Accounting for patient-associated risk for adverse outcomes is critical for hospital administrators to allocate hospital resources and prioritize the care. Among multidimensional factors related to patient outcomes, comorbidity is of major importance because patients with more comorbidities have higher risks of death and complications and use more hospital resources (Fortin, Soubhi, Hudon, Bayliss, & Van den Akker, 2007; Ritchie, 2007). Understanding patient comorbidities is critical for nursing care planning. By separating modifiable patient and provider characteristics from those that are attributable to intrinsic, unmodifiable patient risk (Hessels, Liu, Cohen, Shang, & Larson, 2018; Olson et al., 2013), hospital administrators can appropriately allocate nursing staffing and other resources and provide the most cost-effective care. With accurate assessment of patients' risks, nurses and other clinicians at the point of care will be better informed to integrate patient health data with current evidence and treatment protocols to ensure that the most effective care is rendered.
Comorbidity measurement is critical to hospital performance comparisons and benchmarking. It is important to adjust for potential confounding when describing and comparing patient outcomes between facilities (van Walraven, Austin, Jennings, Quan, & Forster, 2009). If differences among patients in their initial conditions and prognostic expectations are not appropriately identified and accounted for, subsequent patient outcomes may be erroneously attributed to variations in treatment.
For effective nursing care planning and hospital resource allocation, as well as to adjust for potential confounding for hospital comparisons or benchmarking, it is necessary to describe and compare patient populations whenever chronic disease conditions are associated with outcomes such as length of stay (LOS) or death. This becomes especially important for healthcare organizations participating in the Bundled Payment for Care Improvement Initiative. Under this initiative, hospitals have to absorb the financial burden in the event of an unplanned prolonged LOS. Choosing the right risk adjustment measure can help the organization identify patient factors related to increased costs and allocate resources to avoid incurring additional costs.
Among the several risk-adjustment methods available, the Charlson/Deyo and the Elixhauser methods are the most widely used (Ladha et al., 2015). The Charlson Comorbidity Index was developed in 1987 using 19 medical diagnoses from clinical data to predict 1-year mortality for patients with multiple conditions (Charlson, Pompei, Ales, & MacKenzie, 1987). The coding algorithm has been modified several times since its original version. In 1992, a coding algorithm including 17 diagnoses was adapted by Deyo using International Classification of Disease, 9th revision, Clinical Modification (ICD-9-CM) codes from administrative data (Deyo, Cherkin, & Ciol, 1992). The Charlson/Deyo measure has been the most commonly used adaptation of the original Charlson method. More recently, Quan et al. (2005) modified and enhanced the ICD-9-CM coding algorithm for each comorbidity and the performance of the enhanced coding algorithm in predicting in-hospital mortality was tested and verified.
The Elixhauser Comorbidity Index is another commonly used method to predict hospital resource use and in-hospital mortality. First developed using ICD-9-CM codes from hospital administrative data, the original Elixhauser Index included 30 diagnoses (Elixhauser, Steiner, Harris, & Coffey, 1998) and was later modified by Garland et al. (2012) to include 31 diagnoses. In 2009, a weighted composite Elixhauser score, the V W Elixhauser comorbidity score, was generated, and similar discriminating ability of the composite score and 30 individual comorbidities in predicting in-hospital mortality was reported (van Walraven et al., 2009).
In addition to the two risk-adjustment methods, the All Patient Refined Diagnosis-Related Group (APR-DRG) is a widely adopted commercial patient classification system that uses administrative data (Romano & Chan, 2000). The APR-DRG methodology that provides patient classification systems was developed by the New York State Department of Health in collaboration with 3 M Health Information System for the purposes of analyzing and comparing of outcomes for a given diagnostic group (Iezzoni et al., 1995). The APR-DRG expanded the basic DRG structure and added two sets of subclasses to each base APR-DRG: severity of illness (SOI), defined as the extent of physiologic decompensation or organ system loss of function, and risk of mortality (ROM), estimating the likelihood of death. In APR-DRG calculations, patients are first assigned to a base APR-DRG based on primary diagnosis and then separately assigned to one of the four possible levels (minor, moderate, major, and extreme) of the distinct SOI and ROM subclasses; assignment is based on secondary diagnoses and the interaction between diagnoses, age, and selected procedures (Averill et al., 1997; Solutions, 2018). The 3 M ROM subclass was tested in intensive care units and found to have good performance (Baram et al., 2008). The APR-DRG has been adopted by several states to compare hospital performance (Romano & Chan, 2000) and was used in various studies to predict patients' hospital LOS and in-hospital mortality (Hansen et al., 2017; Nante, De Marco, Balzi, Addari, & Buiatti, 2000).
