Results Patients All Biopsies Endplate and Disk Disk Only Paravertebral Soft Tissue All patients True positive 44 5 24 15 True negative 40 12 17 11 False positive 5 2 2 1 False negative 33 8 18 7 Total 122 27 61 34 Only patients not treated with antibiotics True positive 30 4 17 9 True negative 23 10 9 4 False positive 4 2 1 1 False negative 17 4 9 4 Total 74 20 36 18 TABLE 3: Sensitivity, Specificity, and Yield Calculations Patients Yield Sensitivity Specificity All patients All cases 36 57 89 Endplate and disk 19 38 86 Disk only 39 57 89 Paravertebral soft tissue 44 68 92 Only patients not treated with antibiotics All cases 41 64 85 Endplate and disk 20 50 83 Disk only 47 65 90 Paravertebral soft tissue 50 69 80 Note Values are percentages. AJR:205, July 2015 125

TABLE 4: Results of Fisher Exact Test Comparing Yields From Endplate and Disk, Disk, and Paravertebral Soft-Tissue Biopsies Group No antibiotic versus any antibiotic 0.34 All patients Paravertebral soft tissue versus endplate-disk biopsies 0.05 Disk versus endplate-disk biopsies 0.08 Paravertebral soft-tissue versus disk biopsies 0.67 All patients not treated with antibiotics Paravertebral soft tissue versus endplate-disk biopsies 0.09 Disk versus endplate-disk biopsies 0.05 Paravertebral soft-tissue versus disk biopsies 1 Note The analysis was repeated excluding patients who were not being treated with any antibiotic at the time of the biopsy. Bold type indicates p values approaching statistical significance. TABLE 5: Needle Gauge and Yield for Each Type of Biopsy Needle Location 14-Gauge 16-Gauge 18-Gauge 20-Gauge Endplate-disk Total no. of samples 7 (28) 17 (68) 1 (4) 0 (0) True positive (no.) 2 3 0 0 True negative (no.) 3 8 1 0 False positive (no.) 1 1 0 0 False negative (no.) 1 5 0 0 Yield (%) 29 18 0 NA Disk only Total no. of samples 23 (40) 11 (19) 21 (37) 2 (4) True positive (no.) 10 5 8 0 True negative (no.) 7 3 4 2 False positive (no.) 1 0 1 0 False negative (no.) 5 3 8 0 Yield (%) 43 45 38 0 Soft tissue Total no. of samples 9 (33) 4 (15) 12 (44) 2 (7) True positive (no.) 6 2 3 0 True negative (no.) 2 1 5 1 False positive (no.) 0 0 1 0 False negative (no.) 1 1 3 1 Yield (%) 67 50 25 0 All Total no. of samples 39 (36) 32 (29) 34 (31) 4 (4) Sensitivity (%) 72 53 50 0 Specificity (%) 86 92 83 0 Yield (%) 46 31 32 0 Note NA = not applicable. Values in parentheses are percentages. biopsy. There was no statistically significant difference in biopsy yield between patients treated with antibiotics and patients not treated with antibiotics at the time of biopsy (p = 0.34). The yield of the paravertebral soft-tissue biopsies was higher than the yield of disk biopsies and endplate-disk biopsies, but these results were not statistically significant. The yield for paravertebral soft-tissue versus endplate-disk biopsy and disk versus endplate-disk biopsy approached but did not reach statistical significance (p = 0.05 0.09). p TABLE 6: Number of Specimens Obtained for Microbiologic Analysis and Yield for Each Type of Biopsy Needle Location 1 Specimen a 2 or More Specimens a Endplate, disk Total no. of samples 7 (78) 2 (22) Total no. of samples 3 1 True positive (no.) 3 0 True negative (no.) 0 0 False positive (no.) 1 1 False negative (no.) 43 50 Disk Total no. of samples 13 (52) 12 (48) Total no. of samples 5 3 True positive (no.) 3 3 True negative (no.) 1 1 False positive (no.) 4 5 False negative (no.) 39 25 Soft tissue Total no. of samples 4 (44) 5 (56) True positive (no.) 3 0 True negative (no.) 1 4 False positive (no.) 0 0 False negative (no.) 0 1 Yield 75 0 All Sensitivity (%) 67 36 Specificity (%) 88 88 Yield (%) 46 21 Note Table includes 79 samples (including those who were receiving antibiotics at the time of the biopsy) for which the number of specimens obtained was recorded in the radiology report. Values in parentheses are percentages. a Number of samples obtained and sent for microbiologic examination. This number does not include the specimen that was sent for pathologic examination. The gauges of the biopsy needle and the calculated yields are recorded in Table 5. The biopsy needle gauge was not recorded for two of the endplate-disk biopsies, three of the disk-only biopsies, and seven of the soft-tissue biopsies. There was no statistically significant difference in yields based on the gauge of the biopsy needle. The p values were as follows: 14-gauge versus 16-gauge, p = 0.23; 14-gauge versus 18-gauge, p = 0.24; 14-gauge versus 20-gauge, p = 0.13; 16-gauge versus 18-gauge, p = 1; 16-gauge versus 20-gauge, p = 0.56; 18-gauge versus 20-gauge, p = 0.30. There were 17 cases of false-negative results, and the biopsy needle gauge was recorded in 16 of them. Of these 126 AJR:205, July 2015

