We found a difference in concentration for 12 of the 24 synovial fluid proteins tested comparing patients with periprosthetic infection to those with aseptic diagnoses (Table 3). Synovial fluid IL-1α, IL-1β, IL-6, IL-17, G-CSF, and SKALP levels were each further evaluated for sensitivity, specificity, accuracy, NPV, and PPV. Synovial fluid IL-1β and IL-6 demonstrated a sensitivity, specificity, PPV, NPV, and accuracy of 1 (Table 4).
Several biomarkers had superior diagnostic accuracy for infection when compared to current standard tests for infection (Table 5).
The diagnosis of periprosthetic infection is a serious clinical challenge, requiring the use of multiple clinical signs and laboratory tests . The recent literature has highlighted the need for improved diagnostics as evidenced by the number of studies attempting to identify the best combination of laboratory tests predicting periprosthetic infection [3, 4, 9, 14-16]. Additionally, new methods of diagnosis such as PCR [12, 18], biofilm detection [17, 20], and cultures augmented by implant ultrasound  have been described and evaluated for diagnostic purposes. The patient, physician, and economic burdens of infected arthroplasties [5, 6] are expected to grow rapidly with the increase in joint arthroplasties performed each year. For these reasons, the ability to accurately diagnose infection is critical. Currently there are no commercially available synovial fluid immunoassays for the purpose of diagnosing infection, and the only commonly used systemic immunoassay for periprosthetic infection is the CRP. Because the CRP and any other systemic test for infection would be confounded by concomitant infection at another anatomic site or systemic inflammatory disease, a synovial fluid test for infection is intuitively more appealing. We previously reported the genome-wide synovial fluid WBC response to periprosthetic infection , attempting to identify potential biomarkers for infection. The purpose of the current study is to evaluate several potential synovial fluid biomarkers for infection, and compare their diagnostic characteristics to current standard laboratory tests for infection.
We acknowledge several weaknesses and limitations in this study. First, there is no gold standard test or clinical evaluation for infection in the literature. There have been a variety of laboratory tests and combinations of clinical criteria and laboratory tests used as the diagnostic criteria to classify patients previously in the literature. We used the solid medium culture results and/or draining sinus tracts to define the groups as infected or aseptic. Because we compared the markers in the current study to the ESR, CRP level, synovial fluid WBC count, and percent segmented neutrophils, we did not want to include them in the classification of patients. One could argue that weaknesses in the classification of patients could lead to alteration of the primary results of this study; however this is an inherent weakness of any study evaluating a diagnostic test for infection. Second is the relatively small number of patients. As this was a preliminary study to identify potentially useful diagnostic biomarkers for infection, we wanted to eliminate the biomarkers that showed poor diagnostic accuracy before pursuing a larger prospective multicenter study. Despite the small number of patients though, we were able to show differences in the diagnostic potential of many of the biomarkers studied.
We found several synovial fluid biomarkers that were substantially elevated in patients with periprosthetic infection when compared to those patients with aseptic reasons for revision. Several of the synovial fluid biomarkers exhibited fluid levels that were more than 100-fold elevated in patients with periprosthetic infection; a conservative estimate given they were undetectable in the majority of patients without overt infection. Synovial fluid IL-1B, IL1-α IL-6, IL-17, and G-CSF, had excellent diagnostic performance, with accuracy above 0.9 in this study, despite the inclusion of patients with confounding characteristics such as systemic inflammatory disease and preoperative antibiotic treatments.
Furthermore, the synovial fluid biomarkers in this study substantially outperformed many currently used laboratory diagnostics for infection. The diagnostic accuracy of the ESR (0.76) and CRP level (0.82) we found was similar to previous reports in the literature [4, 9, 15]. The synovial fluid IL-1B and IL-6 levels were more accurate in diagnosing infection than the ESR, CRP level, synovial fluid WBC count, or percent segmented neutrophils. Several of the other biomarkers also demonstrated improved accuracy when compared to current standards of testing. We emphasize patients treated with antibiotics and patients with systemic inflammatory disease were not excluded. The synovial fluid biomarkers are especially intriguing for diagnostic use in this group of patients, given their historical exclusion from previous studies evaluating the current standards of diagnostic testing [3, 4, 9, 15, 16]. However a larger group of patients with these confounding factors would be necessary to validate these results.
Our data suggest synovial fluid biomarkers could provide an additional valuable resource for the diagnosis of periprosthetic infection. We have identified biomarkers that appear to accurately identify the local host response to infection. They appear to have improved accuracy over current standards of testing, and may be useful even in a population of patients who have confounding systemic variables. Validation of these results is critical to the clinical application of synovial fluid biomarkers, and planning for a larger prospective multicenter study is currently underway.
We thank Michael Chernick PhD, at the Lankenau Institute for Medical Research, for his kind assistance with aspects of the statistical analysis in this study.
1. Archibeck, MJ., Rosenberg, AG., Sheinkop, MB., Berger, RA. and Jacobs, JJ. Gout-induced arthropathy after total knee arthroplasty: a report of two cases. Clin Orthop Relat Res.
2001; 392: 377-382. 10.1097/00003086-200111000-00049
2. Atkins, BL., Athanasou, N., Deeks, JJ., Crook, DW., Simpson, H., Peto, TE., McLardy-Smith, P. and Berendt, AR. Prospective evaluation of criteria for microbiological diagnosis of prosthetic-joint infection at revision arthroplasty. The OSIRIS Collaborative Study Group. J Clin Microbiol.
1998; 36: 2932-2939.
3. Austin, MS., Ghanem, E., Joshi, A., Lindsay, A. and Parvizi, J. A simple, cost-effective screening protocol to rule out periprosthetic infection. J Arthroplasty.