Researchers have reported that using 30 or 31 individual comorbidities from the Elixhauser method improves discriminating ability to predict mortality (Lieffers, Baracos, Winget, & Fassbender, 2011) or in-hospital mortality (Gutacker, Bloor, & Cookson, 2015; Southern, Quan, & Ghali, 2004; Stukenborg, Wagner, & Connors, 2001) over the Charlson/Deyo method, which uses only 17 individual comorbidities. Researchers have also compared APR-DRG with the Elixhauser method using individual comorbidities to predict mortality among patients hospitalized for hip replacement (Messina et al., 2017) and found that the 3 M ROM slightly outperformed the Elixhauser method for both in-hospital and 30-day mortalities.
However, using 30 or 31 individual diagnosis to describe or model a patient population could be cumbersome. Austin, Wong, Uzzo, Beck, and Egleston (2015, p. 1) suggested that once a valid comorbidity composite score is calculated, “no other information about the comorbidity variables used to create the score is generally needed.” In a study that included individuals diagnosed with localized kidney cancer (age ≥65 years), similar performance of individual comorbidities and composite scores in predicting mortality was reported using either the Charlson/Deyo method or the Elixhauser method (Austin et al., 2015). When describing a population and/or controlling for potential confounding, there are at least three appealing advantages of using composite scores rather than the individual comorbidities:
- being able to provide an overall description of the population;
- reducing the risk of overfitting in small datasets; and
- alleviating computational requirements in large datasets (Thompson et al., 2015; van Walraven et al., 2009).
Hence, it is important to compare various composite scores of risk adjustment measures to identify the most relevant, feasible, and valid one for predicting outcomes in adult hospitalized patients.
Because health claims data and other electronic sources are increasingly used for evaluating patient outcomes (Blumenthal, 2009; Cohen et al., 2015), it is important to recognize the strengths and limitations of these measures for predicting patient outcomes. The aim of this study was to compare the performance of measures commonly used for patient risk adjustment for predicting in-hospital mortality and hospital LOS.
This study was guided by the Quality Health Outcomes Model (Mitchell, Ferketich, & Jennings, 1998) that delineates relations among four components—system, client, interventions, and outcomes. In the model, system refers to organizational structure; intervention refers to processes and activities that clinicians perform; outcomes represent health indicators such as mortality, LOS, and others; and client refers to patient's individual characteristics such as demographics, health status, and comorbidity. In the Quality Health Outcome Model, the effect of medical interventions on outcomes is indirect and mediated by system and client components.
This was a secondary data analysis that included data from all adult (≥18 years) inpatient hospital discharges between January 1, 2013, and December 31, 2014, from a large hospital system in the New York City metropolitan area. The hospital system includes a community hospital and two tertiary/quaternary care hospitals with >2,000 beds and over 100,000 patient admissions annually. All study data, including patients' demographic information, admission date, discharge date and status, and ICD-9-CM codes were extracted retrospectively from the institution's clinical data warehouse server to an institution encrypted network drive using standardized query language by the data manager.
Risk Adjustment Measures Tested
Table 1 provides a summary of the five measures used in this study.
Charlson/Deyo Method: Charlson/Deyo Age-Comorbidity Score
The validated Charlson/Deyo age-comorbidity score represents a patient's clinical comorbidity score. It is measured as a weighted summation of 17 categories of comorbid illnesses and the patient's age point: beginning at age 40 years, one point is added for each 10-year increment (Charlson, Szatrowski, Peterson, & Gold, 1994). The modified and enhanced version of the ICD-9-CM coding algorithm proposed by Quan et al. (2005) was used. Appendix A lists the ICD-9-CM codes and assigned points for each comorbidity and age range. The Charlson/Deyo age-comorbidity score ranges from 0 to 38. Present on admission (POA) indicator is collected in New York State discharge data (Coffey, Milenkovic, & Andrews, 2006) to distinguish POA comorbidities from complications that occurred during the admission (Garrett, 2009). Only comorbidities identified by the POA indicator were used to calculate the Charlson/Deyo score as well as the two composite scores from the Elixhauser method as described below.
