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J Gen Intern Med. May 2004; 19(5 Pt 1): 444–450.
PMCID: PMC1492245

Consistency of Performance Ranking of Comorbidity Adjustment Scores in Canadian and U.S. Utilization Data

Sebastian Schneeweiss, MD, ScD,1,2 Philip S Wang, MD, DrPH,1 Jerry Avorn, MD,1 Malcolm Maclure, ScD,2,3 Raia Levin, MSc,1 and Robert J Glynn, PhD, ScD1,4,5

Abstract

OBJECTIVE

The performance of standard comorbidity scores to control confounding is poorly defined in health care utilization data across elderly populations. We sought to evaluate and rank the performance of comorbidity scores across selected U.S. and Canadian elderly populations using health care utilization databases.

DESIGN

Cross-population validation study.

PARTICIPANTS

Study participants were residents age 65 years or older who had prescription drug coverage through state-funded programs selected from several large health care utilization databases available to the investigators: British Columbia, BC (N=141,161), New Jersey, NJ (N=235,881), and Pennsylvania, PA (N=230,913).

MEASUREMENTS

We calculated 6 commonly used comorbidity scores for all subjects during the baseline year (1994 for NJ and PA, and 1995 for BC). These included scores based on diagnoses (Romano, Deyo, D’Hoore, Ghali) and prescription drugs (CDS-1, CDS-2). The study outcome was 1-year mortality. The performance of scores was measured by c-statistics derived from multivariate logistic regression that included age and gender.

MAIN RESULTS

Across these 4 large elderly populations, we found the same rank order of performance in predicting 1-year mortality after including age and gender in each model: Romano (c-statistic 0.754 to 0.771), Deyo (c-statistic 0.753 to 0.768), D’Hoore (c-statistic 0.745 to 0.760), Ghali (c-statistic 0.733 to 0.745), CDS-1 (c-statistic 0.689 to 0.738), CDS-2 (c-statistic 0.677 to 0.718), and age and gender alone (c-statistic 0.664 to 0.681). Performance was improved by an average of 6% by adding the number of different prescription drugs received during the past year.

CONCLUSIONS

Performance ranking of 6 frequently used comorbidity scores was consistent across selected elderly populations. We recommend that investigators use these performance data as one important factor when selecting a comorbidity score for epidemiologic analyses of health care utilization data.

Keywords: comorbidity adjustment, confounding (epidemiology), prediction, claims data, methods

Comorbidity scores are commonly used tools to efficiently adjust for confounding in epidemiologic studies using health care utilization databases. Comorbidity scores aggregate all relevant comorbidities into a single variable. The attraction of such scores is that they are widely accepted tools that are easy to apply and can simplify data analysis, particularly when multiple hypotheses will be tested or when comorbidities are considered as time-varying confounders. There have been an increasing number of reports on the performance of comorbidity scores in adjusting confounding in observational studies.1 These studies help researchers in selecting the best performing score for their study. However, it is not clear to what extent a score that demonstrated good performance in one population will perform well if applied in studies of other populations and databases.

The construct “comorbidity” reflects the aggregate effect of all clinical conditions a patient might have, excluding the disease of primary interest.2 Because there is no gold standard, researchers have validated measures of comorbidity by how well they predict mortality or other outcomes. The predictive performance of comorbidity scores depends on several factors, including 1) the clinical conditions included in a score and their relative weights, 2) the distribution of comorbid conditions in the source population, 3) the endpoint of a study, such as 1-year mortality, and 4) the accuracy of the administrative data.3 The predictive performance of 2 scores can be validly compared when factors 2 to 4 are held constant. Several studies have explored the predictive validity of individual comorbidity measures in claims data,414 but most of these do not allow direct comparisons.1 Even more important, there are no performance comparisons across population databases that test the generalizability of performance to similar but not identical populations.

We sought to compare the performance of 6 claims-based comorbidity scores in predicting 1-year mortality in subsets of elderly populations (65+) across selected Canadian and U.S. health care utilization databases.

METHODS

Study Populations

The data presented below are derived from 4 elderly populations in Canada and the United States that were available to the investigators:

1) All British Columbia residents age 65 years or older on March 31, 1996, who had at least 1 health care encounter paid for by the Ministry of Health (for a prescription medication, medical service, or hospitalization) during the 4 months prior to the baseline year (April 1, 1995 to March 31, 1996). British Columbia covers prescription medications and medical services costs for all residents 65 years or older through a tax-funded province-wide system, with a dispensing fee of $7.60 per prescription. As part of a larger policy study,15 we obtained linkable claims data for all patients who had filled at least one prescription for an angiotensin-converting enzyme (ACE) inhibitor or calcium channel blocker during the period January 1, 1995 to December 31, 1997. Patients who died or were admitted to long-term care during the baseline year were excluded. The cohort of eligible patients (N=141,161) was followed for 1 year after baseline from April 1, 1996 to March 31, 1997. We assessed comorbidity during the baseline year using several approaches and then measured mortality during the follow-up year. Results from British Columbia were presented previously16 and were used in this study for an international comparison.

