This retrospective cohort study included all non-intensive care unit patients ≥50 years old hospitalized on a medicine service for >24 hours during winter (01/01-03/31, 2016-2017) and summer (07/01-9/30, 2016-2017) months. Hospitalized patients were screened for delirium by registered nurses using the Nursing Delirium Screening Scale (NuDESC) once every 12-hour shift as part of routine care. Prevalent delirium was defined as any NuDESC ≥2 during the hospitalization; severity was calculated as the sum of daily maximum NuDESC scores; and a delirium day was any day with a NuDESC ≥2. A multivariable model examined the relationship between season and delirium severity and prevalence adjusting for the following variables: age, sex, race, admission category, severity of illness quartile.

The cohort included 769 patients in winter and 792 patients in summer. Average age was 72±13 (winter) and 71±13 (summer); distributions of sex and race were similar. A total of 216 patients (28%) experienced delirium in winter, compared with 181 patients (23%) in summer. There were more days where patients experienced delirium in winter compared to summer (2056, 47% vs. 1358, 29%; p<0.0001). In the multivariable model, winter was associated with increased delirium severity (β=1.88, p=0.03, 95% CI 0.22-3.55). We did not find an association between season and delirium prevalence (OR 1.26; p=0.07; 95% CI 0.98-1.60).