V W Elixhauser Method: V W Elixhauser Comorbidity Score and V W Elixhauser Age-Comorbidity Score
Using POA as the indicator to identify comorbidities, we calculated two Elixhauser comorbidity composite scores. More recently, Thompson et al. (2015) proposed a similar weighted composite score that slightly outperformed the V W Elixhauser score. However, the Thompson score uses diagnosis-related group codes to identify comorbidities rather than using POA indicator, and it is not clear if the Thompson score is superior in other datasets. We first used the V W Elixhauser comorbidity score, which is a weighted composite score of 30 individual comorbidities based on the association between each comorbidity and hospital death proposed by van Walraven et al. (2009). The points assigned to each comorbidity ranged from −7 (for drug abuse) to 12 (for metastatic cancer) (Appendix A). The V W Elixhauser comorbidity score ranges from −19 to 89. It does not include age but is highly correlated with a patient's age. Next, we created a V W Elixhauser age-comorbidity score including patient age points based on the same age-point algorithm as the Charlson/Deyo method. As with the Charlson/Deyo method, the modified and enhanced ICD-9 coding algorithm was used (Quan et al., 2005).
APR-DRG method: 3 M ROM and 3 M SOI
The ROM and SOI subclasses were extracted directly from the hospital database as generated by the 3 M Core Grouping Software, which is licensed and maintained by the hospital system. Annual updates to the Grouping Software programs are required to accommodate annual revisions to the ICD diagnosis and procedure codes. In the United States, these revisions become effective October 1 of each year, and thus, the Grouping Software versions applicable to our study period are V30, V31, and V32. The APR-DRGs are proprietary products of the 3 M Health Information System. Notably, ROM and SOI subclasses are determined by both in-hospital complications and POA comorbidities; therefore, all diagnoses—POA and non-POA—were used (Romano & Chan, 2000).
Outcomes and Other Variables
Patient characteristics, including age, gender, admitting hospital, LOS, ICD-9-CM codes, and discharge status, were extracted from hospital discharge data. The outcome variables in this study were in-hospital mortality and LOS.
Descriptive statistics were used to summarize the admission characteristics in our sample. To assess the association between each of the five risk-adjustment measures and in-hospital mortality, we built separate logistic regression models for each measure. As hospital LOS is a typical example of zero-truncated count data with evidence of overdispersion (Zuur, Ieno, Walker, Saveliev, & Smith, 2009), a zero-truncated negative binomial regression model was built to assess the association between each of the five measures and hospital LOS.
To assess the performance of each logistic regression model, we calculated the C-statistic and associated 95% confidence intervals (CIs) to determine the absolute fit. The C-statistic ranges from 0.5 to 1, where 0.5 indicates that the model is no better than a random chance in predicting the outcome and 1 indicates that the model has perfect discriminating ability. Models are typically considered good when the C-statistic is greater than 0.7 and strong when it is greater than 0.8 (Hosmer & Lemeshow, 2000).
It is not common to report R2 measures of model fit for count data (Cameron & Windmeijer, 1996). However, in order to provide a goodness-of-fit measure that can be compared with other or future studies, we assessed the performance of each zero-truncated negative binomial regression model by calculating the likelihood ratio index for each model. Likelihood ratio index measures the proportionate reduction in the log-likelihood due to inclusion of regressors and is calculated as one minus the ratio of the full-model log-likelihood to the intercept-only log-likelihood (Cameron & Windmeijer, 1996). Likelihood ratio index is sometimes proposed as a general pseudo-R2 measure (Cameron & Windmeijer, 1997) with advantages of being reported by statistical packages (Cameron & Windmeijer, 1996), and the ease of calculation based on results provided by general statistical packages for exponential family regression models.
Odds ratios (ORs) from logistic regression and incidence rate ratios (IRRs), calculated as exponentiated regression coefficients from zero-truncated negative binomial regression, are also reported for the size and direction of effects. All statistical analyses were performed using SAS version 9.3 (SAS Institute, 2012) or R (R Core Team, 2013).
Characteristics of the sample and summaries of the risk adjustment measures and outcomes are shown in Table 2. The sample included 123,641 adult admissions, of whom 63,689 (51.5%) were female. The average age was 62.2 (SD = 18.5) years. The in-hospital mortality rate was 2.8%. The median hospital LOS was 5 days, with interquartile range of 3–9 days.
Table 3 shows the model performance for the binary logistic regression models assessing the relationship between each of the risk adjustment measures and in-hospital mortality. All five measures demonstrated good to strong model fit for predicting in-hospital mortality by C-statistics of Charlson/Deyo age-comorbidity score (0.74, 95% CI [0.74, 0.75]), V W Elixhauser comorbidity score (0.80, 95% CI [0.80, 0.81]), V W Elixhauser age-comorbidity score (0.81, 95% CI [0.81, 0.82]), 3 M ROM (0.94, 95% CI [0.93, 0.94]), and 3 M SOI (0.90, 95% CI [0.90, 0.91]). The two V W Elixhauser composite scores outperformed the Charlson/Deyo age-comorbidity score with higher C-statistics. The V W Elixhauser age-comorbidity score had similar model performance to the V W Elixhauser comorbidity score with overlapping 95% CIs of C-statistics. Both 3 M ROM and 3 M SOI subclasses provided statistically superior predictions on in-hospital mortality, with higher C-statistics, and 3 M ROM had the best performance.