2) We identically defined a cohort of Medicare enrollees age 65 years or older who had complete drug coverage either through Medicaid (32%) or the Pharmacy Assistance for the Aged and Disabled program (PAAD) (68%) in New Jersey. Similar to Medicaid, PAAD has no deductible and no maximum benefit, but there is a nominal $2 copayment for each prescription. PAAD has more generous income eligibility criteria than those of Medicaid and therefore includes patients above the poverty level. The program's eligibility criteria were an annual income <$15,700 if single and <$19,250 if married. The baseline year started on January 1, 1994; the follow-up year started on January 1, 1995. All patients had a pharmacy claim during the 4 months prior to the baseline year and had received an ACE inhibitor or calcium channel blocker at least once during the observation period. A total of 108,247 patients were eligible for this study.

3) We identically defined a Medicare/PAAD population in New Jersey without the restriction to patients with ACE inhibitor or calcium channel blocker (N=235,881).

4) We similarly defined a validation sample from Pennsylvania Medicare enrollees age 65 years or older who had complete drug coverage through the Pharmacy Assistance Contract for the Elderly (PACE). The eligibility criteria for PACE were an annual income <$13,000 if single and <$16,200 if married. There was a $6 copayment for each prescription. The baseline year started on January 1, 1995. The follow-up year started on January 1, 1996. All patients had a pharmacy claim during the 4 months prior to the baseline year without the restriction to patients with ACE inhibitor or calcium channel blocker and survived the baseline year (N=230,913).

All personal identifiers were stripped from the data prior to analysis and transformed to untraceable coded study numbers to protect participants’ privacy.

Scores

Original research on the metric properties of comorbidity indices for claims data was identified by a literature search using Medline and HealthStar databases, bibliographies, and expert consultations. We identified 6 distinct indices of comorbidity for use in administrative databases.4,710,12 Four of the 6 scores use diagnostic information from ICD-9 codes and are based on the Charlson Index, which was originally designed for clinical data.17 Two scores are based on outpatient drug utilization data. The composition and development of these scores are summarized elsewhere.1,16 We did not include a list of 30 conditions published by Elixhauser et al.18 Such lists may perform slightly better, but they lose the advantage of simplicity of comorbidity scores that provide a single number per patient. They will therefore be of limited utility in studies that model comorbidity as a time-varying covariate and in studies of moderate or small sample size.

Diagnosis-based Scores.

The Charlson Index is a list of 19 conditions; each has a weight assigned from 1 to 6. The Charlson Index score is the sum of weights for all conditions that a patient has. For the Deyo and Romano implementations of the Charlson Index, the corresponding sets of 5-digit ICD-9-CM diagnoses as delineated in their original publications were used.5,9 The Romano implementation is often called the Dartmouth-Manitoba score. These 2 scores differ only modestly in the ICD-9-CM codes mapping the conditions of the Charlson Index.5 For the D’Hoore implementation of the Charlson comorbidity index, the first 3 digits of the ICD-9 as described in D’Hoore et al. were used.10 The Ghali adaptation of the Charlson Index was calculated with the reduced set of diagnoses and new weights that improved prediction of in-hospital mortality in bypass surgery patients.12

The 4 scores were calculated using ICD-9-CM codes derived from all hospital discharges, which can contain up to 16 diagnoses in British Columbia and 9 diagnoses in New Jersey and Pennsylvania, and all diagnoses associated with ambulatory physician services during the baseline year. Although all 4 diagnosis-based scores were developed for hospital discharge diagnoses, studies have found that including ambulatory diagnoses in addition to hospital discharge diagnoses improves the prediction of mortality.16,19,20

Prescription Medication-based Scores.

The Chronic Disease Score (CDS) uses outpatient pharmacy dispensing data to assign patients to chronic disease groups. An integer weight is given to each comorbidity category represented by selected medication classes, which are summed to an overall score.7 For calculation of the CDS-1, we followed the original coding.7 Drugs that became available since 1992 were assigned to an appropriate category based on the condition for which the medication is prescribed. CDS-2 was developed later and includes more drug classes.8 It is based on the fairly crude 4-digit American Hospital Formulary Services classification of drugs.21 We used the weights that were published for predicting primary care visits.8

Combining several sources of morbidity information, including hospital discharge information, ambulatory or nursing home diagnoses, and pharmacy claims, can reduce residual confounding bias.1920 In earlier studies, we found that combining a diagnosis-based comorbidity score with the number of distinct prescription medications used during the past year was found to be a particularly practical and effective improvement.16,19 We therefore used the number of distinct prescription drugs (distinct chemical entities) dispensed during the baseline year as an additional crude comorbidity measure.

