Nurse staffing levels, missed vital signs observations and mortality in hospital wards: retrospective longitudinal observational study

Background: Low nurse staffing levels are associated with adverse patient outcomes from hospital care, but the causal relationship is unclear. Limited ability to observe patients at risk has been hypothesised as a causal mechanism but this has not been directly tested.

Objectives: This study determines whether adverse outcomes are more likely to occur after patients experience low nurse staffing levels and whether missed vital signs observations mediate any relationship.

Design: Retrospective longitudinal observational study. Multi-level/hierarchical mixed-effects regression models were used to explore the association between Registered Nurse (RN) and Health Care Assistant (HCA) staffing levels and outcomes, controlling for ward and patient factors.

Setting & Participants: 138,133 admissions to 32 general adult wards of an acute Hospital over three years (2012-2015).Main outcomes: Death in hospital, adverse event (death, cardiac arrest or unplanned ICU admission), length of stay, missed vital signs observations.Data sources: patient administration system, cardiac arrest database, e-roster, temporary staff bookings and the Vitalpac system for observations.

Results: 4.1% of patients died and 16% of observations were missed. Over the first five days of stay, each additional hour of care relative to the mean was associated with a 3% reduction in the hazard of death (HR .97 95% CI .94-1.0). Days where HCA staffing fell below the mean were associated with increased hazard of death (HR 1.04 95% CI 1.02-1.07) but the hazard of death increased as cumulative staffing exposures varied from the mean in either direction. Adverse events and length of stay were reduced with higher RNHPPD. Higher RNHPPD were associated with fewer missed observations in high acuity patients (IRR .98 95% CI .97-.99) whereas the overall rate of missed observations was related to overall care HPPD (RN+HCA) but not skill mix. The relationship between low RN staffing and mortality was mediated by missed observations but other relationships between staffing and mortality were not. Changing average skill mix and staffing levels to the levels planned by the trust, involving an increase of .32 RNHPPD and a decrease in HCAHPPD would be associated with an increase in staffing costs of £28 per patient with a net saving of £0.52 per patient after taking into account the value of reduced stays.

Limitations: This was an observational study and there were limitations in the accuracy of our staffing data. Variation in staffing could be influenced by variation in the assessed need for staff.

Conclusions: Higher RN staffing levels are associated with lower mortality and this study provides evidence of a causal mechanism. There may be several causal pathways and the absolute rate of missed vital signs observations cannot be used to guide staffing decisions. Increases in nursing skill mix may be cost effective for improving patient safety.

Future work: More evidence is required to validate approaches to setting staffing levels. Other aspects of missed nursing care should be explored using objective data and the implications of findings about temporary staffing for flexible approaches to staffing need further exploration.

Study registration: ISRCTN17930973Funding details: NIHR Grant number 13/114/17