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Diagnosis of Periprosthetic Joint Infection Using Synovial C-Reactive Protein
The Journal of Arthroplasty Vol. 27 No. 8 Suppl. 1 2012
Winner of the Clinical Award
Diagnosis of Periprosthetic Joint Infection Using Synovial C-Reactive Protein Javad Parvizi, MD, FRCS, James C. McKenzie, BS, and James P. Cashman, MD
Abstract: The diagnosis of periprosthetic joint infection (PJI) is a considerable challenge. This study examines the quantification of C-reactive protein (CRP) in synovial fluid for diagnosis of PJI. Synovial fluid samples were collected prospectively from 63 patients undergoing revision or primary joint arthroplasty. All patients were divided into septic vs aseptic groups. There were 43 patients in the aseptic group and 20 patients in the septic group. There was a statistically significant difference in the mean synovial CRP between the septic cohort at 40 mg/L vs a mean of 2 mg/L for aseptic failure (P b .0001). The sensitivity was 85% with 95% specificity at a threshold of 9.5 mg/L. The area under the curve was 0.92. We believe that synovial CRP assay holds great promise as a diagnostic marker for PJI. Keywords: periprosthetic joint infection, C-reactive protein, synovial CRP. © 2012 Elsevier Inc. All rights reserved.
diagnosing infection [8-10]. These tests have many drawbacks including expense, need for specialized equipment, and the relatively long time required to complete the analysis. Because of these limitations, molecular techniques have not yet become part of standard clinical tests in evaluation of patients' possible PJI [11]. Serum C-reactive protein (CRP) is a simple, inexpensive test that is currently used for diagnosis of infection [12]. However, serum CRP is nonspecific for the diagnosis of localized infection because elevated CRP levels can be present in several noninfectious inflammatory processes. This is, as far as we are aware, the first study that evaluates the role of synovial CRP in the diagnosis of PJI. The aim of the current study was also to determine the threshold, specificity, and sensitivity of synovial CRP for diagnosis of PJI.
Periprosthetic joint infection (PJI) is one of the most common indications for revision hip and knee arthroplasties [1,2]. The overall incidence of PJI after joint arthroplasty is estimated to be between 1% and 3% [3]. The consequences of PJI can result in decreased knee or hip function and diminished patient quality of life and, in some cases, may lead to arthrodesis, amputation, permanent resection arthroplasty, or even death [4]. Diagnosis of PJI can be challenging in some patients because the clinical presentation of PJI can be subtle and because PJI can coexist with other modes of failure for total knee arthroplasty (TKA) and total hip arthroplasty [5]. There are several tests available for diagnosis of PJI, with none of them having an absolute accuracy [6]. It is thus necessary to combine the results of these tests to obtain a more accurate impression of the possible cause of failure for a joint arthroplasty [7]. Numerous researchers have focused on improvement of standards and development of novel molecular methods for
Materials and Methods Institutional review board approval was obtained before recruiting patients into this prospective study. Between September 2010 and June 2011, the study recruited a total of 63 patients, which included 35 patients (29 knee and 6 hip) undergoing revision arthroplasty surgery for aseptic failure, 20 patients (14 knee and 6 hip) undergoing revision arthroplasty for PJI, and 8 patients undergoing primary TKA. All patients undergoing revision arthroplasty surgery at our institution are investigated according to the American Academy of Orthopedic Surgeon's (AAOS) guidelines for
From the Rothman Institute of Orthopedics, Thomas Jefferson University Hospital, 925 Chestnut Street, Philadelphia, Pennsylvania. Submitted December 19, 2011; accepted March 13, 2012. The Conflict of Interest statement associated with this article can be found at doi:10.1016/j.arth.2012.03.018. Reprint requests: Javad Parvizi, MD, FRCS, Rothman Institute Orthopaedics, 925 Chestnut St, 2nd Floor, Philadelphia, PA 19107. © 2012 Elsevier Inc. All rights reserved. 0883-5403/2708-0004$36.00/0 doi:10.1016/j.arth.2012.03.018
12
Diagnosis of PJI Using Synovial CRP Parvizi et al