2008; 23: 65-68. 10.1016/j.arth.2007.09.005
4. Bare, J., MacDonald, SJ. and Bourne, RB. Preoperative evaluations in revision total knee arthroplasty. Clin Orthop Relat Res.
2006; 446: 40-44. 10.1097/01.blo.0000218727.14097.d5
5. Bozic, KJ., Kurtz, SM., Lau, E., Ong, K., Chiu, V., Vail, TP., Rubash, HE. and Berry, DJ. The epidemiology of revision total knee arthroplasty in the United States. Clin Orthop Relat Res.
2010; 468: 45-51. 10.1007/s11999-009-0945-0
6. Bozic, KJ., Kurtz, SM., Lau, E., Ong, K., Vail, TP. and Berry, DJ. The epidemiology of revision total hip arthroplasty in the United States. J Bone Joint Surg Am.
2009; 91: 128-133. 10.2106/JBJS.H.00155
7. Deirmengian, C., Lonner, JH. and Booth, RE Jr. The Mark Coventry Award: white blood cell gene expression: a new approach toward the study and diagnosis of infection. Clin Orthop Relat Res.
2005; 440: 38-44. 10.1097/01.blo.0000185756.17401.32
8. Del Pozo, JL. and Patel, R. Clinical practice. Infection associated with prosthetic joints. N Engl J Med.
2009; 361: 787-794. 10.1056/NEJMcp0905029
9. Della Valle, CJ., Sporer, SM., Jacobs, JJ., Berger, RA., Rosenberg, AG. and Paprosky, WG. Preoperative testing for sepsis before revision total knee arthroplasty. J Arthroplasty.
2007; 22: 90-93. 10.1016/j.arth.2007.04.013
10. Di Cesare, PE., Chang, E., Preston, CF. and Liu, CJ. Serum interleukin-6 as a marker of periprosthetic infection following total hip and knee arthroplasty. J Bone Joint Surg Am.
2005; 87: 1921-1927. 10.2106/JBJS.D.01803
11. Kobayashi, N., Inaba, Y., Choe, H., Iwamoto, N., Ishida, T., Yukizawa, Y., Aoki, C., Ike, H. and Saito, T. Rapid and sensitive detection of methicillin-resistant Staphylococcus periprosthetic infections using real-time polymerase chain reaction. Diagn Microbiol Infect Dis.
2009; 64: 172-176. 10.1016/j.diagmicrobio.2009.01.033
12. Kobayashi, N., Procop, GW., Krebs, V., Kobayashi, H. and Bauer, TW. Molecular identification of bacteria from aseptically loose implants. Clin Orthop Relat Res.
2008; 466: 1716-1725. 10.1007/s11999-008-0263-y
13. Muller, M., Morawietz, L., Hasart, O., Strube, P., Perka, C. and Tohtz, S. Diagnosis of periprosthetic infection following total hip arthroplasty—evaluation of the diagnostic values of pre- and intraoperative parameters and the associated strategy to preoperatively select patients with a high probability of joint infection. J Orthop Surg.
2008; 3: 31. 10.1186/1749-799X-3-31
14. Parvizi, J., Ghanem, E., Sharkey, P., Aggarwal, A., Burnett, RS. and Barrack, RL. Diagnosis of infected total knee: findings of a multicenter database. Clin Orthop Relat Res.
2008; 466: 2628-2633. 10.1007/s11999-008-0471-5
15. Savarino, L., Tigani, D., Baldini, N., Bochicchio, V. and Giunti, A. Pre-operative diagnosis of infection in total knee arthroplasty: an algorithm. Knee Surg Sports Traumatol Arthrosc.
2009; 17: 667-675. 10.1007/s00167-009-0759-3
16. Schinsky, MF., Della Valle, CJ., Sporer, SM. and Paprosky, WG. Perioperative testing for joint infection in patients undergoing revision total hip arthroplasty. J Bone Joint Surg Am.
2008; 90: 1869-1875. 10.2106/JBJS.G.01255
17. Stoodley, P., Kathju, S., Hu, FZ., Erdos, G., Levenson, JE., Mehta, N., Dice, B., Johnson, S., Hall-Stoodley, L., Nistico, L., Sotereanos, N., Sewecke, J., Post, JC. and Ehrlich, GD. Molecular and imaging techniques for bacterial biofilms in joint arthroplasty infections. Clin Orthop Relat Res.
2005; 437: 31-40. 10.1097/01.blo.0000175129.83084.d5
18. Tarkin, IS., Henry, TJ., Fey, PI., Iwen, PC., Hinrichs, SH. and Garvin, KL. PCR rapidly detects methicillin-resistant staphylococci periprosthetic infection. Clin Orthop Relat Res.
2003; 414: 89-94. 10.1097/01.blo.0000087323.60612.5d
19. Trampuz, A., Piper, KE., Jacobson, MJ., Hanssen, AD., Unni, KK., Osmon, DR., Mandrekar, JN., Cockerill, FR., Steckelberg, JM., Greenleaf, JF. and Patel, R. Sonication of removed hip and knee prostheses for diagnosis of infection. N Engl J Med.
2007; 357: 654-663. 10.1056/NEJMoa061588
20. Tunney, MM., Patrick, S., Curran, MD., Ramage, G., Hanna, D., Nixon, JR., Gorman, SP., Davis, RI. and Anderson, N. Detection of prosthetic hip infection at revision arthroplasty by immunofluorescence microscopy and PCR amplification of the bacterial 16S rRNA gene. J Clin Microbiol.
1999; 37: 3281-3290.