Day and night nurse staffing levels and hospital-associated disability in older adults in Japan: a retrospective cohort study

Hospital-associated disability (HAD) in older adults often results from preventable factors. Nurse staffing at the ward level is a key modifiable factor in mitigating HAD. To investigate the relationship between nurse staffing shortfalls during typical day and night shifts and the risk of HAD in older adults in acute care hospitals. Retrospective cohort study. General medical or surgical wards in nine acute care hospitals in Japan. Hospital admissions of patients aged ≥65 years. Electronic claims data and daily ward-level nursing rosters were used to measure nurse staffing shortfalls based on the patient-to-nurse ratio relative to the annual mean. Among 57 498 hospital admissions [23 981 (41.7%) female, median age 76 (interquartile range [IQR] 71-76) years], 30 507 (70%) were functionally independent at admission. Functional deterioration occurred in 15 458 (26.9%) admissions. The median (IQR) patient-to-nurse ratio across the whole day was 6.11 (5.54-6.59), 3.70 (3.34-3.90) for the day shift, and 9.38 (8.57-10.37) for the night shift. The median (IQR) deviation from typical ward-level staffing was -0.21 (-0.64 to 0.20) for the whole day, -0.23 (-0.56 to 0.09) for the day shift, and 0.01 (-0.51 to 0.49) for the night shift. For each patient above the mean patient-to-nurse ratio, the risk of HAD increased by 7%. Attention to the deviations from the usual staffing levels for both day and night shifts may be important in efforts to reduce the risk of HAD in older adults.