The aim of this study was to confirm that Mirra's criterion (≥ 5 Polymorphonuclears (PMNs) per field in 5 high power fields (HPFs)) is not adequate for diagnosis of chronic bone and joint infections (BJIs) due to Cutibacterium acnes (C. acnes). The second objective was to determine if plasma-cell infiltration, that is a classical marker of chronic inflammation, could be useful for the diagnosis of chronic BJIs due to C. acnes.
We retrospectively selected 25 patients from 2009 to 2013 with chronic BJIs due to C. acnes. In addition of Mirra's criterion, the number of plasma-cells (≥5 plasma-cells/5 HPFs, defined as “CRIOAc Lyon's criterion”) was implemented in the histopathological analysis. Patients were defined as infected, if at least one of the two criteria were present.
According to Mirra's and CRIOAc Lyon's histopathological criteria, positive histology was observed in respectively 12 (48%) and 16 (64%) cases. In 1 case the samples were not analyzable. Considering the 12 cases with negative Mirra's criterion, high plasma-cell infiltration (≥5 plasma-cells/5 HPFs; Figure 1) was observed in 6 cases (50%), and low plasma-cells infiltration (2–5 plasma-cells/5 HPFs) was observed in 5 other cases (42%).
Mirra's criterion is not an adequate criterion to defining chronic BJIs [1, 2]. In our study, more cases of chronic BJIs due to C. acnes have been diagnosed using CRIOAc Lyon's criterion than Mirra's criterion. Adding CRIOAc Lyon's criterion might restore some histopathological diagnosis of chronic BJIs due to C. acnes, when a clinical chronic BJI is suspected.
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