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Patient Outcomes After the Introduction of Statewide ICU Nurse Staffing Regulations*

Law, Anica C., MD1,2; Stevens, Jennifer P., MD, MS1,2; Hohmann, Samuel, PhD, MS3,4; Walkey, Allan J., MD, MS5,6,7

doi: 10.1097/CCM.0000000000003286
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Objectives: To assess whether Massachusetts legislation directed at ICU nurse staffing was associated with improvements in patient outcomes.

Design: Retrospective cohort study; difference-in-difference design to compare outcomes in Massachusetts with outcomes of other states (before and after the March 31, 2016, compliance deadline).

Setting: Administrative claims data collected from medical centers across the United States (Vizient).

Patients: Adults between 18 and 99 years old who were admitted to ICUs for greater than or equal to 1 day.

Interventions: Massachusetts General Law c. 111, § 231, which established 1) maximum patient-to-nurse assignments of 2:1 in the ICU and 2) that this determination should be based on a patient acuity tool and by the staff nurses in the unit.

Measurements and Main Results: Nurse staffing increased similarly in Massachusetts (n = 11 ICUs, Baseline patient-to-nurse ratio 1.38 ± 0.16 to Post-mandate 1.28 ± 0.15; p = 0.006) and other states (n = 88 ICUs, Baseline 1.35 ± 0.19 to Post-mandate 1.31 ± 0.17; p = 0.002; difference-in-difference p = 0.20). Massachusetts ICU nurse staffing regulations were not associated with changes in hospital mortality within Massachusetts (Baseline n = 29,754, standardized mortality ratio 1.20 ± 0.04 to Post-mandate n = 30,058, 1.15 ± 0.04; p = 0.11) or when compared with changes in hospital mortality in other states (Baseline n = 572,952, 1.15 ± 0.01 to Post-mandate n = 567,608, 1.09 ± 0.01; difference-in-difference p = 0.69). Complications (Massachusetts: Baseline 0.68% to Post-mandate 0.67%; other states: Baseline 0.72% to Post-mandate 0.72%; difference-in-difference p = 0.92) and do-not-resuscitate orders (Massachusetts: Baseline 13.5% to Post-mandate 15.4%; other states: Baseline 12.3% to Post-mandate 14.5%; difference-in-difference p = 0.07) also remained unchanged relative to secular trends. Results were similar in interrupted time series analysis, as well as in subgroups of community hospitals and workload intensive patients receiving mechanical ventilation.

Conclusions: State regulation of patient-to-nurse staffing with the aid of patient complexity scores in intensive care was not associated with either increased nurse staffing or changes in patient outcomes.

1Center for Healthcare Delivery Science, Beth Israel Deaconess Medical Center, Boston, MA.

2Division of Pulmonary, Critical Care, and Sleep Medicine, Beth Israel Deaconess Medical Center, Boston, MA.

3Vizient, Center for Advanced Analytics, Chicago, IL.

4Department of Health Systems Management, Rush University, Chicago, IL.

5Evans Center for Implementation and Improvement Sciences, Department of Medicine, Boston University School of Medicine, Boston, MA.

6The Pulmonary Center, Boston University School of Medicine, Boston, MA.

7Department of Health Law, Policy and Management, Boston University School of Public Health, Boston, MA.

*See also pp. 1681 and 1683.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website (

Supported, in part, by grants from National Institute on Aging (1F32AG058352 to Dr. Law); Agency for Healthcare Research and Quality (5K08HS024288) and Doris Duke Charitable Foundation (to Dr. Stevens); and National Heart, Lung, and Blood Institute (1R01HL136660, K01HL116768) and Boston University School of Medicine Department of Medicine Career Investment Award (to Dr. Walkey).

