how do you compute peritonitis rate

The ISPD recommends reporting peritonitis rate as the number of episodes per patient-year at risk. The “time at risk” for each patient begins on the first day of planned long-term PD (whether during training, in the hospital, or at home) and continues as long as the patient remains on PD, regardless of setting or caregiver. Total patient days at risk, divided by 365, equals the patient-years experience. To calculate the rate, divide the total number of peritonitis episodes within a period (usually 12 months) by the total patient-years of PD experience during that time. Relapsing peritonitis (same organism, <4 weeks post-treatment) counts as one episode; recurrent peritonitis (different organism or >4 weeks) counts as new. The ISPD target is no more than 0.40 episodes per patient-year, with a goal of >80% peritonitis-free patients annually. An alternative reporting method is one episode per patient-months.
References
Li PK, Szeto CC, Piraino B, de Arteaga J, Fan S, Figueiredo AE, Fish DN, Goffin E, Kim YL, Salzer W, Struijk DG, Teitelbaum I, Johnson DW. ISPD Peritonitis Recommendations: 2016 Update on Prevention and Treatment. Perit Dial Int. 2016 Sep 10;36(5):481-508.