diagnosis of PJI, which includes serum erythrocyte sedimentation rate (ESR) and CRP as screening tests and aspiration of the joint when serologic markers are elevated [13]. For the purpose of this study, confirmation of septic failure coincided with the diagnosis of PJI based on the AAOS guidelines cited above and explained below. Those who did not meet the AAOS guidelines for a diagnosis of PJI and required a revision were considered as aseptic failures. Demographic data including age, gender, body mass index, and Charlson index were collected for comparative analyses. A diagnosis of PJI would be made if 3 of the following 4 thresholds were abnormal: an ESR greater than 30 mm/h, a serum CRP value greater than 10 mg/L, a synovial white blood cell (WBC) count greater than 1760 cells/μL for chronic infection or 10 700 cells/μL for acute infection, or a synovial neutrophil differential percentage greater than 73% for chronic infection or greater than 89% for acute infection [13]. The synovial fluid samples were collected under aseptic conditions. The synovial fluid samples were analyzed for CRP, WBC, and polymorphonuclear differential. The synovial fluid was also placed in red top bottles (serum sample; BD Vacutainer, Franklin Lakes, NJ) and transferred to the hospital laboratory for analysis and measurement of CRP. The synovial CRP was measured, using standard hospital laboratory equipment, in an automated turbidimetric method using a specific reagent kit reacting goat anti-CRP antibody and patient CRP antigen in the sample into sedimentation complexes. The automated machinery used was the Synchron LX System (Beckman Coulter, Inc, Brea, Cal). When determining both serum and synovial fluid CRP values, any value below the calibrated sensitivity of the machinery (any CRP value b5 mg/L) was standardized at a value of 0 for the purposes of statistical analysis. Cultures were not analyzed for this study protocol. Statistical Methods Continuous data values were analyzed for statistical significance with univariate unpaired, 2-tailed student t tests with an α level of .05. We used receiver operating characteristic (ROC) curve analyses to determine whether the CRP concentration in synovial fluid could effectively be used to diagnose PJI. This method assesses the diagnostic strength of a test and calculates the best cutoff point for a differentiating between a positive and negative test result. Sensitivity is plotted against 1-specificity, and the area under the curve (AUC) is calculated from the plot. An AUC of 0.5 indicates that a test has no diagnostic strength; as the AUC increases (to a maximum of 1), the diagnostic strength improves. A test with an AUC greater than 0.9 is considered to be an excellent diagnostic test. The ROC curves were determined with SPSS software version 16.0 (IBM Corporation, New York, NY). Correlation coefficients
13
(r 2) were examined to determine any linear progression between serum and synovial CRP results.
Results The study recruited a total of 63 patients. Synovial fluid analysis for CRP could not be performed in 3 patients because of grossly hemolyzed or excessively viscous synovial fluid specimen. The baseline level of synovial CRP was 1.9 mg/L in the primary joint arthroplasty. Because there was no statistically significant difference between the mean synovial CRP in patients with aseptic failures and primary TKA patients, these patients were combined for the purpose of comparison to form 43 aseptic synovial fluid samples vs 20 samples from patients with PJI. The aseptic and PJI cohorts were statistically similar in terms of demographics with no difference in age (P = .06), gender (P = .60), body mass index (P = .25), Charlson index (P = .38), and hip or knee joint type (P = .16) (Table 1). Nine patients (9.5%) in the study population had possible rheumatoid arthritis or fibromyalgia, but their synovial CRP fluid values fell within the median range of the corresponding septic or aseptic cohort. There was a statistically significant difference in the mean synovial CRP between the infected cohort at 40 mg/L vs a mean of 2 mg/L for patients with aseptic failure (P b .0001). There was also a statistically significant difference between the groups for serum CRP, ESR, synovial WBC count, and neutrophil differential (P b .001 for all) (Table 2). Serum CRP correlated strongly with synovial fluid CRP in PJI (r 2 = 0.72). The ROC curve showed that a CRP threshold of 9.5 mg/L had 85% sensitivity and 95.2% specificity as a diagnostic marker for PJI. The AUC was 0.923 (Fig.). During the course of the study, 2 false negatives in which a patient diagnosed with PJI according to the AAOS guidelines were below the 9.5 mg/dL threshold, and 2 false positives in which a patient not considered for diagnosis of PJI occurred. In the case of such occurrences, verification of PJI would require consideration of alternate testing modalities such as the use of intraoperative culture collection and consideration of other clinical parameters. The plurality of diagnostic testing can help confirm or deny unique cases in which the CRP screening does not provide accurate results.
