Adjusted percentage change in inpatient BSI frequency in spring, summer, and autumn compared with winter.*

BSI = bloodstream infection; CI = confidence interval; IQR = interquartile range.*Percentage change = 100×(relative BSI frequency−1). Estimated using Poisson mixed-effects regression models with random intercepts to account for within-site correlation, natural cubic splines (with 7 degrees of freedom) to adjust for long-term trends, and adjustment for census region.