Although relatively rare (under 5%), surgical site infections are the most common complication of planned operations (hospital admissions for what is termed “elective surgery”). Other complications (such as needing to be re-operated on, or even dying) are also rare, but may also be related to underlying infections, even when these cannot be directly identified by growing micro-organisms from patient samples. These complications cause ill-health and are extremely distressing for patients. Some predictors of complications are known – for example, making sure patients glucose levels remain in normal ranges, and that they do not get too hot or too cold reduces the risk of complications. However, recent studies in other infections including tuberculosis and malaria suggest that the relative numbers of some different types of blood cells might also be related to poor outcomes. We want to find out whether these blood cell values could also be a predictor of complications following planned operations, and whether this could be a useful way to identify patients at high risk, who could then be managed more intensively to reduce complications.