Table 1. Demographic Data for the Study Population Age (y) Body mass index (kg/m2) Gender Joint type Charlson index
Infected (n = 20)
Aseptic (n = 43)
P
68.2 (52-94) 32.6 (24.2-45.1) 42% male 14 knees, 6 hips 1.5 (0-4)
62.5 (42-83) 30.7 (19.5-42.7) 38% male 37 knees, 6 hips 1.2 (0-4)
.06 .25 .7 .16 .38
The mean values and the ranges are shown.
14 The Journal of Arthroplasty Vol. 27 No. 8 Suppl. 1 September 2012 Table 2. The Serum and Synovial Markers Infected (n = 20) Aseptic (n = 43) ESR (mm/h) Serum CRP (mg/L) [percentage with elevated serum CRP] Synovial WBC (cells/μL) Synovial neutrophil percentage Synovial CRP (mg/L)
P
84 (28-120) 98 (3-263) [95]
34 (9-115) b.001 1.1 (0-125) [23] b.001
40 560 84 (72-100)
705 30 (1-80)
.03 b.001
40 (0-103)
2 (0-13)
b.001
The mean values and the ranges are shown.
Discussion Despite the availability of numerous modalities, diagnosis of PJI remains a challenge [1]. Currently, simple serologic tests, analysis of joint aspirate, and culture of fluid or tissues are the mainstay for diagnosis of PJI [3]. Efforts have been made in recent years to develop molecular tests that may carry better accuracy for diagnosis of PJI [14]. Many of these tests are not clinically feasible at the present time because of requirements for specialized equipment and time necessary to obtain results. Laboratory testing for synovial fluid biomarkers of inflammation has proven promising [15]; however, these can be prohibitively expensive or difficult to access. Some assays proposed include staphylococcal immunoglobin M enzyme-linked immunosorbent assay and bacterial polymerase chain reaction as well as several other inflammatory cytokines [15-17]. Interleukin-6 has also been proposed by numerous authors as a potential marker for PJI; however, it is not currently widely available [6,18].
Fig. Receiver operator characteristic curve for synovial fluid CRP (AUC, 0.923).
This study, first of its kind to our knowledge, was designed to evaluate the role of a simple and rapid test, namely, the synovial CRP, for diagnosis of PJI. Previously, synovial CRP has been studied as a possible diagnostic test to distinguish inflammatory arthritis from noninflammatory arthritis in the knee [14]. Zamani et al [19] analyzed the synovial CRP and found that synovial CRP assay distinguished accurately between osteoarthritis and inflammatory arthritis such as rheumatoid arthritis, crystal-induced arthritis, and even septic arthritis. They did not observe statistically significant differences in the synovial CRP level between patients with inflammatory arthritis and patients with septic arthritis. C-reactive protein production has also been demonstrated at other sites, such as the kidney [20], respiratory tract [21], and by other tissues such as adipocytes [22] and neurons [23]. C-reactive protein was originally discovered by Tillett and Francis [24] in 1930 as a substance in the serum of patients with acute inflammation that reacted with the C polysaccharide of pneumococcus. It has a quick onset and peak and dissipates quickly after the insult. C-reactive protein binds to phosphocholine on microbes. It is thought to assist in complement activation and enhances phagocytosis by macrophages, which express a receptor for CRP [25]. Serum CRP has been shown to be a valuable test for diagnosis of PJI. Ghanem et al [7] examined the sensitivity and specificity of serum ESR and CRP in PJI and found that when used in combination with cutoff values of 30 mm/h and 20.5 mg/L, respectively, they had a sensitivity of 96% and a specificity of 59%. Greidanus et al [26] found similar results with higher specificity and an accuracy of 0.88. Measurement of CRP is already a routine clinical test that can be performed rapidly and inexpensively. We performed our investigation using routine laboratory equipment that assays serum