Weekend Hospital Admission, Acute Kidney Injury, and Mortality

Discussion

AKI is a relatively common condition that continues to be associated with high mortality.¹² Although some studies have suggested a recent trend toward improved outcomes in the setting of AKI, it is unclear whether these observations are due to overall improvements in the quality of health care or if they have resulted from differences over time in the way AKI is recognized and defined.^13,14 Hospital admission on a weekend may delay the provision of some elements of care, including diagnostic and therapeutic interventions.^8–10 We observed an increased risk of death with weekend admission for patients with AKI that was most evident for admissions to smaller hospitals. Furthermore, weekend admissions were associated with heightened mortality when AKI complicated a wide array of acute medical conditions.

The excess risk of in-hospital death among patients admitted with AKI on a weekend was modest when expressed in relative terms (ranging from 7% to 17% depending on hospital size, compared with admission on a weekday). However, when projected across the U.S. health care system, the high incidence of AKI translates into a large increase in deaths. On the basis of these estimates, we project that approximately 4000 additional deaths per year occur in patients with AKI admitted on the weekend in the United States, representing 1 additional death for every 65 weekend admissions with AKI. Admission on a weekend thus appears to be a clinically relevant risk factor for mortality among patients with AKI.

Little is known about the association between time of admission and outcomes for AKI. For other complex acute medical conditions with high mortality, increased weekend mortality has been felt to be a consequence of reduced levels of staffing.¹ In patients admitted with acute myocardial infarction on the weekend, higher mortality appears to be mediated by reduced access to invasive cardiac procedures.^8,10 Conversely, weekend admissions to critical care units have not been associated with increased mortality, an observation perhaps attributable to more constant levels of access to staff and availability of medical procedures in these settings.^15–17 Few studies have examined how timing of assessment or institution of treatment specific to the management of AKI affects clinical outcomes. Observational studies examining the effects of timing of nephrology consultation and institution of renal replacement therapy on clinical outcomes in patients with AKI have produced conflicting results.^18–20 However, delays in appropriate diagnosis and management in several medical conditions (e.g., reperfusion for ST elevation myocardial infarction,^10,21 early management of severe sepsis²²) have been shown to affect clinical outcomes, suggesting that processes of acute medical care may be influential.

There are several reasons why we may have observed a higher risk of death among patients with AKI admitted on the weekend. On the basis of recorded diagnoses and procedures, patients admitted on a weekend appeared to be more severely ill than those admitted on a weekday. However, higher mortality with weekend admission persisted in analyses adjusted for demographic characteristics, recorded comorbidities, and provision of mechanical ventilation and across most of the acute medical conditions that co-existed with AKI.

The reasons for excess mortality among patients admitted with AKI on a weekend are uncertain, although delays in assessment, diagnosis, and management of other acute medical conditions on weekends have been previously described.⁹ We found that patients admitted with AKI on a weekend were less likely to receive dialysis than those admitted on a weekday, despite their greater severity of illness. In many centers across the United States, access to dialysis may be reduced during the weekend, and the availability of trained personnel may differ on weekends, particularly on Sundays.^23,24 If nephrology consultation and the provision (and timing) of renal replacement therapy affect survival in the setting of AKI, improved weekend access to specialist care or dialysis could potentially reduce AKI-related mortality for patients admitted on weekend days; however, further research is required to test this hypothesis. Process-of-care factors may be accentuated in smaller hospitals where we observed the more pronounced increases in risk of death with weekend admission. Because small hospitals were more likely than large hospitals to transfer patients with AKI to another short-term hospital (6.2% versus 2.8% of admission with AKI), and patients who are transferred are likely sicker, the accentuated “weekend effect” in smaller hospitals may be greater than reported here (presumably having discharged a fraction of their sicker patients to medium-sized or larger hospitals).

Our study has several limitations. First, we identified our study population on the basis of the presence of International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes for AKI. These codes for AKI have high specificity but poor sensitivity.²⁵ Although this may result in exclusion of some patients with AKI from our study, this is unlikely to produce selection bias because weekend and weekday admissions are unlikely to differ with respect to the coding of AKI. Indeed, if we failed to identify some patients with AKI (as we suspect), then we would also have underestimated the absolute number of deaths attributable to weekend admission with AKI. Second, classification of weekend admission was defined as a Saturday or Sunday admission and ignored the fact that the provision of health care services on a Friday evening or in the early morning hours on Monday (e.g., midnight to 6:00 a.m.) are likely more similar to those provided on a Saturday or Sunday, respectively. If relevant, the “peri-weekend” misclassification is likely to lead to underestimation of the excess risk of death among patients with weekend admission, thereby biasing our results toward the null. Third, we were only able to identify in-hospital mortality and may have misclassified the outcome of patients who died after transfer to another acute, intermediate, or skilled nursing facility. However, because the length of hospital stay was similar for weekend and weekday admissions, and discharge to intermediate care and skilled nursing facilities was more common among patients admitted on a weekend than on a weekday, these minor differences are unlikely to account for the observed associations. Fourth, despite multivariable adjustment, we cannot exclude residual confounding by unmeasured characteristics that may have influenced our results. However, the elevated adjusted rate of death with weekend admission across seven co-existing conditions strengthens our findings.

This study was conducted using a large administrative health care database. Although this data source provides a large sample of hospital admissions sampled from across the United States, there are limitations to the information available, and data on access to consultants, emergency room waiting times, intensive care unit admissions, and severity of illness scores are not included. Future studies that capture these relevant prognostic and health services utilization variables are required to better understand the relation between time of admission with AKI, management, and clinical outcomes. Given the nature of our study data, the underlying reasons for increased weekend mortality with AKI could not be determined. Further studies are needed to compare the effect of differences in clinical practice related to access and timing of diagnostic and therapeutic interventions for the management of AKI. Studies should focus on health practices amenable to improvement, including the provision of intravenous fluids and/or diuretics, diagnostic testing, and timing of nephrologist consultation and initiation of dialysis.

In conclusion, for patients hospitalized with AKI, mortality was significantly higher among patients admitted on a weekend as compared with those admitted on a weekday. This disparity was most evident among admissions with AKI in smaller hospitals. These findings highlight the need to further investigate the availability and timing of provision of diagnostic and therapeutic strategies to patients hospitalized with AKI.