Impact of Nurse to Patient Ratio Explanation

Nurse-Patient Ratio on Care Quality

Nurse Patient Ratio

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The Impact of Nurse to Patient Ratio on Healthcare Quality

The Impact of Nurse to Patient Ratio on Healthcare Quality

It would be hard to understate the importance of a high nurse to patient ratio (NPR) for patient and staff safety, as well as quality of care. While there are a number of different nursing factors that can influence these outcomes, including nursing education, experience, skills mix, contact time, frequency of interactions, and type of inpatient unit, the NPR has been the focus of considerable interest in part because it can be easily quantified (reviewed by Sidani, Manojlovich, and Covel, 2010). This review will examine the empirical evidence for the importance of NPR in determining patient and staff safety, as well as quality of care. Towards this goal, research articles obtained from the Library of Medicine will be reviewed in detail and compared to the findings of more recent research studies. This approach is intended to establish a historical foundation for this topic and then use it for elaborating on the different variables that influence the predictive value of the NPR.

Patient Safety

When the Institute of Medicine (2000) published their report on patient safety, To Err is Human, the American public woke up to the hidden reality that hospitals can be bad for patient health. The authors of this report estimated that between 44,000 and 98,000 people died each year as a result of medical errors. To put these numbers into perspective, this is more deaths than those caused by vehicle accidents, breast cancer, or AIDS.

In the aftermath of the Institute of Medicine report, Aiken and colleagues (2002) published a large study in the Journal of the American Medical Association revealing NPR to be a significant contributor to medical errors. Included in the study were 168 Pennsylvania general hospitals encompassing 232,342 patients and 10,184 nurses. The NPRs were broken down into less than or equal to 1:4 (11.9%), 1:5 (38.1%), 1:6 (24.4%), 1:7 (17.3%), and 1:8 or greater (8.3%). Based on the discharge abstracts for the patients included in the study, 23.2% experienced a major complication that emerged after being admitted to the hospital and another 2.0% died within a month. All patients were admitted for surgery: orthopedic (51.2%), gastrointenstinal (36.4%), and other (12.4%).

The main factors investigated in the study by Aiken and colleagues (2002) were the relationship between NPR and nurse burnout and patient adverse events. Their data reveal that for every patient increase in the NPR, nurses were 23% and 15% more likely to report feeling burned out and dissatisfied with their job experience, respectively. Every increase in NPR by one patient also increased the risk that a patient would die by 7%. To put this last estimate in perspective, increasing the NPR from 4:1 to 8:1 would result in 18.2 and 5.0 excess deaths per 1000 patients with and without complications, respectively. While there are some limitations to this study, including a hospital selection bias and the inclusion of only a few confounding factors, the large representative sample the authors were able to obtain created a high degree of confidence in their findings.

In support of Aiken and colleague’s (2002) findings, a number of studies have investigated the same issue during the years since the Pennsylvania hospital study was published. However, none have conducted a controlled study of an intentional change in staffing levels. To get around these limitations, researchers have taken advantage of below target staffing levels to see if this tended to increase adverse outcomes for patients within the same hospital. By taking this approach, such variables as differences in organizational attitudes, the quality of nursing staff, and other variables would be minimized or eliminated.

This approach was taken by Needleman and colleagues (2011) at a major medical center when they examined almost 200,000 records for adult patient admissions to see whether there was an association between units operating with below target staffing levels and increased patient mortality. They also controlled for a number of other confounding factors, such as day vs. night shift and type of unit, and purposely selected a well-respected, high quality tertiary care hospital with a low NPR. Taking this approach allowed them to ascertain whether subtle changes in nurse staffing levels had a negative impact on patient safety. They also controlled for the impact that patient turnover would have on nursing load and patient mortality.

