Clinical, radiological and microbiological criteria have to be taken into account for the evidence of periprosthetic joint infection. This multidisciplinary approach is necessary, as there is still no reference diagnostic standard [28]. While Erythrocyte Sedimentation Rate (ESR) or serum C-reactive Protein (CRP) have not proved as secure preoperative infection markers, microbiological culture of synovial fluid or tissue samples is the present diagnostic gold standard, although molecular methods, biomarkers, such as alpha-defensin or sonication of the removed prosthesis are suggested to improve diagnostic performance [29, 30]. Varying specimen, such as synovial fluid, biopsies from bone or periprosthetic tissues or the sonication fluid from the removed prosthesis itself show different performances in different diagnostic procedures and complicate the comparison between existing studies [31]. Culture has a limited sensitivity and specificity due to previous antibiotic usage or contaminating skin-microorganisms, which are difficult to distinguish from true pathogens. Before conventional culture results are available, often a long time period of up to 14 days is needed for a conclusive result. Therefore, PCR-techniques have been focused on in recent years, having shown to detect PJI and providing information about causative pathogen in a timely manner [32]. Here, we inform of a investigation comparing conventional microbiological culture with two different PCR-methods using prospectively collected specimen from patients with suspected periprosthetic joint infection. In the present study the overall concordance rate between the mPCR and conventional culture with respect to the detection of any bacterial species was 75.6%. These results seem to be consistent with previous studies on PCR-techniques. Morgenstern et al. reports a concordance rate of 82% in 116 patients of septic and aseptic cases [33]. This is also in accordance with the results reported by Sigmund et al., showing a concordance rate between culture and mPCR about 85.6% in 90 patients [32]. Borde et al. conducted a investigation with 54 patients by comparing culture with 16S-rDNA-PCR and mPCR, reporting a concordance rate of 82% concerning culture and mPCR [34]. mPCR results in our study were also compared to the 16S-rDNA-PCR, and notwithstanding differences in the identity of the bacterial species detected an overall concordance rate of 82.9% was determined. Borde et al. report a concordance rate between culture and 16S-rDNA-PCR about 96% in their study. Our findings seem to be accordant with the existing data on PCR-techniques regarding diagnosis of PJI. Hirschebeth et al. report a sensitivity of 66.7% and a specificity of 100% for the Unyvero mPCR when compared to cultural method in a study with 62 specimen from 31 patients [35]. In our study we calculated a sensitivity of the Unyvero® mPCR of 33% and specificity of 91%. The sensitivity and specificity of the 16S-rDNArDNA PCR was calculated with 55% and 97%. Using conventional culture as reference, the positive predictive value for the presence of bacterial DNA in the sample and thus infection was calculated with 57% for the Unyvero® mPCR in our study, while Hirschebeth et al. reports 100% using sonication and synovial fluids [35]. Likewise, 16S-rDNA-PCR can identify pathogens in synovial fluid with a sensitivity and specificity of 84% and 89%, in sonication fluid of 81% and 96% as well as a high sensitivity and specificity in patients on antibiotics [36]. Restrictions of this sensitive technique are current its higher cost compared to culture and the susceptibility to contamination leading to false positive findings. In our study, the 16S-rDNA-PCR detected a Methylobacterium in one case, while the Unyvero mPCR remained without bacterial detection and the culture showed S. epidermidis. As well as its normal habitats in soil and water, Methylobacterium has also been recognized as a contaminant of DNA extraction kit reagents, which may result in its erroneous appearance in nucleic acid amplification techniques [37]. To reduce the false-positive rates, attempts have been made by establishing a detection threshold, but this could decrease the sensitivity of these tests [34, 38]. In addition, also a low bacterial inoculum found in positive conventional culture samples might be probably linked to a negative PCR-result [39]. Another potential limitation of the commercially mPCR test is to narrow only on the identification of a pre-selected list of causative pathogens. In particular, the Unyvero i60 ITI® is a mPCR dedicated to the diagnosis of prosthetic joint infections, considering only pathogens that are considered to the most relevant in causing PJI, including P. acnes. In our study only 13.3% (2/15) were positive in culture for bacteria not included in the Unyvero i60 ITI-panel (1x Micrococcus luteus, 1x Parvimonas micra). The simultaneous use of mPCR and culture could be a more sensitive diagnostic tool. At present, the mPCR cannot yet replace conventional microbiological culture, which has still to be considered as the standard of care in preoperative diagnostic of periprosthetic infection.
Our study has therefore limitations. To start with the overall number of patients (n = 51) and the number of culture-positive cases (n = 14) is rather low. Hence the concordance rate of 75.6% is predominantly based on concordant negative results (n = 30). Future investigations, with higher count of patients will be needed to more reliably establish sensitivity and specificity of mPCR or 16S-rDNA-PCR. Second, our study group contains different locations of possible infected implants including hip, knee and shoulder, as well as different types of specimen, such as synovial fluid or periprosthetic tissue. Forthcoming studies should focus on special locations and identically specimen.