Lower RN staffing levels increase risk for inpatient death, according to new study

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Hospitalised patients were more likely to die when registered nurse (RN) staffing levels fell below the ward average, according to a retrospective analysis of data from a large acute care hospital. Each day spent in an RN-understaffed ward over a 3-year period conferred a 3% rise in mortality risk, whereas each additional hour of care provided by an RN was associated with a 3% reduction in the chances of dying, the study found.

By contrast, a high level of nursing assistant staffing did not reduce mortality risk and in fact was associated with an increased risk for death, the authors write in BMJ Quality and Safety.

The study, “Nurse staffing, nursing assistants and hospital mortality: retrospective longitudinal cohort study” was led by Peter Griffiths, PhD, from the Faculty of Health Sciences at the University of Southampton in the United Kingdom.

The researchers studied the effect of periods of low RN and nursing assistant staffing on risk for death among patients in general hospital wards.

They identified 138,133 patients who spent 1 or more days in one of 32 general adult medical and surgical wards of a large acute care hospital between April 2012 and March 2015. Patient data were extracted from the hospital's electronic administration system and an electronic system used to record patients' vital signs. These data were linked to nurse staffing data that included information on shifts worked, location, hours, and grade.

For their analysis, the investigators considered staffing levels with respect to hours per patient day for each staff group (RNs and unlicensed nursing assistants). The primary study outcome was in-hospital mortality during the first 5 days of stay, when patients are most likely to be acutely ill. A secondary outcome was death within the first 30 days of admission.

Of the admitted patients (mean age, 67 years), 50% had no comorbidities, and 35% had a Charlson Comorbidity Index of 5 or greater. In total, 5662 patients died, most commonly from pneumonia (4.2%).

The mean staffing levels across all wards were 4.75 RN hours per patient per day and 2.99 nursing assistant hours per patient per day. The mean skill mix was 61% RN. During the first 5 days of hospitalisation, the full patient cohort experienced a mean of 1.93 days of low RN staffing and a mean of 1.94 days of low nursing assistant staffing.

Each day spent on a ward that was understaffed by RNs increased patients' risk of dying by 3% (adjusted hazard ratio [aHR], 1.03; 95% confidence interval [CI], 1.01 - 1.06), and each day of exposure to low nursing assistant staffing increased the risk for death by 4% (aHR, 1.04; 95% CI, 1.02 - 1.07). On wards where there was high patient turnover (admissions per RN exceeding 125% of the mean for that ward), the risk of dying rose by 5% (aHR, 1.05; 95% CI, 1.01 - 1.09). There was "no significant association between admissions per nursing assistant and death," the researchers write.

Similar patterns were observed for deaths within 5, 10, and 30 days of admission, "although for deaths within 5 days the effect of low RN staffing was greater and the effect of low nursing assistant staffing smaller and non-significant," the authors note.

Neither the addition of variables to account for weekend admissions or weekend stays nor the consideration of the possible co-occurrence of low staffing with seasonal infections changed the association patterns meaningfully.

To explore the effect of absolute variations in staffing levels on mortality risk, the researchers calculated the sum of staffing in hours per patient day relative to the mean for each ward for each patient for each of the first 5 days.

"This gives an indication of the average staffing experienced by the patient, relative to what was normal for each ward," the authors explain. The adjusted hazard ratio decreased by 3% for every additional RN hour per patient (aHR, 0.97; 95% CI, 0.94 - 1.00). No association emerged with additional nursing assistant hours.

When the researchers treated staffing as a continuous variable, they observed a linear effect for the association between RN staffing and mortality (more RN hours per patient reduced mortality), but not for nursing assistant staffing. "The hazard of death was increased when patients were exposed to either more than average or less than average nursing assistant hours over the course of their stay," the authors observe.

In addition to confirming the results of previous research linking RN staffing and patient outcomes, the study findings add important insights, the authors state. "First, the association is demonstrated at a patient level with a longitudinal association between variation in staffing and outcome, providing important confirmation that the hospital-level cross-sectional associations reflect individual patient exposures," they write. "Second, we have demonstrated that the relationship appears to be linear with no threshold effect over the range of variation that we observed."

These results suggest that staffing measures that combine RN and nursing care assistant hours into one mixed-skill measure are "unwarranted," the authors write. "Although our findings lend no support to a policy of compensating for deficits in the RN workforce by employing more nursing assistants, these results do show that an adequate number of assistants is important for maintaining patient safety."

From a policy perspective, the findings highlight the potential benefits of increasing the availability of RNs on acute hospital wards and the need for a policy to rectify the shortage of RNs and to retain existing RNs within the workforce, the authors explain. Increasing the numbers of lesser-trained nursing staff in the workforce is unlikely to remedy RN shortages, they add.

The authors stress that, as an observational study, causation cannot be established and should not be inferred. Further, because the investigation involved only one hospital, the findings may not be generalisable across all hospitals.