BACKGROUND: Cross-sectional studies of hospital-level administrative data have shown an association between lower levels of staffing of registered nurses (RNs) and increased patient mortality. However, such studies have been criticized because they have not shown a direct link between the level of staffing and individual patient experiences and have not included sufficient statistical controls. METHODS: We used data from a large tertiary academic medical center involving 197,961 admissions and 176,696 nursing shifts of 8 hours each in 43 hospital units to examine the association between mortality and patient exposure to nursing shifts during which staffing by RNs was 8 hours or more below the staffing target. We also examined the association between mortality and high patient turnover owing to admissions, transfers, and discharges. We used Cox proportional-hazards models in the analyses with adjustment for characteristics of patients and hospital units. RESULTS: Staffing by RNs was within 8 hours of the target level for 84% of shifts, and patient turnover was within 1 SD of the day-shift mean for 93% of shifts. Overall mortality was 61% of the expected rate for similar patients on the basis of modified diagnosis-related groups. There was a significant association between increased mortality and increased exposure to unit shifts during which staffing by RNs was 8 hours or more below the target level (hazard ratio per shift 8 hours or more below target, 1.02; 95% confidence interval [CI], 1.01 to 1.03; P<0.001). The association between increased mortality and high patient turnover was also significant (hazard ratio per high-turnover shift, 1.04; 95% CI, 1.02 to 1.06; P<0.001). CONCLUSIONS: In this retrospective observational study, staffing of RNs below target levels was associated with increased mortality, which reinforces the need to match staffing with patients' needs for nursing care. (Funded by the Agency for Healthcare Research and Quality.).
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