Dr. Law’s institution received funding from the National Institute on Aging; the Agency for Healthcare Research and Quality (AHRQ); and the National Heart, Lung, and Blood Institute. She received support for article research from the National Institutes of Health. Dr. Stevens was supported by grant number K08HS024288 from the AHRQ. The content is solely the responsibility of the authors and does not represent the official views of the AHRQ. The remaining authors have disclosed that they do not have any potential conflicts of interest.

Address requests for reprints to: Anica C. Law, MD, Center for Healthcare Delivery Science, Division of Pulmonary, Critical Care, and Sleep Medicine, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, MA 02215. E-mail:

Nurses are critical to ensuring the safe care of patients. However, studies investigating patient outcomes associated with nurse staffing have shown mixed results (1–8). Whereas multiple observational studies have identified associations between lower patient-to-nurse ratios (higher nurse staffing) and lower rates of complications within ICUs (1–3), interventions to increase nurse staffing on general wards have not shown improvements in patient outcomes (4–8). Observational findings from the ICU—where complex nursing needs often differ from general wards—have not been explored using experimental or quasiexperimental study designs.

In response to concerns that unregulated patient-to-nurse ratios in the ICU may lead to patient harm (9); Massachusetts became the first state to mandate 1:1 or 2:1 patient-to-nurse staffing in the ICU guided by an acuity tool in March of 2016. The passage of the Massachusetts mandate created a natural experiment to evaluate the downstream effects of statewide policies applied to nurse staffing in the ICU. We hypothesized that Massachusetts ICU nurse staffing regulations would result in decreased complications and mortality for critically ill patients, when compared with patients admitted to ICUs across the country unaffected by Massachusetts regulations.

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On June 30, 2014, Massachusetts General Law c. 111, § 231, “An Act relative to patient limits in all hospital intensive care units” was signed into law (10). The law established that 1) “in all intensive care units the patient assignment for the registered nurse shall be 1:1 or 1:2” and 2) that this determination should be based “on the stability of the patient as assessed by [an] acuity tool and by the staff nurses in the unit.” Regulations governing the implementation of the law were unanimously approved as 958 Code of Massachusetts Regulations 8.00 (11) on June 10, 2015. Academic medical centers, as defined by the Center for Health Information and Analysis (12), were given a compliance date of March 31, 2016. All other hospitals were required to comply with mandate requirements by January 31, 2017. Hospitals were required to report on four ICU quality measures, including central line-associated bloodstream infections, catheter-associated urinary tract infections, hospital-acquired pressure ulcers, and patient falls with injury.

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Data Source and Cohort Definitions

We conducted a retrospective cohort study of patients 18–99 years old admitted to ICUs at 246 medical centers and affiliated hospitals in the United States. Data were abstracted from Vizient (Clinical Data Base/Resource Manager and Action-OI databases), a collection of administrative claims and billing information from the largest network of healthcare systems in the United States. We used difference-in-difference methods to compare outcomes in academic Massachusetts and non-Massachusetts comparator hospitals during three time periods relative to the nurse staffing compliance date for academic hospitals: a “Baseline” period: April 1, 2014, to March 31, 2015; a “Preparation” period: April 1, 2015, to March 31, 2016; and a “Post-mandate” period: April 1, 2016, to March 31, 2017 (Fig. 1; Supplemental Methods, Supplemental Digital Content 1,

Figure 1

Figure 1

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The primary outcome was the change in risk-standardized mortality ratio during the Baseline and Post-mandate periods for patients hospitalized within Massachusetts academic ICUs, when compared with patients hospitalized in non-Massachusetts academic ICUs. Risk-standardized hospital mortality was calculated by Vizient 2016 Mortality Risk Adjustment Models (13). To contextualize changes in standardized mortality ratios, we also reported changes in the component observed and expected mortality rates.

