**3.1.1 Sample origin (hip versus knee)**

Joint aspiration prior to revision arthroplasty is widely utilized. For knee patients the procedure is comparatively straightforward, whereas hip aspiration may impose a higher risk of iatrogenic infection. Thus, it is often argued that invasive diagnostic samples from hip patients should be obtained only if a there is a high probability of infection (Bozic et al., 2009, 2010).

Regarding periprosthetic tissue biopsies, the diagnostic sensitivity of pre-operative sampling may be lower in hip infections compared with knee infections (Fink et al., 2008; Meermans & Haddad, 2010; Williams et al., 2004), possibly because infected tissue is more difficult to assess without dislocating the joint.

### **3.1.2 Underlying systemic diseases**

The definitive identification of microorganisms is especially important in patients with systemic inflammatory diseases because, as mentioned before, inflammation markers can be elevated in aseptic implant failure. At the same time, the differentiation between an infecting and a contaminating agent is challenging in these patients (see 3.7).
