Negative microbiological cultures are often interpreted as evidence against infection. In fracture-related infection, this assumption is increasingly difficult to sustain. Clinical definitions now explicitly recognise infection in the absence of microbiological confirmation, raising the question of why cultures fail and how those results should be interpreted.
Summary
A consistent minority of fracture-related infections are culture-negative despite confirmatory clinical features
Low bacterial load, biofilm localisation, and patchy spatial distribution limit culture sensitivity
Host inflammatory markers may remain normal in low-grade infection
Negative cultures reflect biological constraints rather than diagnostic failure
Why this matters
Fracture-related infection complicates approximately 1–2% of closed fractures and up to 30% of open fractures, with substantial consequences for healing, function, and need for revision surgery (Baertl et al., 2021; Rupp et al., 2024). Reliable diagnosis is therefore critical. When cultures are negative, clinicians face uncertainty: whether infection is absent, missed, or biologically concealed.
The 2018 consensus definition of fracture-related infection addressed this problem directly by allowing diagnosis in the absence of microbiological confirmation when other confirmatory criteria are present (Metsemakers et al., 2018; Morgenstern et al., 2018).
What the evidence shows
A definable subgroup of fracture-related infections remains culture-negative
Validation studies of the fracture-related infection definition consistently show that approximately 5–15% of cases classified as infection have negative cultures despite appropriate sampling (Onsea et al., 2022; Vanvelk et al., 2023). These patients do not behave like aseptic cases. They show recurrence patterns, delayed healing, and need for further intervention consistent with infection rather than mechanical failure alone (Onsea et al., 2022).
This observation underpins the inclusion of the category “infection likely” in the fracture-related infection framework (Metsemakers et al., 2018).
Culture sensitivity is limited by bacterial distribution and load
Microbiological cultures depend on sampling tissue that contains a sufficient number of viable organisms capable of growth under laboratory conditions. In chronic or delayed fracture-related infection, bacteria may be present in low numbers, unevenly distributed, or predominantly localised on implant or bone surfaces rather than in surrounding soft tissue (Onsea et al., 2018).
Dudareva et al. showed that even with multiple deep tissue samples and strict diagnostic thresholds, culture sensitivity for fracture-related infection remained approximately 60–68%, despite high specificity (Dudareva et al., 2021). Importantly, this included patients without recent antibiotic exposure, supporting a biological rather than iatrogenic explanation for negative cultures.
Biofilm behaviour limits detectability
Low-virulence organisms commonly implicated in fracture-related infection, such as coagulase-negative staphylococci and Cutibacterium acnes, preferentially persist in biofilm at the bone–implant interface (Onsea et al., 2018). In this state, bacteria may be metabolically less active and less likely to be recovered through routine tissue culture.
This behaviour explains why microbiological yield can remain low even when infection is clinically established, and why adjunctive techniques such as sonication improve yield without achieving complete sensitivity (Bellova et al., 2021).
Host inflammatory markers may remain normal
Low-grade fracture-related infection often produces a limited systemic inflammatory response. CRP and leukocyte counts may be normal or only mildly elevated, reflecting a localised process rather than systemic infection (Govaert et al., 2020).
Histopathology adds diagnostic information but is also spatially constrained. Neutrophil infiltration may be present in some regions and absent in others, meaning sampled sections can appear negative despite infection elsewhere along the bone–implant interface (Morgenstern et al., 2018).
Mechanisms behind the pattern
Culture-negative fracture-related infection emerges from the interaction of three constraints.
First, bacteria are unevenly distributed and often sequestered in biofilm.
Second, bacterial metabolic activity may be low, reducing culturability.
Third, sampling captures only a fraction of a spatially complex interface.
These constraints persist even with appropriate sampling strategies and experienced microbiological processing.
Practical implications for clinical interpretation
Negative cultures do not exclude fracture-related infection when confirmatory clinical or radiological signs are present (Metsemakers et al., 2018)
Microbiological results should be interpreted alongside clinical, histological, and imaging findings rather than in isolation
Culture-negative infection represents a recognised biological pattern, not a diagnostic anomaly
Diagnostic frameworks explicitly accommodate uncertainty rather than requiring microbiological proof in all cases
Common pitfalls
Treating negative cultures as evidence of absence rather than limited detectability
Attributing culture negativity primarily to laboratory or sampling error
Expecting systemic inflammatory markers to be elevated in all infections
Assuming culture-negative cases behave like aseptic non-unions
Closing note
Culture-negative fracture-related infection is not a failure of microbiology but a reflection of how bacteria persist within bone and implants. Recognising this shifts interpretation away from blame and towards integration of biological, clinical, and diagnostic signals.
References
Baertl S et al. Fracture-related infection. Bone Joint Res. 2021.
Foster AL et al. A population-based epidemiological and health economic analysis of fracture-related infection. Bone Joint J. 2024.
Metsemakers WJ et al. Fracture-related infection: a consensus on definition from an international expert group. Injury. 2018.
Morgenstern M et al. Diagnostic challenges and future perspectives in fracture-related infection. Injury. 2018.
Onsea J et al. Validation of the diagnostic criteria of the consensus definition of fracture-related infection. Injury. 2022.
Vanvelk N et al. Diagnosis of fracture-related infection in patients without clinical confirmatory criteria: an international retrospective cohort study. Injury. 2023.
Onsea J et al. Accuracy of tissue and sonication fluid sampling for the diagnosis of fracture-related infection: a systematic review and critical appraisal. J Bone Jt Infect. 2018.
Dudareva M et al. Providing an evidence base for tissue sampling and culture interpretation in suspected fracture-related infection. J Bone Joint Surg Am. 2021.
Govaert GAM et al. Diagnosing fracture-related infection: current concepts and recommendations. J Orthop Trauma. 2020.
Bellova P et al. Sonication in the diagnosis of fracture-related infections (FRI): a retrospective study on 230 retrieved implants. J Orthop Surg Res. 2021.