We evaluated two clinical secondary outcomes: 1) a composite in-hospital complication rate composed of four complications for which reporting was mandated (central line-associated bloodstream infections, catheter-associated urinary tract infections, hospital-acquired pressure ulcers, and patient falls with injury) (1–3); and 2) rates of do-not-resuscitate (DNR) orders. Previous studies showed associations between ICU staffing and DNR rates that may influence mortality (14). We also assessed for changes in nurse staffing before and after the mandate compliance date.

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Statistical Analysis

Difference-in-difference approaches can be used to evaluate effects of policy interventions introduced at a specific time and can achieve unbiased effect estimates when temporal trends are the same within the intervention and comparison groups (despite differences in group characteristics) (15). After establishing parallel baseline trends in mortality between Massachusetts and non-Massachusetts hospitals in the 2 years preceding the compliance date (p for difference in trends = 0.58), we investigated the effects of the nurse staffing mandate for risk-standardized mortality using generalized linear models with a Poisson distribution and the natural log of expected mortality as an offset variable (16). Difference-in-difference estimates were evaluated using similar methods for unadjusted hospital mortality, expected mortality, complications, and DNR orders. Three Massachusetts hospitals (11 ICUs) and 33 hospitals (88 ICUs) outside of Massachusetts reported nurse staffing data during both the Baseline and Post-mandate period; we calculated the ratio of total patient-days to the total nurse-days worked by licensed nurses to yield patient-to-nurse ratios for the Baseline and Post-mandate period and compared the change in nurse staffing using t tests.

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Sensitivity and Subgroup Analysis

We performed four sensitivity and subgroup analyses. First, we used a controlled interrupted time series approach (17 , 18) to evaluate the change in unadjusted mortality after implementation of the nursing mandate among academic hospitals in Massachusetts when compared with outside of Massachusetts. The controlled interrupted time series design allowed an alternative approach to assess changes to mortality at the date of mandate implementation that did not depend on risk adjustment using a standardized mortality ratio. We assessed the difference in monthly mortality rates for MA and non-MA hospitals in the 2 years before and 1 year after the legislation compliance date. Due to the presence of first-order autocorrelation (Durbin-Watson test = 1.5; p = 0.026), we used an exact maximum likelihood to account for autocorrelated errors and seasonality. Models included independent variables for time (i.e., number of months from start of follow-up), the onset of the policy intervention (i.e. pre- vs postimplementation), and time after the policy intervention (19). Polynomial models were also explored for best fit.

Second, we conducted a difference-in-difference analysis for risk-standardized and unadjusted mortality in nonacademic community hospitals. Previous analyses have shown that nurse staffing can vary between academic and community hospitals (20), and that state nursing mandates may affect hospitals differentially depending on prelegislation staffing levels (4 , 5). Nonacademic community hospitals were required to comply with mandate requirements by January 31, 2017 date; therefore, two time periods were compared: February 1, 2015, to June 30, 2015, and February 1, 2017, to June 30, 2017, for Massachusetts and non-Massachusetts comparator nonacademic hospitals.

Third, we examined the effect of the nurse staffing mandate in the subgroup of mechanically ventilated patients (using International Classification of Diseases [ICD], 9th Edition and ICD, 10th Edition procedure codes) (21). The presence of mechanical ventilation accounts for a significant increase in acuity score on a tool such as the Therapeutic Intervention Scoring System-76 (22), and therefore, patients requiring mechanical ventilation may be more likely to have nurse staffing increased as a result of the mandate.

Fourth, we performed a post hoc exploratory analysis to evaluate changes in patient outcomes within Massachusetts alone using interrupted time series analysis (17 , 18). This analysis was conducted to evaluate whether the changes in nurse staffing after implementation of state regulations were associated with changes in patient outcome, without relying on assumptions that Massachusetts was comparable to other states. Methods were similar to the controlled time series approach, with exact maximum likelihood used to account for autocorrelated errors (Durbin-Watson test = 1.4; p = 0.01) and seasonality, independent variables for time, the onset of the policy intervention, and time after the policy intervention (19) and polynomial models explored for best fit.

