**2.3.2 Microscopic detection of inflammatory cells**

**Joint aspiration fluid.** Total and differential white blood cell counts in synovial fluid are routinely determined in many settings. Some studies of knee patients have reported that total leukocyte counts or neutrophils percentages which exceed a certain cutoff level are highly indicative of arthroplasty infection. However, the thresholds differ considerably between studies (Della Valle et al., 2007; Ghanem et al., 2008; Trampuz et al., 2004).

In contrast, there are less data available for hip patients because aspiration of this joint is more prone to complications and is therefore only recommended if there is substantial clinical or laboratory evidence for infection (Della Valle et al., 2010; Schinsky et al., 2008).

**Frozen tissue sections.** Neutrophils are the predominant histomorphologic factor in periprosthetic infection (Krenn et al., 2011; Morawietz et al., 2006). As a consequence, the histologic diagnosis of probable infections is based on the tissue neutrophil concentration, as defined by i) the number of neutrophils in a high-power (400x) microscopic field, and ii) the minimum number of fields (usually 10) containing that concentration of neutrophils. The available studies report 5 or 10 neutrophils per high-power field as suitable thresholds for diagnosis of arthroplasty infection (Banit et al., 2002; Della Valle et al., 2007; Fehring & McAlister, 1994; Fink et al., 2008; Frances Borrego et al., 2007; Ko et al., 2005; Lonner et al., 1996; Nunez et al., 2007; Schinsky et al., 2008). Patients with inflammatory arthropathy, which often display tissue infiltration by neutrophils in the absence of infection, were excluded in some of these investigations (Fehring & McAlister, 1994; Ko et al., 2005; Pandey et al., 1999; Schinsky et al., 2008). However, all in all there is not enough information to enable a clear-cut preference of the lower or the higher threshold.

Despite the considerable advances in recent years with respect to the histomorphologic characterization of periprosthetic infections, it is not possible to treat affected patients sufficiently unless the causative microorganisms are identified precisely. Thus, customized local and systemic antibiotic therapy of a known infectious agent is inherently superior to calculated therapy because treatment failure arising from antibiotic resistance can be avoided (Bejon et al., 2010).

Infections in Hip and Knee Arthroplasty:

**3.2.2 Joint aspiration fluid** 

Challenges to and Chances for the Microbiological Laboratory 443

absorb nearly as much material as can be harvested from tissue biopsies or joint fluid, which alone would account for inferior sensitivity. Furthermore, as most etiologic agents of arthroplasty infections belong to the normal skin flora, it is hardly possible to discern between infectious strains and contaminants using swab material. In summary, tissue swabs

The overall significance of culture from pre-operative synovial fluid to detect periprosthetic infection is valued as high. However, sensitivity may be reduced if the infection does not involve the synovia or if the concentration of planktonic bacteria in the fluid is limited due to a mature biofilm. Furthermore, false positive results from skin flora occur (Barrack & Harris, 1993; Della Valle et al., 2007; Eisler et al., 2001; Fink et al., 2008; Lachiewicz et al.,

Analysis of tissue biopsies offers the advantage that multiple samples can be obtained from different locations within the suspicious area. Repeated isolation of bacteria (e. g., isolation of the same organism in at least 2 tissue samples) increases the probability of infection. Thus, there are several reports in which higher sensitivity of tissue culture compared with synovial fluid culture is observed (Fink et al., 2008; Meermans & Haddad, 2010; Roberts et

Pre-analytical sampling errors have been claimed to contribute significantly to falsenegative culture results, with highly sensitive PCR techniques being a means to overcome these drawbacks (Achermann et al., 2010). Guidelines devised by the German Society of Hygiene and Microbiology have proposed that periprosthetic sample material intended for microbiological cultivation should be processed within one hour post-drawing. Although such stringent demands are not realistic for the routine setting, it is indeed crucial to establish a standardized work flow between the clinic and the laboratory regarding the procedures of sample drawing, transportation to the laboratory, and specimen processing. The organizational structure in our laboratory comprises a courier service as well as evening and weekend laboratory duty, which ensures that over 95% of culture samples are

The efforts made to obtain significant sample material are futile if the laboratory process is

The simplest way of tissue sample processing is mincing by a scalpel. This allows efficient investigation of multiple samples from each patient (see 3.2.3). If carried out under a laminar air flow workbench it is not highly prone to contamination (Atkins et al., 1998; Schäfer et al.,

Some authors favor scraping the surface of the explanted material. This has been reported to be more sensitive than tissue culture but also liable to contamination (Bjerkan et al., 2009;

cannot be recommended to assess prosthetic infections reliably.

1996; Malhotra & Morgan, 2004; Williams et al., 2004).

al., 1992; Sadiq et al., 2005; Williams et al., 2004).

**3.3 Logistics between clinic and laboratory** 

processed within 6 hours post-operatively (see 4.1.1).

not optimized. However, costs and benefits should be well-balanced.

**3.4 Sample processing** 

**3.4.1 Native material** 

Neut et al., 2003).

2008; Trampuz et al., 2006, 2007).

**3.2.3 Periprosthetic tissue biopsies** 