Staffing levels were generally found to be near target levels for most units, but 16% were more than 8 hours below target, including over 19% of ICUs (Needleman et al., 2011). Differences between daytime and nighttime staffing were limited to step-down and general care units, with higher staffing levels in the daytime. After adjusting for selected confounding factors, the authors found that the risk of death increased significantly [Hazard Ratio (HR) = 1.02, 95% CI, 1.01-1.03, p < 0.001] if staffing levels dropped more than 8 hours below target levels or a shift experienced high patient turnover (HR = 1.04, 95% CI, 1.03-1.10, p < 0.001). Both results are consistent with the theory that any factors that increase nursing workloads will tend to have a negative impact on patient safety, including NPR.

The study by Needleman and colleagues (2011) revealed how patient safety can be degraded when the NPR does not meet target goals in a high-quality hospital, but it also revealed that more subtle changes in nurse workload can still have a significant negative impact on patient mortality rates. What this implies is that NPR functions as a healthcare quality indicator because it has a dramatic impact on nurse workload.

One of these aspects is the ratio of nursing training and expertise, such that an increased percentage of well-trained and experienced nurses would tend to decrease negative patient outcomes. This implies that more nursing assistants will not make up for a lack of RNs and LPNs. A longitudinal and cross-sectional study in Australia examined the effect of an equivalent RN:LPN:NA ratio in randomly selected medical, surgical, ICU, and emergency units in 19 hospitals (Duffield et al., 2011). The percent RNs ranged from a low of 45% to a high of 100%. The nursing workload and demand was calculated, such that a score of 100 indicated a balance. Among all units, about 25% had scores of 100 or less, which implies that nursing workload was above recommended levels for 75% of all units, with the highest score reaching 250. NPRs varied from 1:6 to 1:10, but the RN to patient ratio averaged around 1:8.

The longitudinal arm of the study revealed that over the 5-year study period, units that experienced a higher percentage of RNs (including clinical nursing specialists) had lower rates of patient adverse events (Duffield et al., 2011). The three patient outcomes significantly associated with a low RN to patient ratio was bedsores, pneumonia, and sepsis (p ? 0.01). Other factors that significantly influenced patient outcomes in a negative manner were patient turnover rates, temporary vs. permanent nursing staff, threats of physical violence, and patient acuity. All these factors tend to reduce the effective nurse staffing level for a unit because they increase the workload on nursing staff. These findings reveal that the importance of NPR to patient safety depends on more than simply having a nurse attending to a patient’s needs, but also the skills level of the nurse.

A similar study was conducted by Patrician and colleagues (2011a) in 13 U.S. military hospitals. They discovered that the percentage of RNs made a difference on patient safety when measured in terms of falls, falls with injury, and medical errors, but in a more nuanced way. The analysis was broken down into three types of medical units: medical-surgical, step-down, and critical care. Each 10% increase in RN percentage reduced falls in medical-surgical and critical care units by 30% and 36%, respectively. The training and experience of the RN also mattered, since each 10% decrease in the presence of a civilian RN increased the chance of a fall by 33-48% and medical errors by up to 67%. The other RNs on staff were military, contractor, and reservist. The reason civilian nurses provided a significant advantage to patient safety was because their average experience level was 14 years, compared to 5 years for military nurses. A higher NPR reduced both falls and falls with injury. In addition, most falls occurred during night shifts. Reduced medical errors were associated with more RNs and civilian RNs per shift, higher NPR, and night shift.

Patrician and colleagues (2011a) admit that there are several weaknesses to their study, including a reliance on the more controversial incident reports. Such reports are viewed by many researchers as an invalid measure because incidents tend to be underreported for fear of reprisal and litigation. The other weaknesses mentioned were not adjusting for differences in the risk of falling and medical errors for patients, but the authors argue that dividing up the data by unit type tended to provide an equivalent effect.

Staff Safety

While incident reports may be susceptible to underreporting, the same is probably less true for needle stick injuries since the risk of blood borne disease is on the minds of everyone today. Patrician and colleagues (2011b) examined the same military hospital data and looked at the interaction between NPR and nurse skills mix (RNs vs. LPNs vs. NAs) and the incidence of needle stick injuries. The rate of needle stick injuries was equivalent between the three types of units examined, which were medical-surgical, step-down, and critical care; however, higher NPR (except for step-down units), percentage of RNs, civilian RNs, and night shift was significantly associated with a lower risk of needle stick injury. These findings suggest that NPR is important for staff safety too.