A B Fig. 2 75-year-old man new 10 on a scale of 10 back pain and history of cholecystectomy for cholangitis 2 months earlier. A, Sagittal reformatted unenhanced CT image of lumbar spine shows loss of T12-L1 intervertebral disk height and erosions of adjacent endplates (arrow) concerning for diskitis-osteomyelitis. B, Axial unenhanced CT image with patient prone during percutaneous biopsy 6 days after A shows spring-loaded soft-tissue biopsy needle (arrow) with tray open in T12-L1 intervertebral disk. Pathology report noted fragments of fibrocollagenous tissue and acute and chronic inflammation. Cultures from biopsy grew Klebsiella organisms. needles, 31% (5/16) were 14 gauge, 19% (3/16) were 16 gauge, 44% (7/16) were 18 gauge, and 0% (0/16) were 20 gauge. The distribution of needle gauges in the false-negative subgroup was not substantially different from the overall needle gauge distribution (p = 0.59). The number of specimens obtained at biopsy and used specifically for microbiologic analysis and the calculated yields are recorded in Table 6. This sample number does not include the core specimen that was sent for pathologic examination. A large number of samples were not included in the analysis (79/122 [65%]) because this information was not recorded in the report. There was no statistically significant difference in yield based on the number of specimens obtained and submitted for microbiologic examination (p = 0.11). Discussion Percutaneous needle-guided biopsy for diskitis-osteomyelitis is commonly attempt- A B C Fig. 3 72-year-old woman with severe acute upper back pain radiating to shoulders and around chest and 2-month history of fevers, increasing back pain, and 10 pound (4.5 kg) weight loss. A C, Sagittal T1-weighted (A), T2-weighted fat-suppressed (B), and T1-weighted fat-suppressed (C) gadolinium-enhanced MR images of thoracic spine show loss of height and increased T2 signal intensity and enhancement of T5-6 disk (arrow), cortical erosions with low T1 signal intensity, high T2 signal intensity, enhancement of adjacent endplates and essentially entirety of each vertebral body, and high T2 signal intensity and enhancing anterior and posterior paravertebral soft tissue centered at this level concerning for diskitis-osteomyelitis. Anterior epidural abscess (asterisk) spans T5 T10. (Fig. 3 continues on next page) AJR:205, July 2015 127

D E Fig. 3 (continued) 72-year-old woman with severe acute upper back pain radiating to shoulders and around chest and 2-month history of fevers, increasing back pain, and 10 pound (4.5 kg) weight loss. D, Axial T1-weighted fat-suppressed gadolinium-enhanced MR image of thoracic spine shows marked enhancement of T5 vertebral body and overlying anterior paravertebral soft tissues (asterisk). E, Axial unenhanced CT image with patient prone during percutaneous biopsy 1 day after A D shows springloaded soft-tissue biopsy needle (arrow) with tray open in paravertebral soft tissues adjacent to T5 vertebral body. Pathology report noted presence of acute and chronic osteomyelitis but did not mention disk, bone, or bone fragments. Cultures from biopsy grew α-hemolytic Streptococcus organisms. ed to help direct antibiotic therapy. However, the microbiologic yield is low. In a 10- year retrospective survey, the yield was 36%, similar to the yield of 36 65% reported by other authors [11, 12]. In addition, the yields from the endplate-disk (18 20%), disk-only (39 47%), and paravertebral soft-tissue (44 50%) biopsies were not statistically different. The