CRP. This is a quick and inexpensive laboratory test, being performed in about an hour and costing approximately $17. Our findings demonstrate that the measurement of CRP levels in synovial fluid rather than the serum increases the diagnostic accuracy of this test in identifying PJI. We also found a high correlation between serum CRP and synovial fluid CRP (r 2 = 0.72). Catterall et al [27] also identified correlation between synovial fluid CRP and serum CRP in patients with acute knee trauma. They postulated that diffusion of serum CRP into the joint could be responsible for an elevated synovial fluid CRP in the context of an elevated serum CRP. This mechanism may also apply to patients with PJI. In the context of infection, synovial permeability caused by local inflammation may allow for high levels of serum CRP to diffuse into the joint elevating the synovial CRP. We used standardized institutional criteria for the diagnosis of PJI, which uses serum ESR, CRP, synovial
Diagnosis of PJI Using Synovial CRP Parvizi et al
neutrophil count, and differential and culture results. There were clear differences in each of the aforementioned parameters between the infected and aseptic cohorts. Because of this, we were confidently able to determine that the 2 groups were true reflections of aseptic and septic failures. To this end, we were able to reduce the risk of α and β errors. Although 1 criticism of this study may relate to its relatively small sample size, there was a clear and statistical separation in the mean value of synovial CRP between the infected and noninfected cohorts. The study also achieved its other objective of determining the appropriate cutoff level for synovial CRP indicative of PJI. It appears that same threshold level as serum (10 mg/L) may also apply to synovial CRP, another testimony to the fact that diffusion of CRP from circulating blood may be the mechanism elevating synovial CRP. Based on statistical analysis and the use of ROC curve, we believe that a threshold of 9.5 mg/L is the most appropriate level resulting in an AUC of 0.923, indicating excellent diagnostic accuracy and sensitivity and specificity of 85% and 95%, respectively. Although future studies are needed to confirm our findings in a larger cohort, we believe that an assay for synovial fluid CRP levels may serve as a simple and costeffective method for diagnosing PJI in the future, particularly because most hospital laboratories have the ability to perform CRP testing in serum currently.
References 1. Kurtz SM, Ong KL, Lau E, et al. Prosthetic joint infection risk after TKA in the Medicare population. Clin Orthop Relat Res 2010;468:52. 2. Ong KL, Kurtz SM, Lau E, et al. Prosthetic joint infection risk after total hip arthroplasty in the Medicare population. J Arthroplasty 2009;24(6 Suppl):105. 3. Parvizi J, Ghanem E, Sharkey P, et al. Diagnosis of infected total knee: findings of a multicenter database. Clin Orthop Relat Res 2008;466:2628. 4. Volin SJ, Hinrichs SH, Garvin KL. Two-stage reimplantation of total joint infections: a comparison of resistant and non-resistant organisms. Clin Orthop Relat Res 2004;94. 5. Canner GC, Steinberg ME, Heppenstall RB, et al. The infected hip after total hip arthroplasty. J Bone Joint Surg Am 1984;66:1393. 6. Berbari E, Mabry T, Tsaras G, et al. Inflammatory blood laboratory levels as markers of prosthetic joint infection: a systematic review and meta-analysis. J Bone Joint Surg Am 2010;92:2102. 7. Ghanem E, Antoci Jr V, Pulido L, et al. The use of receiver operating characteristics analysis in determining erythrocyte sedimentation rate and C-reactive protein levels in diagnosing periprosthetic infection prior to revision total hip arthroplasty. Int J Infect Dis 2009;13:e444. 8. Zimmerli W, Trampuz A, Ochsner PE. Prosthetic-joint infections. N Engl J Med 2004;351:1645.