An important caveat to the findings of Patrician and colleagues (2011) is that needle stick injuries can still be underreported to some extent, but less likely if the offending needle has been contaminated with a patient’s blood. Accordingly, 79% of the needle stick incident reports in the study involved contaminated needles. While needle sticks with uncontaminated needles may be underreported, this statistic was still sufficient to reveal the protective effect of a low NPR and better trained and more experienced nurses.

Quality of Care

Some of the most vulnerable patients are located in neonatal ICUs. For this reason, researchers interested in the impact of nursing care quality on health outcomes have studied these patients and their caregivers. Mefford and Alligood (2011) examined the impact of NPR and skills mix, among other variables, on the health outcomes of NICU and intermediate care nursery (IMCN) patients. The outcome measures were bronchopulmonary dysplasia, intraventricular hemorrhage, periventricular leukomalacia, retinopathy of prematurity, nosocomial infection, length of stay, and duration of mechanical ventilation, supplemental oxygen, and parenteral nutrition. NPR was found to be significantly (p ? 0.01) and inversely associated with morbidity in the NICU, but not in the IMCN, although there were several non-significant results that were trending towards significance. The only NICU outcome not reaching significance or trending towards significance was periventricular leukomalacia. A similar result was obtained when RNs were the primary caregivers in both the NICU and IMCN, such that the presence of RNs improved patient outcomes. These findings support the importance of NPR and skills mix for the quality of care provided to the youngest patients. The major limitations for these findings are the retrospective design and the limited sample size (single university teaching hospital).

The limitations of the above study can be moderated somewhat as additional independent studies are published. Sink and colleagues (2011) also examined patient outcomes for NICU patients in a university hospital setting as a function of NPR and found that the amount of time patients spent in the SpO2 target range (85-92%) declined significantly (p < 0.001) from 38% to 22% to 15% as NPR increased from 1:1 to 1:2 to 1:3/1:4, respectively. It should be noted that only 6% of the time was the NPR at 1:1 during the study period and the patient sample size was small at just 14. Additional findings were presented, but the small sample size precludes drawing any general conclusions about statistical interactions between ventilation method, NPR, and time spent in the SpO2 target range. However, these results tend to support the findings of Mefford and Alligood (2011) that NPR has a significant impact on patient outcomes assuming that all other variables are equal. An important aspect of the NICU studies is that the patient population could not have had a significant influence on the study’s findings.

Extending this approach to adults, Schwab and colleagues (2012) examined the patient outcome of nosocomial infections for almost 160,000 patients being cared for in 182 ICU across Germany. The independent variables were NPR, bed to patient ratio, and nurse to ventilated patient ratio. Both bed to patient ratio and nurse to ventilated patient ratio, but not NPR, significantly (p < 0.05) predicted the prevalence of nosocomial infections. Another potentially relevant finding is that ICUs with the highest bed occupancy had significantly (p < 0.05) lower infection rates, which would be consistent with the theory that nurse contact time with patients increases quality of care (Despins, Scott-Cawiezell, and Rouder, 2009). In other words, higher bed occupancy may create more opportunities for nurses to glance in the direction of other patients to ensure that they are stable. However, such a finding contradicts the notion that more patients per unit increases nurse workload, thereby decreasing patient safety and quality of care (Needleman et al., 2011; Duffield et al., 2011). The main limitation of this study was nurse staffing levels were based on annual reports, which tend to obscure reduced staffing levels due to holidays, vacation time, and sick days.

The reasons that a low NPR would contribute to poor quality of care may be more systemic than a few indicators, like NPR, can represent. To capture a more holistic view of the work climate that contributes to a low NPR, Paquet and colleagues (2013) looked at a number of psychosocial factors and their interactions with NPR, length of stay, and nosocomial infection rates. What they found was a low NPR was significantly (p < 0.01) associated with poor patient outcomes as a component of a poorly functioning workplace climate. Accordingly, higher rates of absenteeism, turnover, and overtime were also significantly (p < 0.01) associated with low NPR. It should be noted that the data used for this analysis represents an intermediate time point in a longer, 4-year study and therefore should be considered preliminary.


