**4.1.1 Samples and diagnosis**

An overview of the laboratory procedures is given in Table 2.


Table 2. Overview over the laboratory methods used to detect arthroplasty infection.

The definitive diagnosis of arthroplasty infection is established with multiple tissue biopsies taken from the periprosthetic membrane and other macroscopically conspicuous sites during revision surgery. We have experienced that the inflammation process can be assessed with high reliability if 5 samples each are obtained for culture and for histologic analysis. The definition that i) growth of indistinguishable bacteria in ≥ 2 specimens or ii) microbial growth in one specimen combined with a histology score of 3+ (≥ 5 neutrophils per high-power field in 10 fields) (Atkins et al., 1998; Pandey et al., 1999; Virolainen et al., 2002) has proven feasible with respect to the clinical outcomes (Fink et al., 2008, 2009).

Until now we use native tissue biopsies for culture. Tissue mincing is simple to perform and not too prone to contamination if carried out under a laminar air flow workbench. In our opinion, the prolonged incubation period of 14 days we carry out before cultures are cleared is a decisive measure. The persuasiveness of this approach has been shown in detail in 3.6.2. Although it would be interesting to compare the allegedly most sensitive sonication culture method directly with prolonged tissue cultivation, we doubt that the cumbersome and potentially contamination-prone sonication concept would prove significantly superior to our own approach.

As we are convinced that prolonged cultivation over 14 days is the key to detecting infecting organisms with optimal sensitivity, we also currently refrain from using PCR techniques on a routine basis.

If overnight storage of unprocessed samples is necessary, which occurs in less than 5% of cases in our setting, tissue specimens are kept at 4°C and processed at the laboratory the next morning with highly reproducible results. Synovial fluid, when inoculated into pediatric blood culture vials immediately post-drawing, is stable for at least 24 hours at room temperature. Subsequent supplementation with the appropriate enhancing medium, which is necessary to cultivate blood-free sample fluids, can then be done at the laboratory.

Taken together, our diagnostic measures have contributed significantly to the high eradication rates we observe with the treatment of both hip and knee arthroplasty infections (Fink et al., 2008, 2009).
