The incidence of SSI varies widely between hospitals and between surgical categories in NHS hospitals in England (Public Health England (PHE)2013). This may be because of differences in case ascertainment, clinical parctices and case mix. Categories with the highest degree of wound contamination have a higher incidence of infection For example, large bowel surgery has an incidence rate for SSI of 10.6%, while categories generally considered as associated with clean wounds, such as knee and hip surgery are lower-with an SSI incidence of less than 1%