A statistical approach to an outbreak of endophthalmitis following cataract surgery at a hospital in the West of Scotland

The number of cases of endophthalmitis following cataract surgery caused co nsiderable concern in a West of Scotland hospital throughout 1998 and early 1999. A multi-disciplinary team including infection control nurses, doctor s, public health officials, epidemiologists and statisticians was set up to investigate the situation. This paper examines the statistical issues surr ounding the investigation. A method based on the Poisson distribution showe d that the number of cases,vas significantly higher than expected. Fisher's Exact Test and Logistic Regression were then applied to the data from two related case control studies. These analyses showed that a higher risk of e ndophthalmitis was associated,vith being female, having a vitrectomy or hav ing a previous history of respiratory disease. Finally, a method,vas devise d to enable staff to recognize more quickly when the number of cases of end ophthalmitis was becoming higher than expected. The method should find appl ication in other clinical situations where the probability of rare events i s known. (C) 2001 The Hospital Infection Society.