Earlier studies investigating the impact of NPR on patient safety helped awaken the medical field to the importance of having enough nurses staffing wards, but in the years since, investigators have begun to ask more nuanced questions about what factors influence the impact of NPR on patient safety, staff safety, and quality of care. For example, simply crowding wards with caregivers regardless of the level of nursing training and experience cannot replace the protective effect of RNs. Another aspect is discriminating the needs of patients in terms of contact time with RNs, which tends to increase the chances that an impending adverse event can be averted. In summary, a high NPR is an important quality of care indicator that must be qualified in terms of skill mix and patient need.


Aiken, Linda H., Clarke, Sean P., Sloane, Douglas M., Sochalski, Julie, and Silber, Jeffrey H. (2002). Hospital nurse staffing and patient mortality, nurse burnout, and job dissatisfaction. Journal of the American Medical Association, 288(16), 1987-1993.

Despins, Laurel A., Scott-Cawiezell, Jill, and Rouder, Jeffrey N. (2009). Detectiion of patient risk by nurses: A theoretical framework. Journal of Advanced Nursing, 66(2), 465-474.

Duffield, Christine, Diers, Donna, O’Brien-Pallas, Linda, Aisbett, Chris, Roche, Michael, King, Madeleine et al. (2011). Nursing staffing, nursing workload, the work environment and patient outcomes. Applied Nursing Research, 24, 244-255.

Institute of Medicine. (2000). To Err is Human: Building a Safer Health System. Online: National Academy Press. Retrieved 18 Apr. 2013 from

Mefford, Linda C. And Alligood, Martha R. (2011). Evaluating nurse staffing patterns and neonatal intensive care unit outcomes using Levine’s conservation model of nursing. Journal of Nursing Management, 19, 998-1011.

Needleman, Jack, Buerhaus, Peter, Prankratz, Shane, Leibson, Cynthia L., Stevens, Susanna R., and Harris, Marcelline. (2011). Nurse staffing and inpatient hospital mortality. New England Journal of Medicine, 364, 1037-1045.

Paquet, Maxime, Courcy, Francois, Lavoie-Tremblay, Melanie, Gagnon, Serge, and Maillet, Stephanie. (2013). Psychosocial work environment and prediction of quality of care indicators in one Canadian health center. Worldviews on Evidence-Based Nursing, published online April 30, 2012, 1-13.

Patrician, Patricia A., Donaldson, Nancy, Loan, Lori, Bingham, Mona, McCarthy, Mary, Brosch, Laura R. et al. (2011a). The association of shift-level nurse staffing with adverse patient events. Journal of Nursing Administration, 41(2), 64-70.

Patrician, Patricia A., Pryor, Erica, Fridman, Moshe, and Loan, Lori. (2011b). Needlestick injuries among nursing staff: Association with shift-levels staffing. American Journal of Infection Control, 39(6), 477-482.

Schwab, F., Meyers, E., Geffers, C., and Gastmeier, P. (2012). Understaffing, overcrowding, inappropriate nurse: Ventilated patient ratio and nosocomial infections: Which parameter is the best reflection of deficits. Journal of Hospital Infection, 80, 133-139.

Sidani, Souraya, Manojlovich, Milisa, and Covel, Christine. (2010). Nurse dose: Validation and refinement of a concept. Research and Theory for Nursing Practice: An International Journal, 24(3), 159-171.

Sink, David W., Hope, Shelly Ann E., and Hagadorn, James I. (2011). Nurse:patient ratio and achievement of oxygen saturation goals in premature infants. Archives of Disease in Children. Fetal and Neonatal Edition, 96(2), F93-F98.

Tremblay, Melanie, Gagnon, Serge, and Maillet, Stephanie. (2013). Psychosocial work environment and prediction of quality of care indicators in one Canadian health center. World Views on Evidence-Based Nursing, published online ahead of print April 30, 2012, 1-13.

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