Higher Charlson age-comorbidity scores were associated with higher risk of in-hospital mortality: With a one-unit increase in the Charlson age-comorbidity score, the odds of in-hospital mortality increased by 28% (OR = 1.28, 95% CI [1.27, 1.29]). Higher V W Elixhauser comorbidity scores (OR = 1.12, 95% CI [1.12, 1.12]), and higher V W Elixhauser age-comorbidity scores (OR = 1.12, 95% CI [1.12, 1.12]) were also associated with higher risk of in-hospital mortality. Greater odds of mortality were associated with higher risk category measured by 3 M ROM. The results of in-hospital mortality rate by risk category of 3 M subclasses are presented in Table 4. Inpatients who scored minor ROM had the minimal in-hospital mortality rate (0.02%), followed by those who scored moderate ROM (0.14%) and those who scored major ROM (2.45%). Inpatients who scored extreme risk of morality had the highest in-hospital mortality rate (28.5%). The same pattern of in-hospital mortality rate was also found in the 3 M SOI.
Table 5 shows the model performance for the zero-truncated negative binomial regression models assessing the relationship between each of the risk adjustment measures and hospital LOS. The Charlson/Deyo age-comorbidity score, V W Elixhauser comorbidity score, and V W Elixhauser age-comorbidity score were weak predictors of hospital LOS (likelihood ratio index = 0.3%, 1.2%, and 1.1%, respectively). The 3 M SOI and 3 M ROM were better in predicting hospital LOS (likelihood ratio index = 6.2% and 4.3%, respectively). Among the five measures, 3 M SOI had the best predictive performance.
Higher Charlson age-comorbidity scores (IRR = 1.042, 95% CI [1.041, 1.045]) were associated with longer hospital stays: With a one-unit increase in the Charlson age-comorbidity score, the hospital LOS increased by 4.2%. Higher V W Elixhauser comorbidity scores (IRR, 1.029, 95% CI [1.028, 1.030]) and higher V W Elixhauser age-comorbidity scores (IRR = 1.025, 95% CI [1.025, 1.026]) were also associated with longer hospital stays. Longer hospital stays were associated with a greater risk category of 3 M SOI. As compared to inpatients who scored minor SOI, inpatients with extreme SOI had the longest hospital stay (IRR = 5.30, 95% CI [5.22, 5.39]), followed by those with a major SOI (IRR = 2.30, 95% CI [2.27, 2.33]) and those with moderate SOI (IRR = 1.44, 95% CI [1.42, 1.46]). The same pattern was also found in 3 M ROM.
We compared the performance of five measures developed from three methods to predict inpatients' in-hospital mortality and hospital LOS using large-scale electronic healthcare data. Previous researchers have reported the Elixhauser method outperformed the Charlson/Deyo method to predict mortality (Lieffers et al., 2011) or in-hospital mortality by using individual comorbidities (Gutacker et al., 2015; Southern et al., 2004; Stukenborg et al., 2001). However, to our knowledge, no previous research has compared the performance of the weighted composite scores from these two methods in adult inpatients with different medical conditions and various age ranges in in-hospital mortality or LOS. We found using composite scores that the Elixhauser method still outperformed the Charlson/Deyo method.
The performance of the Elixhauser method may be because the Elixhauser's ICD-9-CM coding algorithm captured more comorbidities on more patients as compared to the Charlson/Deyo method (Southern et al., 2004). However, it is important to note that the differences in predictive power between the Charlson/Deyo and the two Elixhauser composite scores were small; the C-statistics for predicting in-hospital mortality were 0.74, 0.80, and 0.81 for the Charlson/Deyo, V W Elixhauser comorbidity score, and V W Elixhauser age-comorbidity score, respectively. Other researchers also reported C-statistics of 0.7–0.8 for predicting in-hospital mortality using these methods, with differences of <0.1 between the Charlson/Deyo and Elixhauser methods (Southern et al., 2004; Stukenborg et al., 2001). More recently, researchers in Taiwan reported C-statistics of 0.76 in predicting postillness mortality in patients with thoracic empyema using the Charlson Method (Wu, Liu, Lee, Kuo, & Hsieh, 2018). In addition, in alignment with the Charlson/Deyo age-comorbidity score, we created the new V W Elixhauser age-comorbidity score by adding age points to the Elixhauser composite score; the new score had similar accuracy to the V W Elixhauser comorbidity score in predicting in-hospital mortality and LOS.
