Joint replacement is now considered as one of the most effective procedures in orthopaedics, providing pain relief and functional restoration while significantly improving the quality of life for millions of people worldwide [1,2]. However, the risk of periprosthetic joint infection (PJI) remains, a serious complication that occurs in 1-2% of primary arthroplasties and 4% of revision arthroplasties, resulting in a significant increase in morbidity and mortality [3,4].

The diagnosis of PJI is challenging because there is no definitive test. Therefore, it is necessary to combine subjective symptoms with objective findings from physical examination, along with the results of various laboratory tests, including synovial fluid cell counts, synovial and serum inflammatory markers, and microbiologic cultures [[5], [6], [7]]. Furthermore, the Musculoskeletal Infection Society (MSIS) and the European Bone and Joint Infection Society (EBJIS) have developed major and minor criteria to support a more accurate diagnosis of PJI [8,9]. Among the major criteria, the detection of two positive periprosthetic cultures with phenotypically identical organisms or the presence of a sinus tract communicating with the joint is deemed sufficient for diagnosing PJI. In addition, this must be complemented by secondary criteria, such as: a) elevated serum levels of C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR), and b) an increased white blood cell (WBC) count or alterations in the leukocyte esterase test strip.

CRP and ESR are widely used due to their high sensitivity, while WBC count in synovial fluid is a direct indicator of infection. However, these markers have limitations, including low specificity, particularly in low-grade infections or in patients with systemic inflammation unrelated to infection [10,11]. Due to the need for more specific diagnostic tools, there has been growing interest in natural antimicrobial peptides, such as human β-defensin-1 (hBD-1). Unlike other synovial biomarkers, such as α-defensin and calprotectin, hBD-1 has not been extensively studied; however, it shows promising specificity and sensitivity in PJI. Although these biomarkers have been extensively investigated as universal PJI diagnostic biomarkers, they are not useful for differentiating between pathogens [[12], [13], [14]]. HBD-1 is constitutively expressed in synovial tissue and plays a crucial role in innate immunity. In addition to its diagnostic potential, hBD-1 interacts directly with microbial surfaces and modulates inflammatory cascades [15]. This reflects both pathogen virulence and host defense strategies. Therefore, studying pathogen-specific hBD-1 responses provides insight into the dynamics of host-pathogen interactions that underpin infection persistence and clinical outcomes [16].

Our group previously demonstrated that synovial hBD-1 has higher sensitivity and specificity than CRP, ESR and WBC count [17]. Therefore, hBD-1 may serve as a valuable complementary marker, particularly in ambiguous or culture-negative cases. The etiology of PJI is variable, and Gram-positive cocci such as Staphylococcus epidermidis (S. epidermidis), Staphylococcus aureus (S. aureus), and species of the genera Enterococcus and Streptococcus are among the most commonly identified pathogens. Gram-positive bacilli, such as Cutibacterium, and gram-negative bacilli, including Pseudomonas aeruginosa (P. aeruginosa) and the Enterobacteriaceae family, have also been isolated [3,18,19].

The expression of inflammatory markers varies by pathogen species and virulence. This variability may help differentiate between Gram-positive and Gram-negative sepsis, allowing for an earlier diagnosis and more effective treatment [20,21]. Therefore, recognizing pathogen-specific biomarker responses is clinically meaningful, as it may improve diagnostic algorithms and inform the selection of empiric antibiotic therapy.

To our knowledge, no previous study has examined how hBD-1 and conventional biomarkers change in synovial fluid based on the genus of the infecting bacteria in PJI. Based on the above, the purpose of this retrospective study was to evaluate how specific pathogens affect the levels of inflammatory markers such as synovial hBD-1 and routinely used markers such as ESR, serum CRP and WBC count to assess their usefulness as early predictors of PJI, which could significantly improve diagnosis and clinical management.