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9. Bottner F, Wegner A, Winkelmann W, et al. Interleukin-6, procalcitonin and TNF-alpha: markers of peri-prosthetic infection following total joint replacement. J Bone Joint Surg Br 2007;89:94. 10. Worthington T, Dunlop D, Casey A, et al. Serum procalcitonin, interleukin-6, soluble intercellular adhesin molecule-1 and IgG to short195 chain exocellular lipoteichoic acid as predictors of infection in total joint prosthesis revision. Br J Biomed Sci 2010;67:71. 11. Bauer TW, Parvizi J, Kobayashi N, et al. Diagnosis of periprosthetic infection. J Bone Joint Surg Am 2006; 88:869. 12. Parvizi J, Ghanem E, Menashe S, et al. Periprosthetic infection: what are the diagnostic challenges? J Bone Joint Surg Am 2006;88(Suppl 4):138. 13. Parvizi J, Della Valle CJ. AAOS Clinical Practice Guideline: diagnosis and treatment of periprosthetic joint infections of the hip and knee. J Am Acad Orthop Surg 2010;18:771. 14. Gallo J, Raska M, Dendis M, et al. Molecular diagnosis of prosthetic joint infection. A review of evidence. Biomed Pap Med Fac Univ Palacky Olomouc Czech Repub 2004; 148:123. 15. Deirmengian C, Hallab N, Tarabishy A, et al. Synovial fluid biomarkers for periprosthetic infection. Clin Orthop Relat Res 2010;468:2017. 16. Artini M, Romano C, Manzoli L, et al. Staphylococcal IgM enzyme-linked immunosorbent assay for diagnosis of periprosthetic joint infections. J Clin Microbiol 2011; 49:423. 17. Kalogianni DP, Goura S, Aletras AJ, et al. Dry reagent dipstick test combined with 23S rRNA PCR for molecular diagnosis of bacterial infection in arthroplasty. Anal Biochem 2007;361:169. 18. Di Cesare PE, Chang E, Preston CF, et al. Serum interleukin-6 as a marker of periprosthetic infection following total hip and knee arthroplasty. J Bone Joint Surg Am 2005;87:1921. 19. Zamani B, Jamali R, Ehteram H. Synovial fluid adenosine deaminase and high sensitivity C-reactive protein activity in differentiating monoarthritis. Rheumatol Int 2012; 32:183. 20. Jabs WJ, Logering BA, Gerke P, et al. The kidney as a second site of human C-reactive protein formation in vivo. Eur J Immunol 2003;33:152. 21. Gould JM, Weiser JN. Expression of C-reactive protein in the human respiratory tract. Infect Immun 2001;69:1747. 22. Calabro P, Chang DW, Willerson JT, et al. Release of C-reactive protein in response to inflammatory cytokines by human adipocytes: linking obesity to vascular inflammation. J Am Coll Cardiol 2005;46:1112. 23. Yasojima K, Schwab C, McGeer EG, et al. Human neurons generate C reactive protein and amyloid P: upregulation in Alzheimer's disease. Brain Res 2000;887:80. 24. Tillett WS, Francis T. Serological reactions in pneumonia with a non-protein somatic fraction of Pneumococcus. J Exp Med 1930;52:561. 25. Gewurz H, Mold C, Siegel J, et al. C-reactive protein and the acute phase response. Adv Intern Med 1982; 27:345. 26. Greidanus NV, Masri BA, Garbuz DS, et al. Use of erythrocyte sedimentation rate and C-reactive protein
16 The Journal of Arthroplasty Vol. 27 No. 8 Suppl. 1 September 2012 level to diagnose infection before revision total knee arthroplasty. A prospective evaluation. J Bone Joint Surg Am 2007;89:1409.
27. Catterall JB, Stabler TV, Flannery CR, et al. Changes in serum and synovial fluid biomarkers after acute injury (NCT00332254). Arthritis Res Ther 2010;12:R229.
Winner of the Clinical Award
Diagnosis of Periprosthetic Joint Infection Using Synovial C-Reactive Protein Javad Parvizi, MD, FRCS, James C. McKenzie, BS, and James P. Cashman, MD
Abstract: The diagnosis of periprosthetic joint infection (PJI) is a considerable challenge. This study examines the quantification of C-reactive protein (CRP) in synovial fluid for diagnosis of PJI. Synovial fluid samples were collected prospectively from 63 patients undergoing revision or primary joint arthroplasty. All patients were divided into septic vs aseptic groups. There were 43 patients in the aseptic group and 20 patients in the septic group. There was a statistically significant difference in the mean synovial CRP between the septic cohort at 40 mg/L vs a mean of 2 mg/L for aseptic failure (P b .0001). The sensitivity was 85% with 95% specificity at a threshold of 9.5 mg/L. The area under the curve was 0.92. We believe that synovial CRP assay holds great promise as a diagnostic marker for PJI. Keywords: periprosthetic joint infection, C-reactive protein, synovial CRP. © 2012 Elsevier Inc. All rights reserved.
diagnosing infection [8-10]. These tests have many drawbacks including expense, need for specialized equipment, and the relatively long time required to complete the analysis. Because of these limitations, molecular techniques have not yet become part of standard clinical tests in evaluation of patients' possible PJI [11]. Serum C-reactive protein (CRP) is a simple, inexpensive test that is currently used for diagnosis of infection [12]. However, serum CRP is nonspecific for the diagnosis of localized infection because elevated CRP levels can be present in several noninfectious inflammatory processes. This is, as far as we are aware, the first study that evaluates the role of synovial CRP in the diagnosis of PJI. The aim of the current study was also to determine the threshold, specificity, and sensitivity of synovial CRP for diagnosis of PJI.
