From serum inflammatory markers to fluid, tissue, and molecular assays: current advances in the laboratory diagnosis of bone and joint infections.

Bone and joint infections (BJIs), including periprosthetic joint infection (PJI), fracture-related infection (FRI), and osteomyelitis, present persistent diagnostic challenges driven by biofilm formation and a high incidence of culture-negative cases. Traditional diagnostic modalities relying on peripheral serum markers and conventional cultures are often limited by insufficient specificity or prolonged turnaround times. This narrative review critically evaluates recent advances in laboratory diagnosis for bone and joint infections, with particular attention to disease-specific applicability across periprosthetic joint infection, fracture-related infection, native vertebral osteomyelitis, diabetic foot osteomyelitis, and other osteomyelitis-related conditions. Current evidence indicates that while traditional serum inflammatory markers are valuable for initial screening, their susceptibility to aseptic inflammatory confounders precludes standalone diagnostic confirmation. In contrast, localized sampling demonstrates significant superiority: novel synovial fluid biomarkers, notably calprotectin and alpha-defensin, accurately reflect the infection microenvironment and offer exceptional diagnostic specificity. At the tissue level, the integration of multiple deep-tissue sampling with preprocessing techniques like sonication has substantially enhanced the recovery of occult biofilm-encased pathogens. Furthermore, targeted and untargeted molecular assays, including multiplex PCR panels, broad-range bacterial PCR, amplicon-based sequencing, and untargeted shotgun metagenomic sequencing, have expanded the diagnostic toolkit for culture-negative, low-virulence, and polymicrobial infections. The diagnostic framework for BJIs has decisively shifted from the pursuit of a solitary "silver bullet" marker toward multimodal, culture-independent assay panels and artificial intelligence-assisted risk stratification algorithms. Future clinical breakthroughs will depend heavily on the global standardization of disease definitions, robust external validation of predictive models, and the seamless integration of advanced laboratory techniques into multidisciplinary team (MDT) workflows.