Our study further extends the current literature by testing both the 3 M ROM and SOI subclasses in adult inpatients with different medical conditions and by comparing those two subclasses with the Charlson/Deyo and Elixhauser methods using composite scores rather than individual comorbidities. The high predictive power of both the 3 M ROM and SOI subclasses was verified in our study, with C-statistics of 0.94 and 0.90 for predicting in-hospital mortality. Previous researchers found that the 3 M ROM slightly outperformed the Elixhauser method using individual comorbidities for both in-hospital and 30-day mortalities (Messina et al., 2017). Ours is the first study to compare the Charlson/Deyo and Elixhauser weighted composite scores with the 3 M ROM and 3 M SOI subclasses for predicting in-hospital mortality and LOS. We found that both 3 M subclasses outperformed Charlson/Deyo and Elixhauser methods in predicting these two outcomes.
We also found that the 3 M ROM had the best predictive power for in-hospital mortality and the 3 M SOI was best in predicting hospital LOS. This is not surprising because the 3 M SOI subclass was structured for evaluating resource use, whereas the 3 M ROM was developed primarily for evaluating patient mortality (Averill et al., 1997). However, the high predictive performance of the two APR-DRG subclasses may be because they include all diagnoses, both POA and non-POA (Romano & Chan, 2000). When the 3 M ROM subclass was tested by including only POA diagnoses, the predictive power was significantly less; the C-statistic dropped from 0.83–0.85 to 0.74 (Romano & Chan, 2000). Further, complications developed after admission are also directly affected by the care patients receive during the hospitalization. Previous researchers using 2005 California discharge data and vital statistics death files found that substantial effects on hospital rankings of performance were reported when incorporating POA indicators into risk-adjusted models (Goldman, Chu, Bacchetti, Kruger, & Bindman, 2015). Thus, APR-DRG methods may not be suitable for comparisons of hospital performance or for clinical research studies in which it is important to control for SOI at admission.
Our findings have significant clinical practice implications. Hospitals are now mandated to submit standardized outcome measures. Risk adjustment is critical to conduct and fairly interpret benchmarking of organizations and provider performance. Our findings suggest that the Elixhauser method is slightly better than the Charlson/Deyo method in predicting in-hospital mortality and LOS, and caution should be applied when using APR-DRG to ensure the POA indicator is factored into the calculation. However, because the differences between the risk adjustment methods are small, hospital administrators may choose one based on the best available sources. The risk adjustment is also important in plan of care. Risk profiles generated by these approaches can help clinicians prioritize tasks and develop personalized care based on individual patient risk score and needs. Nurse managers and administrators can also use this information to assign nursing care, distribute direct-care resources, and provide needed support services. However, it is important to note that these comorbidity measures have been developed primarily to predict death and hospital LOS, taking only medical diagnoses into account. None of these measures consider functional capacity, which may have significant effects on patient outcomes and recovery. Future research should include these important factors and should also assess their predictive value for other outcomes such as rehospitalization or cost.
It is important to note that these study findings are not generalizable to pediatric populations, as the diagnoses included in these indices, such as chronic pulmonary disease and congestive heart failure, are not commonly found in children. It is also important to note that these measures were developed for use in acute-care settings. Caution needs to be applied when they are used in other healthcare settings such as long-term care, nursing homes, or home healthcare.
In conclusion, all of the measures examined in this study had good discrimination for in-hospital mortality. The composite scores from the Elixhauser method slightly outperformed the Charlson/Deyo method. Between the two composite scores from the Elixhauser method, the age-comorbidity score might be a better choice when the purpose is to identify the most parsimonious model—particularly with small datasets. The advantage of the V W Elixhauser comorbidity score is that the effects of the age and comorbidity composite score can be examined separately.
Although the APR-DRG method performed better in predicting both in-hospital mortality and hospital LOS, the 3 M ROM and SOI are usually calculated after hospital discharge and therefore are not appropriate for use at admission for planning care. Further, the inclusion of complications developed after hospital admission in the calculation of APR-DRG makes it less appealing than the Charlson/Deyo and Elixhauser methods, which also have the advantage of being readily available and free of cost. Health services researchers and hospital administrators must choose the best measure based on the available datasets, sources, and needs.
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