Authors’ Disclosure Statement: Dr. Barsoum reports that he receives royalties from Stryker, Exactech, and Zimmer Biomet; a patent from Zimmer Biomet; consultant fees from Stryker; stock/stock options from iVHR, Peer Well, Custom Orthopaedic Solutions, and Otismed; research support from Stryker, Zimmer Biomet, DJO, and Orthosensor; and other financial support from KEF Healthcare. Dr. Higuera reports that he receives personal fees from Zimmer Biomet, Covance, and Pfizer; research support/grants and personal fees from KCI; and research support/grants from Stryker, Myoscience, CD Diagnostics, The Orthopaedic Research and Education Foundation (OREF), and Pacira. He is on the editorial review board of The American Journal of Orthopedics. The other authors report no actual or potential conflict of interest in relation to this article.
α-Defensin, a natural peptide produced and secreted by neutrophils in response to pathogens, has antimicrobial and cytotoxic properties,38-40 signals for the secretion of various cytokines, and acts as a chemoattractant for various immune cells.41 Deirmengian and colleagues6 found that α-defensin was consistently elevated in patients with PJI. α-Defensin is extremely accurate in diagnosing PJI; it has sensitivity ranging from 97% to 100% and specificity ranging from 96% to 100%.6,27,42 Moreover, α-defensin was effective in diagnosing PJI caused by a wide spectrum of organisms, including various low-virulence bacteria and fungi.43
Leukocyte Esterase
Leukocyte esterase is an enzyme produced and secreted by neutrophils at sites of active infection.7,44 Testing for this enzyme is performed with a colorimetric strip and was originally performed for the diagnosis of urinary tract infections.44,45 In a study conducted by Parvizi and colleagues,7 this strip was used to test for leukocyte esterase in synovial fluid samples; a ++ reading was found to have sensitivity of 80.6% and specificity of 100% in diagnosing knee PJI. Similarly, De Vecchi and colleagues45 found sensitivity of 92.6% and specificity of 97%.
Other Synovial Markers
Research has identified numerous molecular biomarkers that may be associated with the pathogenesis of PJI. Although several (eg, cytokines) have demonstrated higher levels in synovial fluid in patients with PJI than in normal controls, only a few have had clinically relevant diagnostic utility.6 Deirmengian and colleagues6 screened 43 synovial fluid biomarkers that potentially could be used in the diagnosis of PJI. Besides the cytokine α-defensin, 4 other biomarkers—lactoferrin, neutrophil gelatinase-associated lipocalcin, neutrophil elastase 2, and bactericidal/permeability-increasing protein—had accuracy of 100%. In addition, 8 cytokines and biomarkers (IL-8, CRP, resistin, thrombospondin, IL-1β, IL-6, IL-10, IL-1α) had area under the curve values higher than 0.9. Studies have also evaluated the diagnostic utility of metabolic products such as lactate, lactate dehydrogenase, and glucose; their accuracy was comparable to that of serum CRP.32
Serum Markers
In addition to the synovial fluid cytokines, several serum inflammatory cytokines have been studied as potential targets in diagnosing infection. Serum IL-6 has had excellent diagnostic accuracy46 and, when combined with CRP, could increase sensitivity in diagnosing PJI; such a combination (vs either test alone) could be useful in screening patients.47,48 Biomarkers such as tumor necrosis factor α and procalcitonin are considered very specific for PJI and may be useful in confirmatory testing.48 Evidence also suggests that toll-like receptor 2 proteins are elevated in the serum of patients with PJI and therefore are a potential diagnostic tool.49
Limitations of Synovial Cytokines
The literature suggests that some synovial fluid cytokines have promise.6 However, the best biomarker or combination of biomarkers is yet to be determined. Results have been consistent with α-defensin and other cytokines but mixed with IL-6 and still others32,42,50 (Table 2).
Table 2.
In addition, the techniques for detecting these biomarkers are not fully standardized, limiting their generalizability. PJI diagnostic tests based on biomarkers are expensive, require special expertise, and are limited to only a few centers. Apart from synovial leukocyte esterase, none of the newly investigated biomarkers are included in current guidelines.11 Given the lack of consensus and guidelines, biomarkers are rarely used to guide treatment decisions. However, with the increase in supportive evidence, incorporation of biomarkers into the general PJI guidelines is expected.
Information on the utility of synovial biomarkers in detecting persistent infection is limited. Frangiamore and colleagues50 found that IL-1 and IL-6 levels decreased between the stages of 2-stage revision. Unfortunately, none of the synovial fluid cytokines investigated (IL-1, IL-2, IL-6, IL-8, Il-10, interferon γ, granulocyte macrophage-colony stimulating factor, tumor necrosis factor α, IL-12p70) satisfactorily detected resolution of infection in the setting of prior treatment for PJI. Although cytokines are expected to be elevated in the presence of infection, the internal milieu at the time of stage 2 of the revision makes diagnosis of infection difficult. In addition, presence of spacer particles and recent surgery may activate immune pathways and yield false-positive results. Furthermore, antibiotic cement spacers may suppress the microorganisms to very low levels and yield false-negative results even if these organisms remain virulent.19