All statistical testing was through SAS 9.4 (SAS Institute, Cary, NC), with two-tailed and performed with α equals to 0.05. A sample size of 7,000 was required to detect a 10% relative change in mortality with α equals to 0.05 and power of 0.90. A sample size of 10,000 was required to detect a 25% relative change in complications with α equals to 0.05 and power of 0.90. This study of deidentified composite hospital data was deemed exempt from review by the Boston University Medical Institutional Review Board.

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Cohort Characteristics

During the Baseline period, we identified 29,754 ICU admissions in the six academic hospitals in Massachusetts and 572,951 ICU admissions at 114 academic hospitals outside of Massachusetts. In the Preparation period between passage of the staffing law and mandated implementation date, we identified 29,553 ICU admissions in Massachusetts and 580,837 ICU admissions outside of Massachusetts. In the Post-mandate period, we identified 30,058 ICU admissions in Massachusetts teaching hospitals and 567,608 ICU admissions at Association of American Medical Colleges teaching hospitals outside of Massachusetts. Characteristics of the six cohorts are shown in Table 1.



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Nurse Staffing Changes

Patient-to-nurse ratios decreased in Massachusetts after nurse staffing legislation (Baseline: 1.38 ± 0.16 to Post-mandate: 1.28 ± 0.15, p = 0.006) and outside of Massachusetts in the same time periods (Baseline: 1.35 ± 0.19 to Post-mandate: 1.31 ± 0.17, p = 0.002), without significant difference-in-differences for change in nurse staffing (–0.05 [95% CI, –0.03 to 0.13]; p = 0.20).

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Mortality, Complications, and Advance Directives

After implementation of nurse staffing regulations, risk-standardized mortality remained stable in Massachusetts (1.20 [95% CI, 1.16–1.24] to 1.15 [1.11–1.19]; p = 0.11) but decreased outside of Massachusetts (1.15 [1.14–1.16] to 1.09 [1.08–1.10]; p < 0.001), without a significant difference-in-difference (odds ratio [OR], 1.01; 95% CI, 0.96–1.06; p = 0.69) (Fig. 2). Observed mortality (OR, 0.96; 95% CI, 0.91–1.00; p = 0.09) and expected mortality (OR, 0.95; 95% CI, 0.89–1.00; p = 0.06) also did not change significantly in Massachusetts after nursing legislation, when compared with outside of Massachusetts (Fig. 2).

Figure 2

Figure 2

The rate of complications was stable between the Baseline period and the Post-mandate period in Massachusetts (0.68–0.67%; p = 0.88) and outside of Massachusetts (0.72–0.72%; p = 0.83), with no significant difference in trends (p = 0.92). The proportion of ICU patients with a DNR status increased in Massachusetts (13.5–15.4%; p < 0.001) and outside of Massachusetts (12.3–14.5%; p < 0.001), without a significant difference-in-difference (p = 0.07).

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Sensitivity Analyses

Sensitivity analyses showed similar results to the primary analysis. Controlled interrupted time series analysis did not demonstrate a significant change in observed mortality in Massachusetts when compared with ICUs outside of Massachusetts following the Massachusetts policy implementation (β = –0.81; p = 0.23) (Supplemental Fig. 1, Supplemental Digital Content 1, Similarly, analysis of nonacademic community hospitals did not show a significant difference-in-difference of risk-standardized mortality (OR, 0.99; 95% CI, 0.91–1.13; p = 0.89). Risk-standardized mortality in Massachusetts decreased from 1.29 (95% CI, 1.21–1.37) to 1.14 (95% CI, 1.07–1.22) (p = 0.01), whereas outside of Massachusetts, the mortality index decreased similarly from 1.10 (95% CI, 1.11–1.19) to 0.96 (95% CI, 0.93–0.99) (p < 0.001) among nonacademic hospitals. Analysis of complication rates and DNR status revealed results similar to academic hospitals (Supplemental Results, Supplemental Digital Content 1, Mechanically ventilated patients also did not show a significant difference-in-difference in risk-standardized mortality (OR, 1.02; 95% CI, 0.96–1.08; p = 0.57), rate of complications (p = 0.42), or in rates of DNR orders (p = 0.31). Analysis of interrupted time series within Massachusetts alone was also not associated with a significant change in observed mortality in Massachusetts (adjusting for first-order autocorrelation, β for immediate change in mortality = –1.13; p = 0.18), although some quadratic models suggested increasing mortality trends after implementation (Supplemental Fig. 2, Supplemental Digital Content 1,

