A statistical method for estimating the proportion of cases resulting from cross-transmission of multi-resistant pathogens in an intensive care unit.

Multi-resistant bacteria are an increasing challenge for infection control in hospitals and the proportion of patients newly colonized with multi-resistant bacteria during their hospital stay can be used to assess the effectiveness of infection control measures. Current laboratory methods for estimating this require expensive additional tests. We propose an alternative statistical method to estimate the proportion of cases resulting from transmission in a hospital from the distribution of time intervals between subsequent cases. A prerequisite for the application of this method is the existence of records from regular screening of the patients during their hospital stay, usually performed in intensive care units (ICUs). We describe the method and present an example of its application using records of two multi-resistant pathogens collected in an ICU over a three-year period. The estimated proportion of cases resulting from transmission was 0.73 (95% CI: 0.56-0.90) for meticillin-resistant Staphylococcus aureus and 0.45 (95% CI 0.15-0.75) for imipenem-resistant Pseudomonas aeruginosa. The method proposed here can be used for retrospective evaluation of clinical records in order to evaluate the effectiveness of infection control measures in low endemicity settings.