Study Endpoint

The primary study endpoint was all-cause mortality during the follow-up year. We used British Columbia vital statistics information to identify deaths in Canada, and both Medicare and Medicaid eligibility files in the U.S. populations.22 We used complete 1-year follow-up information on all patients.

Data Quality

The accuracy of pharmacy claims data within Medicaid, PAAD, and PACE is considered very good.23 The strength and limitations of diagnostic coding in Medicaid and Medicare have been frequently examined.2426 In British Columbia, pharmacy dispensing data including medication, strength, and the number of units are entered into a computer network by the pharmacist when a prescription is filled, and underreporting and misclassification appear to be minimal.27 Although previous reports indicate reasonable levels of accuracy and completeness of diagnostic coding,28 misclassification of ICD-9-CM diagnoses in British Columbia, New Jersey, and Pennsylvania is probably similar to that seen in research using other administrative databases.25,2932

Data Analysis

One-year mortality was modeled by multivariate logistic regression models that included age, gender, and each of the comorbidity scores. The scores and number of distinct prescription drugs used in the previous year were modeled as continuous variables; earlier studies showed no improvement in predictive performance compared to grouping scores in 3 categories.16 Second-degree polynomials were considered for age and the scores, and were removed from the final models if they did not significantly improve the model fit (likelihood ratio test P < .01). The area under the receiver operating characteristic (ROC) curve or c-statistic was calculated, as it is a widely accepted measure of discrimination.33 The c-statistic ranges from 0 to 1, with 1 indicating a perfect prediction and 0.5 indicating a chance prediction. For example, the Framingham Heart Study equations predict the incidence of coronary heart disease based on age, blood pressure, smoking, diabetes, and LDL and HDL levels with a c-statistic of 0.77.34 c-statistics between 0.7 and 0.8 are generally considered as acceptable and between 0.8 and 0.9 as excellent.35 Higher values are rarely observed in studies of the general population and are described as outstanding. It is assumed that in most cases, a higher c-statistic is a marker for better control for confounding.16 Asymptotic 95% confidence limits were calculated for c-statistics.36

We calculated the relative improvement in percent between c-statistics of 2 scores beyond the predictive power of age and gender alone as {[(c1cb)−(c2cb)]/(c1cb)} * 100, with cb being the c-statistic of the corresponding baseline age-gender adjusted logistic regression model(s).36 Because the comparison of absolute performance depends on many factors that vary among populations and studies, we chose to compare the relative ranking of scores across the 4 study populations.

RESULTS

Population

At the beginning of the baseline year, the BC population was on average 75.4 years old (standard deviation ± 6.7), and 58% were female. The U.S. populations were comparable in age but had higher proportions of female enrollees (78%, 77%, and 80.7%; Table 1). The distributions of comorbidity indices during the baseline period are shown in Table 2. There were 17,690 deaths in the total NJ population (7.5%), and 20,684 deaths in PA (7.7%). In the cardiovascular subpopulations, 5,569 deaths occurred during the follow-up year in BC (3.9%), and 7,236 in NJ (6.7%).

Table 1
Characteristics of Four Elderly Populations in British Columbia, New Jersey, and Pennsylvania During the Baseline Year
Table 2
Distributions of Six Comorbidity Scores During the Baseline Year in Elderly in British Columbia, New Jersey, and Pennsylvania

Performance

The Romano adaptation of the Charlson Index performed best among the 6 scores at predicting 1-year mortality consistently across all 4 populations, with c-statistics ranging from c=0.754 in the cardiovascular disease population of NJ to 0.771 in the total BC population (Table 3). The performance of Deyo's version was not statistically significantly different from the Romano implementation. The 3-digit ICD-based D’Hoore score and Ghali's adaptation consistently performed less well, and both CDS-1 and CDS-2 performed even less well (Table 3). In all 4 study populations the same rank order of predictive performance (c-statistic) was found: Romano ≥ Deyo > D’Hoore > Ghali > CDS-1 > CDS-2. On average, scores predicted mortality better in the restricted BC cohort than in the identically defined restricted Medicare cohort, mostly due to better prediction by the baseline age-gender model. However, the c-statistics of the 2 drug-based scores (CDS-1 and CDS-2) were 56% and 65% higher in the BC population compared to a similarly defined NJ population adjusted for the predictive power of age and gender alone. Combining the number of different prescription drugs used in the past year with the Romano score improved predictive power consistently by an average of 6% in all 4 study populations. Modeling scores as tertiles or including quadratic terms did not improve prediction.