Periprosthetic joint infection (PJI) is one of the most common indications for revision hip and knee arthroplasties [1,2]. The overall incidence of PJI after joint arthroplasty is estimated to be between 1% and 3% [3]. The consequences of PJI can result in decreased knee or hip function and diminished patient quality of life and, in some cases, may lead to arthrodesis, amputation, permanent resection arthroplasty, or even death [4]. Diagnosis of PJI can be challenging in some patients because the clinical presentation of PJI can be subtle and because PJI can coexist with other modes of failure for total knee arthroplasty (TKA) and total hip arthroplasty [5]. There are several tests available for diagnosis of PJI, with none of them having an absolute accuracy [6]. It is thus necessary to combine the results of these tests to obtain a more accurate impression of the possible cause of failure for a joint arthroplasty [7]. Numerous researchers have focused on improvement of standards and development of novel molecular methods for
Materials and Methods Institutional review board approval was obtained before recruiting patients into this prospective study. Between September 2010 and June 2011, the study recruited a total of 63 patients, which included 35 patients (29 knee and 6 hip) undergoing revision arthroplasty surgery for aseptic failure, 20 patients (14 knee and 6 hip) undergoing revision arthroplasty for PJI, and 8 patients undergoing primary TKA. All patients undergoing revision arthroplasty surgery at our institution are investigated according to the American Academy of Orthopedic Surgeon's (AAOS) guidelines for
From the Rothman Institute of Orthopedics, Thomas Jefferson University Hospital, 925 Chestnut Street, Philadelphia, Pennsylvania. Submitted December 19, 2011; accepted March 13, 2012. The Conflict of Interest statement associated with this article can be found at doi:10.1016/j.arth.2012.03.018. Reprint requests: Javad Parvizi, MD, FRCS, Rothman Institute Orthopaedics, 925 Chestnut St, 2nd Floor, Philadelphia, PA 19107. © 2012 Elsevier Inc. All rights reserved. 0883-5403/2708-0004$36.00/0 doi:10.1016/j.arth.2012.03.018
12
Diagnosis of PJI Using Synovial CRP Parvizi et al
diagnosis of PJI, which includes serum erythrocyte sedimentation rate (ESR) and CRP as screening tests and aspiration of the joint when serologic markers are elevated [13]. For the purpose of this study, confirmation of septic failure coincided with the diagnosis of PJI based on the AAOS guidelines cited above and explained below. Those who did not meet the AAOS guidelines for a diagnosis of PJI and required a revision were considered as aseptic failures. Demographic data including age, gender, body mass index, and Charlson index were collected for comparative analyses. A diagnosis of PJI would be made if 3 of the following 4 thresholds were abnormal: an ESR greater than 30 mm/h, a serum CRP value greater than 10 mg/L, a synovial white blood cell (WBC) count greater than 1760 cells/μL for chronic infection or 10 700 cells/μL for acute infection, or a synovial neutrophil differential percentage greater than 73% for chronic infection or greater than 89% for acute infection [13]. The synovial fluid samples were collected under aseptic conditions. The synovial fluid samples were analyzed for CRP, WBC, and polymorphonuclear differential. The synovial fluid was also placed in red top bottles (serum sample; BD Vacutainer, Franklin Lakes, NJ) and transferred to the hospital laboratory for analysis and measurement of CRP. The synovial CRP was measured, using standard hospital laboratory equipment, in an automated turbidimetric method using a specific reagent kit reacting goat anti-CRP antibody and patient CRP antigen in the sample into sedimentation complexes. The automated machinery used was the Synchron LX System (Beckman Coulter, Inc, Brea, Cal). When determining both serum and synovial fluid CRP values, any value below the calibrated sensitivity of the machinery (any CRP value b5 mg/L) was standardized at a value of 0 for the purposes of statistical analysis. Cultures were not analyzed for this study protocol. Statistical Methods Continuous data values were analyzed for statistical significance with univariate unpaired, 2-tailed student t tests with an α level of .05. We used receiver operating characteristic (ROC) curve analyses to determine whether the CRP concentration in synovial fluid could effectively be used to diagnose PJI. This method assesses the diagnostic strength of a test and calculates the best cutoff point for a differentiating between a positive and negative test result. Sensitivity is plotted against 1-specificity, and the area under the curve (AUC) is calculated from the plot. An AUC of 0.5 indicates that a test has no diagnostic strength; as the AUC increases (to a maximum of 1), the diagnostic strength improves. A test with an AUC greater than 0.9 is considered to be an excellent diagnostic test. The ROC curves were determined with SPSS software version 16.0 (IBM Corporation, New York, NY). Correlation coefficients
13
(r 2) were examined to determine any linear progression between serum and synovial CRP results.