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We performed an analysis of the effect of a Massachusetts law mandating the use of a risk score within ICUs to assign patient-to-nurse ratios of either 2:1 or 1:1, based upon patient acuity and anticipated care intensity. Increases in nurse staffing in Massachusetts after institution of statewide regulations were not significantly higher than staffing trends in states that did not pass ICU nurse staffing legislation. We did not identify improvements in patient outcomes associated with the Massachusetts nursing regulations. Results were robust to multiple sensitivity and subgroup analyses.

Our findings may be considered from two perspectives. First, the difference-in-difference analyses comparing changes in Massachusetts after the implementation of ICU nurse staffing regulations to changes in other states were analogous to an intention-to-treat analysis of the Massachusetts nurse staffing policy legislation. Our results suggest that the Massachusetts nursing regulations were not associated with changes in staffing or patient outcomes when compared with secular trends. Second, when examined apart from secular changes in other states, modest nurse staffing increases in Massachusetts (the change from 1.38 to 1.28 patients per nurse is approximately one extra nurse per 20-bed ICU per 12-hr shift) remained unassociated with changes in hospital mortality.

Few states have passed laws regulating nurse staffing. California previously mandated maximum patient-to-nurse ratios within acute care hospitals but did not use scoring systems to determine 1:1 staffing levels. Although California regulations were associated with an increase in nurse staffing, outcomes of hospitalized did not improve after the California law, findings similar to Massachusetts ICUs (4 , 6 , 8 , 23).

Previous observational studies of nurse staffing have identified associations between lower patient-to-nurse ratios and improved outcomes (1 , 2 , 24 , 25). However, these observational studies were unable to distinguish the causal effect of nurse staffing on patient outcomes—patients in hospitals with a higher commitment to nursing staffing may also benefit from higher overall hospital commitment to a quality of care and nurse retention, improved nurse-physician collaboration, or other predictors of patient safety (1 , 24 , 26–29). In the absence of large randomized trials to determine the causal effect of nurse staffing on patient outcomes, studies of policy interventions provide natural experiments to evaluate the effectiveness of interventions. Whereas baseline demographic differences between Massachusetts and non-Massachusetts hospitals reflect existing underlying demographic differences in the population, the difference-in-difference analysis compares within state changes and therefore is likely robust to baseline demographic differences. Multiple factors may have contributed to the minimal changes in nurse staffing and observed lack of improvement in patient outcomes associated with Massachusetts’ ICU nurse staffing regulations, including the following: 1) adequate nurse staffing and competent nurse assignment determination before the mandate; 2) significant leeway given to hospitals in both the determination of the acuity/severity tool to be used to guide nursing assignments and unclear implementation of the tool in guiding nursing assignments; 3) multidisciplinary staffing factors important for patient outcomes in addition to nursing care (pharmacists, respiratory therapists, among others); 4) secular trends of increased ICU nurse staffing outside of Massachusetts; and 5) a diluted effect on mortality due to unchanged or increased hospital-ward mortality as a result of earlier transfer from the ICU to the floor in the setting of ICU nurse staffing limitations. Hospitals used different tactics in designing or choosing acuity tools to determine nurse staffing, which may reflect uncertainty as to how to optimally distribute nursing resources.

