The audit cycle has four parts: setting standards, testing practice and outcome against these standards, correcting practice where it falls short, and re-auditing to confirm that standards are now met. The last two steps complete the audit cycle or 'close the loop'. Infection control can be audited for whether written guidelines exist for each procedure, whether these guidelines are acceptable and whether they are followed in practice. This form of audit is relatively simple provided there are authoritative model guidelines available, produced by organizations such as the Hospital Infection Society. The best outcome measure of infection control is the nosocomial infection rate, but this is the most difficult to audit in practice. If audit results are to be compared between hospitals or between different time periods in the same hospital, infection rates should be adjusted for the underlying risks of infection in the patient population. This type of audit can be done by repeated prevalence surveys.