Results The study recruited a total of 63 patients. Synovial fluid analysis for CRP could not be performed in 3 patients because of grossly hemolyzed or excessively viscous synovial fluid specimen. The baseline level of synovial CRP was 1.9 mg/L in the primary joint arthroplasty. Because there was no statistically significant difference between the mean synovial CRP in patients with aseptic failures and primary TKA patients, these patients were combined for the purpose of comparison to form 43 aseptic synovial fluid samples vs 20 samples from patients with PJI. The aseptic and PJI cohorts were statistically similar in terms of demographics with no difference in age (P = .06), gender (P = .60), body mass index (P = .25), Charlson index (P = .38), and hip or knee joint type (P = .16) (Table 1). Nine patients (9.5%) in the study population had possible rheumatoid arthritis or fibromyalgia, but their synovial CRP fluid values fell within the median range of the corresponding septic or aseptic cohort. There was a statistically significant difference in the mean synovial CRP between the infected cohort at 40 mg/L vs a mean of 2 mg/L for patients with aseptic failure (P b .0001). There was also a statistically significant difference between the groups for serum CRP, ESR, synovial WBC count, and neutrophil differential (P b .001 for all) (Table 2). Serum CRP correlated strongly with synovial fluid CRP in PJI (r 2 = 0.72). The ROC curve showed that a CRP threshold of 9.5 mg/L had 85% sensitivity and 95.2% specificity as a diagnostic marker for PJI. The AUC was 0.923 (Fig.). During the course of the study, 2 false negatives in which a patient diagnosed with PJI according to the AAOS guidelines were below the 9.5 mg/dL threshold, and 2 false positives in which a patient not considered for diagnosis of PJI occurred. In the case of such occurrences, verification of PJI would require consideration of alternate testing modalities such as the use of intraoperative culture collection and consideration of other clinical parameters. The plurality of diagnostic testing can help confirm or deny unique cases in which the CRP screening does not provide accurate results.
Table 1. Demographic Data for the Study Population Age (y) Body mass index (kg/m2) Gender Joint type Charlson index
Infected (n = 20)
Aseptic (n = 43)
P
68.2 (52-94) 32.6 (24.2-45.1) 42% male 14 knees, 6 hips 1.5 (0-4)
62.5 (42-83) 30.7 (19.5-42.7) 38% male 37 knees, 6 hips 1.2 (0-4)
.06 .25 .7 .16 .38
The mean values and the ranges are shown.
14 The Journal of Arthroplasty Vol. 27 No. 8 Suppl. 1 September 2012 Table 2. The Serum and Synovial Markers Infected (n = 20) Aseptic (n = 43) ESR (mm/h) Serum CRP (mg/L) [percentage with elevated serum CRP] Synovial WBC (cells/μL) Synovial neutrophil percentage Synovial CRP (mg/L)
P
84 (28-120) 98 (3-263) [95]
34 (9-115) b.001 1.1 (0-125) [23] b.001
40 560 84 (72-100)
705 30 (1-80)
.03 b.001
40 (0-103)
2 (0-13)
b.001
The mean values and the ranges are shown.
Discussion Despite the availability of numerous modalities, diagnosis of PJI remains a challenge [1]. Currently, simple serologic tests, analysis of joint aspirate, and culture of fluid or tissues are the mainstay for diagnosis of PJI [3]. Efforts have been made in recent years to develop molecular tests that may carry better accuracy for diagnosis of PJI [14]. Many of these tests are not clinically feasible at the present time because of requirements for specialized equipment and time necessary to obtain results. Laboratory testing for synovial fluid biomarkers of inflammation has proven promising [15]; however, these can be prohibitively expensive or difficult to access. Some assays proposed include staphylococcal immunoglobin M enzyme-linked immunosorbent assay and bacterial polymerase chain reaction as well as several other inflammatory cytokines [15-17]. Interleukin-6 has also been proposed by numerous authors as a potential marker for PJI; however, it is not currently widely available [6,18].