Table 3
Prediction of One-year Mortality by Six Comorbidity Scores Measured One Year Earlier in the Four Study Populations

DISCUSSION

Among 4 elderly North American populations we found a consistent performance ranking of comorbidity scores in predicting 1-year mortality using claims data. This finding is reassuring for researchers who want to select the best performing standard score to control for comorbidity. Based on these data it can be assumed with greater certainty that the performance ranking can be generalized to other large general populations age 65 years or older, or large subgroups thereof, such as cardiovascular patients.

Comorbidity scores are useful because they are easy to apply and save time and resources, a major issue when analyzing massive health care databases and testing multiple hypotheses. They can also increase the efficiency of statistical inference, which may become an issue in claims data when analyzing small population subgroups or when comorbidities are modeled as time-varying covariates in longitudinal studies. However, adjusting for a score should not be regarded as successfully controlling for all confounding caused by comorbidity,37 because even scores with improved performance impose a functional relation between comorbidities and outcome. The narrower a subpopulation is defined, the more likely a score developed for a more general population will perform insufficiently. Scores are useful for preliminary analyses to indicate the direction and magnitude of confounding, which can guide decisions about further adjustment. It remains unclear how much more confounding can be controlled by using traditional multivariate modeling techniques to control comorbidity.

We found that diagnosis-based scores consistently performed better than medication-based scores at predicting future mortality. This is consistent with earlier findings that sicker patients are less likely to be treated for comorbid conditions.38 In particular, medications with some preventive effects, such as oral antidiabetic agents39 or lipid-lowering drugs,40 are less frequently prescribed in very sick patients, causing them to appear artificially “healthier” in medication-based scores. Another limitation of medication-based scores is that their list of drugs must be updated regularly as new drugs and drug classes become available.

In earlier studies, we had found that the number of distinct medications received during the previous year was a better predictor of mortality than the CDS-1.16 This finding is consistently true among our 4 large study populations. It is not surprising that the Ghali score in our sample of elderly beneficiaries did not perform as well as in the original publication because its weights were developed in patients undergoing bypass surgery.1,12

Numeric differences in the performance between some scores were small because the c-statistic (or area under the ROC curve) has a limited sensitivity to detect additional improvements in prediction once a certain level is reached.41 Nevertheless, if researchers had the option to select one score, we would recommend choosing the numerically best performing even if the absolute difference in c is small. In practice, often not all desirable data sources (5-digit ICD-9-CM diagnostic data plus pharmacy dispensing data) are available. Based on our ranking, researchers can pick the next best alternative and discuss the increased potential for residual confounding compared to the better performing scores.

A c of 0.77 means that for all possible pairs of individuals who died with individuals who lived, 77% of the time the model correctly attributed a higher risk of death to the person who died than to the person who lived. It has been shown how predictive validity can be translated into confounder adjustment.16 From several examples4244 and text books,35 it appears that a c above 0.7 is a model with acceptable performance and above 0.8 with excellent predictive performance; however, the meaningfulness of any level of performance depends on what is otherwise achievable using other methods. Large investments, such as additional data and analyses, yield only small numeric gains in c above 0.75. Whether those gains are worth their price depends on the benefits of a “truer” analysis and the costs of error, issues that are unique to each problem studied.

The data for our study are now several years old; however, we are not aware of important changes in diagnostic coding for Medicare patients or changes to the structure and interpretation of relevant data fields. In the unlikely event that there were significant changes, they would affect the absolute performance in predicting mortality (c-statistic) but should not change the ranking in any systematic way.

Our study was limited to predicting mortality, the ultimate outcome of care. However, the ranking of scores may be different when predicting other health care outcomes, such as annual expenditures or number of services. The study was further limited to health care utilization databases from 3 states/provinces that were available to the authors and originally requested for other purposes. Although this selection may not be representative for both countries, it provides sufficient variability in characteristics of patients, health care systems, and recording practice for claims. The Pennsylvania database had more extensive coding of diagnoses related to ambulatory visits and services and the British Columbia data had more discharge diagnoses available. This may have affected the absolute level of performance, which is the reason why a relative ranking is the most valid approach to compare the performance of scores across populations.

We recommend that investigators use these performance data as one important factor when selecting a comorbidity score for epidemiologic analyses of health care utilization data. The Charlson-based Romano and Deyo scores using published weights in combination with a simple count of the different prescription drugs received during the past year appear to be well performing comorbidity measures in epidemiologic studies.

Acknowledgments

Supported by grants R01-HS10881 from the U.S. Agency for Healthcare Research and Quality, and R03-AG19463, R03-AG18395, and R01-AG18833 from the National Institute on Aging.

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