sensitivities of the endplate-disk, disk-only, and paravertebral soft-tissue biopsies were also comparable. These results support the concept that in the setting of diskitis-osteomyelitis, the paravertebral softtissue changes are not merely reactive but are due to actual infection by bacteria and can be confidently biopsied. The lower yield of the endplate-disk biopsies than of the paravertebral soft-tissue biopsies may be due at least in part to the greater technical difficulties that endplate biopsy presents. The sampled bone may be close to the endplate but may not be in direct contact with the disk. In addition, the paravertebral soft tissues may have a more abundant vascular supply, which may be more conducive to bacterial growth. When diskitis-osteomyelitis is suspected, IV antibiotics may be delayed if the patient s condition is stable so that the microbiologic yield of percutaneous biopsy will not be affected. Conventional wisdom states that patients being treated with antibiotics have frequent false-negative culture results and advises against starting antibiotic therapy before spinal biopsy, if possible [5, 18]. However, the literature is inconsistent on this point. In a series of 20 patients, Rankine et al. [19] found a biopsy yield of 25% for patients being treated with antibiotics at the time of biopsy versus a biopsy yield of 50% for patients not being treated with antibiotics at the time of biopsy. In contrast, in a series of 92 patients, 14 of whom were undergoing antibiotic therapy at the time of biopsy, Sehn et al. [20] did not find a substantial difference in yield between the antibiotic subgroup and with the entire cohort (29% versus 30%, p = 0.86). The results of Marschall et al. [11] also suggest that antibiotics do not have an effect on microbiologic yield. Our biopsy yield did not change substantially when the patients treated with antibiotics were included, but this finding may have occurred because the number of patients in the antibiotic group was small compared with the total patient group, similar to the groups in the study by Sehn et al. [20]. No definitive conclusions about antibiotic therapy and percutaneous biopsy yield can be drawn from our results, because our study might have lacked sufficient power. However, including the patients treated with antibiotics did not have any apparent effect on our results. We more closely examined the falsepositive and false-negative cases of the patients not treated with antibiotics. Of the four false-positive cases, two were Propionibacter acnes, which grew only in thioglycolate broth. A single positive culture tube for Propionibacter acnes is a conundrum, but both of these patients had low clinical suspicion of infection and symptoms that resolved without antibiotic treatment. In two cases, only one colony of coagulase-negative staphylococci grew. In both of these cases, subsequent imaging and clinical follow-up did not show changes consistent with infection, even though the patients were not treated with antibiotics. In all of these cases, the assumption was that the positive culture result was due to a contaminant, either in the process of the biopsy or in handling the specimen, because both of these organisms colonize the skin. For the 17 false-negative cases, it was unclear why these biopsies did not also result in positive microbiologic results given that all of these biopsies had histologic findings of infection. One possibility may be the type or gauge of needle used for biopsy, although the distribution of needle gauges was not significantly different from the overall distribution (p = 0.59). Another possibility may be related to the quality of the specimen sent for microbiologic rather than pathologic examination, such as first versus second specimen or proximity to the focus of infection or imaging abnormality. This information was not recorded in the dictated reports but would be important to standardize in a future prospective study. In every category there was a trend toward lower yield with a smaller-gauge biopsy specimen, but this finding did not reach statistical significance. To our