Fig. Receiver operator characteristic curve for synovial fluid CRP (AUC, 0.923).
This study, first of its kind to our knowledge, was designed to evaluate the role of a simple and rapid test, namely, the synovial CRP, for diagnosis of PJI. Previously, synovial CRP has been studied as a possible diagnostic test to distinguish inflammatory arthritis from noninflammatory arthritis in the knee [14]. Zamani et al [19] analyzed the synovial CRP and found that synovial CRP assay distinguished accurately between osteoarthritis and inflammatory arthritis such as rheumatoid arthritis, crystal-induced arthritis, and even septic arthritis. They did not observe statistically significant differences in the synovial CRP level between patients with inflammatory arthritis and patients with septic arthritis. C-reactive protein production has also been demonstrated at other sites, such as the kidney [20], respiratory tract [21], and by other tissues such as adipocytes [22] and neurons [23]. C-reactive protein was originally discovered by Tillett and Francis [24] in 1930 as a substance in the serum of patients with acute inflammation that reacted with the C polysaccharide of pneumococcus. It has a quick onset and peak and dissipates quickly after the insult. C-reactive protein binds to phosphocholine on microbes. It is thought to assist in complement activation and enhances phagocytosis by macrophages, which express a receptor for CRP [25]. Serum CRP has been shown to be a valuable test for diagnosis of PJI. Ghanem et al [7] examined the sensitivity and specificity of serum ESR and CRP in PJI and found that when used in combination with cutoff values of 30 mm/h and 20.5 mg/L, respectively, they had a sensitivity of 96% and a specificity of 59%. Greidanus et al [26] found similar results with higher specificity and an accuracy of 0.88. Measurement of CRP is already a routine clinical test that can be performed rapidly and inexpensively. We performed our investigation using routine laboratory equipment that assays serum CRP. This is a quick and inexpensive laboratory test, being performed in about an hour and costing approximately $17. Our findings demonstrate that the measurement of CRP levels in synovial fluid rather than the serum increases the diagnostic accuracy of this test in identifying PJI. We also found a high correlation between serum CRP and synovial fluid CRP (r 2 = 0.72). Catterall et al [27] also identified correlation between synovial fluid CRP and serum CRP in patients with acute knee trauma. They postulated that diffusion of serum CRP into the joint could be responsible for an elevated synovial fluid CRP in the context of an elevated serum CRP. This mechanism may also apply to patients with PJI. In the context of infection, synovial permeability caused by local inflammation may allow for high levels of serum CRP to diffuse into the joint elevating the synovial CRP. We used standardized institutional criteria for the diagnosis of PJI, which uses serum ESR, CRP, synovial
Diagnosis of PJI Using Synovial CRP Parvizi et al
neutrophil count, and differential and culture results. There were clear differences in each of the aforementioned parameters between the infected and aseptic cohorts. Because of this, we were confidently able to determine that the 2 groups were true reflections of aseptic and septic failures. To this end, we were able to reduce the risk of α and β errors. Although 1 criticism of this study may relate to its relatively small sample size, there was a clear and statistical separation in the mean value of synovial CRP between the infected and noninfected cohorts. The study also achieved its other objective of determining the appropriate cutoff level for synovial CRP indicative of PJI. It appears that same threshold level as serum (10 mg/L) may also apply to synovial CRP, another testimony to the fact that diffusion of CRP from circulating blood may be the mechanism elevating synovial CRP. Based on statistical analysis and the use of ROC curve, we believe that a threshold of 9.5 mg/L is the most appropriate level resulting in an AUC of 0.923, indicating excellent diagnostic accuracy and sensitivity and specificity of 85% and 95%, respectively. Although future studies are needed to confirm our findings in a larger cohort, we believe that an assay for synovial fluid CRP levels may serve as a simple and costeffective method for diagnosing PJI in the future, particularly because most hospital laboratories have the ability to perform CRP testing in serum currently.
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