Our findings should be interpreted in the context of limitations. First, although our analysis is based on deidentified individual patient data, risk-standardized mortality ratios were averaged across hospitals. However, interrupted time series analysis of observed mortality rates, which did not incorporate risk adjustment, corroborated the risk-standardized mortality findings. Second, nursing regulations may have had heterogeneous effects within certain patient populations or larger effects at hospitals with lower premandate staffing. However, results were similar in a subgroup of mechanically ventilated patients who were more likely to be affected by acuity score-based nurse staffing, as well as in nonacademic medical centers with generally higher patient-to-nurse ratios (5). Further studies may seek to evaluate for heterogeneity of policy effects across other patient populations, such as across type of ICU. Third, measured complications were rare (< 1% of patients), but sample size calculations suggested sufficient power to detect differences in complication rates. The complications chosen for the composite analysis were the four complications for which reporting to the state was mandated, but given rarity, were likely not ideal measures of improvements in nurse-staffing-sensitive ICU morbidity. The low rates of complications may reflect that other external forces aside from state legislation (e.g., Center for Medicaid Services and the Joint Commission) have already maximally reduced occurrence rates. Further studies should evaluate other candidate measures of nurse-staffing-sensitive morbidity; for example, Kane et al (24) identified unplanned extubations as a measure potentially sensitive to nurse staffing. Unfortunately, few academic hospitals contributed data regarding nurse staffing and therefore power to detect changes in nurse staffing was limited—although effect estimates suggest small differences in staffing changes after the legislation. Fourth, Massachusetts instituted both maximum patient-to-nurse ratios and the use of patient severity scores in a bundled approach; we were unable to distinguish effects of the patient-to-nurse ratio mandate from the regulation of nurse staffing based upon patient severity scores. It is possible that our findings are the result of suboptimal nursing resource allocation as a result of the acuity tool alone, and that using other methods of nurse staffing distribution may yield different outcomes. Fifth, we did not have data on costs attributable to this legislation, although costs associated with development and implementation of severity score-based nurse staffing and reporting compliance in response to new mandates may be substantial (30). Sixth, other outcomes important to patients (such as patient and family satisfaction, emotional harms, or postintensive care syndrome) and nurses (job satisfaction or creation of a healthier work environment leading to higher nurse retention rates) may be sensitive to nurse staffing regulations and deserve further investigation. A longer follow-up comparison period may also reveal that state legislation similar to MA may prophylactically prevent decrease in nurse staffing or may more readily facilitate appropriate increases in nurse staffing in response to rising patient complexity. Conversely, unintended adverse consequences (longer emergency department wait times or floor-to-ICU transfer times due to limits on ICU staffing, reduction in staffing with more experienced nurses) should be investigated as well. Finally, results may differ among hospitals that chose not to submit information to the Vizient database. Participation in Vizient is voluntary in nature and may reflect a sample of improvement-focused hospitals; however, all MA academic centers submitted data to Vizient, decreasing the likelihood that our findings do not represent MA hospital academic medical center outcomes.

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In conclusion, Massachusetts General Law c. 111, § 231, which regulates the staffing assignments of nurses to patients in the ICU based upon patient acuity tools, failed to demonstrate improvements in patient mortality or complication rates among critically ill patients, potentially due to small effects on nurse staffing. Given the excess costs of enacting and enforcing statewide legislation to implement acuity-guided ICU nurse staffing, future efforts to regulate patient-to-nurse ratios should carefully first consider the extent of the effects of the mandate on nurse staffing levels. In line with previous studies outside of the ICU, our study of ICU nurse staffing adds further evidence suggesting that statewide legislation to mandate nurse staffing strategies may not effectively improve patient outcomes.

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We thank Jane Foley, RN, BSN, MHA, Associate Chief Nurse at Beth Israel Deaconess Medical Center, for her insights.

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hospital mortality; personnel staffing and scheduling; policy

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