knowledge, there is no substantial infectious disease literature on the quantity of biopsy tissue and the resulting microbiologic yield, and this question should be addressed in subsequent studies. Of note, the yield for 20-gauge biopsies was 0%, but there were only four samples in this category. The yield of biopsies with only one specimen was higher than the yield of biopsies with two or more specimens, but this result did not reach statistical significance. It is difficult to draw any conclusions about this information, because the quality of the sample might have also contributed to the operator s decision about the number of samples to obtain. In 65% of cases, the number of biopsy specimens obtained was not recorded, which resulted in a small sample number. The question whether additional samples would increase the microbiologic yield could be addressed in subsequent studies. There were limitations to our study. Because the study was retrospective, there was a possibility of selection bias. In particular, patients selected for soft-tissue biopsy might have had more aggressive infections, which tends to cause more paravertebral soft-tissue infection and inflammation. These patients might have had a greater pathogen burden and a greater likelihood of having a positive microbiologic yield at per- 128 AJR:205, July 2015

cutaneous needle biopsy. Conversely, patients who were not selected for paraspinal soft-tissue biopsy might have had less aggressive or less advanced infection, possibly also a smaller pathogen burden, and therefore a lower likelihood of positive microbiologic yield at percutaneous needle biopsy. Imaging findings were not part of the inclusion criteria for patients in the study. In addition, images were not rereviewed to correlate the severity of inflammatory changes at biopsy and the type of biopsy specimen obtained. Our study design allowed us to add only paravertebral soft-tissue changes as a target for biopsy but did not allow us to compare the yields of the different biopsy locations. Any future prospective studies should evaluate the severity of the infection and paravertebral soft-tissue changes at the time of the biopsy and evaluate soft-tissue, disk, and endplate-disk biopsies. Although we used a relatively long time period, many biopsies had to be eliminated because of either a lack of pathologic results or a lack of clinical and radiologic follow-up. As a result, the sample size was reduced to 122 biopsy specimens, which is not a small number but may have insufficient power for conclusions about antibiotic therapy. This study would be best performed as a prospective study with clear records of the patients preprocedural antibiotic therapy and clinical status at biopsy; with a group in which each patient undergoes endplate, disk, and paravertebral soft-tissue biopsy; and with the specimens sent separately for pathologic and microbiologic analysis. A standardized biopsy protocol in a prospective study would also eliminate some of the uncertainties about needle gauge and number of tissue samples. Given updates regarding indolent infections such as Propionibacterium acnes, holding the culture more than 5 7 days may increase the microbiologic yield [21, 22]. 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Spinal infections: diagnostic tests and imaging studies. Clin Orthop Relat Res 2006; 444:27 33 19. Rankine JJ, Barron DA, Robinson P, Millner PA, Dickson RA. Therapeutic impact of percutaneous spinal biopsy in spinal infection. Postgrad Med J 2004; 80:607 609 20. Sehn JK, Gilula LA. Percutaneous needle biopsy in diagnosis and identification of causative organisms in cases of suspected vertebral osteomyelitis. Eur J Radiol 2012; 81:940 946 21. Schäfer P, Fink B, Sandow D, Margull A, Berger I, Frommelt L. Prolonged bacterial culture to identify late periprosthetic joint infection: a promising strategy. Clin Infect Dis 2008; 47:1403 1409 22. Butler-Wu SM, Burns EM, Pottinger PS, et al. Optimization of periprosthetic culture for diagnosis of Propionibacterium acnes prosthetic joint infection. J Clin Microbiol 2011; 49:2490 2495 AJR:205, July 2015 129