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Survival Advantage of Hispanic Patients Initiating Dialysis in the United States Is Modified by Race

Bhamidipati V.R. Murthy^*, Donald A. Molony^* and Austin G. Stack^*,

^* Division of Renal Diseases and Hypertension, Department of Internal Medicine, University of Texas Medical School at Houston, Houston, Texas; and Regional Kidney Center, Department of Internal Medicine, Letterkenny General Hospital, North Western Health Board, Letterkenny, Ireland

Address correspondence to: Dr. Bhamidipati V.R. Murthy, Department of Internal Medicine, Division of Renal Diseases and Hypertension, University of Texas Health Science Center at Houston, 6431 Fannin Street, MSB 4.148, Houston, TX 77030. Phone: 713-500-6868; Fax: 713-500-6882; E-mail: bhamidipati.murthy{at}uth.tmc.edu

	Abstract

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Differences in survival have been reported among ethnic groups in the general population. Whether these extend to patients with ESRD is unclear. Using national data, mortality risks of ethnic groups who began dialysis treatment in the United States between May 1, 1995, and July 31, 1997, were compared over 2 yr. Patients were classified as Hispanic or non-Hispanic and then subclassified by race forming six race-specific subgroups: Hispanic white, black, and other and non-Hispanic white, black, and other. Mortality rates for Hispanics compared with non-Hispanics were 19.2 versus 26 per 100 patient-years at risk for those with diabetes and were 14.7 versus 22.7 per 100 patient-years at risk for those without diabetes. For those with diabetes, adjusted mortality risks for Hispanics versus non-Hispanics were 30% lower (95% confidence interval [CI], 26 to 34%). In subgroup analysis, mortality risks for Hispanic whites and Hispanic blacks were 35% (95% CI, 31 to 39%) and 33% (95% CI, 12 to 48%) lower than non-Hispanic whites and were similar in magnitude to those of non-Hispanic blacks (32% lower; 95% CI, 29 to 35%) and non-Hispanic other (33% lower; 95% CI, 28 to 39%). Interestingly, mortality risks for Hispanic others were not significantly different from non-Hispanic whites. For those without diabetes, adjusted mortality risks for Hispanics versus non-Hispanics were 17% lower (95% CI, 9 to 23%), and subgroup analysis yielded similar patterns to those of individuals with diabetes. The survival advantage of Hispanic over non-Hispanic patients who receive chronic dialysis treatment in the United States is not consistent across subgroups and is modified by race. Cultural and genetic differences as well as variation in the access and delivery of care before and while on dialysis may account for these differences.

	Introduction

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Health outcomes for ethnic minorities have become an increasingly important area of concern for health care providers and public policy leaders in the United States. This is due, in part, to the increasing number of reports that have documented significantly greater comorbidity risk profiles for Hispanics and other ethnic minorities compared with white populations (1). In addition, Hispanics have become the fastest growing ethnic minority in the general US population (2), as well as the fastest growing ethnic group among the adult ESRD population (3). Accordingly, it is important that we assess the utilization of health care services among these groups, address any disparities that exist in the receipt of such services, and evaluate any potentially negative consequences on morbidity and mortality.

There is a relative paucity of data on health-related outcomes among Hispanics with chronic kidney disease in the United States. A recent report by Frankenfield et al. (4) demonstrated greater survival of adult Hispanic ESRD patients who were treated with hemodialysis (HD) compared with non-Hispanic whites. Moreover, in their study, the delivery of adequate ESRD care, as measured by an achieved solute clearance (Kt/V) of 1.2, an average hemoglobin 11g/dl, or the proportion of patients with an arteriovenous fistula, was at least as good for Hispanic as for non-Hispanic white patients (4). These observations suggested that the health care provided to adult Hispanic patients who were treated with HD and associated survival experiences were similar if not better than other racial or ethnic groups.

Contrasting, Kausz et al. (5) observed that Hispanics and other minorities receive comparatively less care before ESRD initiation compared with whites. In a nationally representative cohort of new ESRD patients in the United States, they observed that Hispanic patients as well as other ethnic minority groups were less likely to be initiated on dialysis at the recommended level of GFR when compared with non-Hispanic white patients. Their findings suggest that disparities exist in the provision of health care during the pre-ESRD period, and these health care disparities may have an impact on subsequent survival. In view of these conflicting observations, we explored survival differences between Hispanic and non-Hispanic subgroups among incident dialysis patients in the United States, in the context of differences in measurable baseline comorbidity and delivered pre-ESRD care.

	Materials and Methods

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Study Design and Data
The study was a historical prospective cohort of new ESRD patients in the United States who were initiated on dialysis between May 1, 1995, and July 31, 1997. Baseline data on all patients at study entry were obtained from the Center for Medicare and Medicaid Services (CMS) Medical Evidence Form 2728. The CMS form is a government document that is completed for all new patients who are initiated on dialysis (6). It is a validated data collection instrument (7), and data arising from it have been published in several previous studies (5,8–10). The form records data on several sociodemographic characteristics and captures information on a broad list of comorbid conditions, laboratory values, and indicators of pre-ESRD care recorded at or near ESRD onset.

Definition of Ethnicity and Race
The CMS form incorporates two questions that collect information on ethnicity and race. The first classifies patients as Hispanic-Mexican, Hispanic other, and non-Hispanic. The second categorizes patients into one of nine racial groups: white, black, American Indian/Alaskan Native, Asian, Pacific Islander, Mid-East/Arabian, Indian subcontinent, other, and unknown. For the purposes of this study, ethnicity was classified as either Hispanic or non-Hispanic and race was classified in one of three principal groups: white, black, and other. The "other" category comprised a relatively small fraction of the entire cohort and was included to facilitate comparisons.

The study start date for all incident patients was defined as day 90 of ESRD. This was based on the fact that many patients who are younger than 65 yr do not become eligible for Medicare for up to 90 d and therefore may have incomplete claims data before this. For the current study, the Medical Evidence data set was merged with mortality and transplantation data from the US Renal Data System Standard Analysis Files. Approval for the current study was obtained from the University of Texas Institutional Review Board.

Patient Population
There were 158,685 patients, aged 18 and over, who were initiated on dialysis from May 1, 1995, to July 31, 1997. Patients were excluded from the analysis if they had received a renal transplant within the first 90 d of ESRD initiation, when modality assignment could not be determined at day 90 of ESRD, or when they had died within the first 90 d (n = 19,648). An additional 38,419 patients were excluded because of missing data on demographic or clinical variables. After these exclusions, 100,618 adult patients were available for this analysis.

Statistical Analyses
As survival differences have been reported among racial subgroups of ESRD patients (11,12), we believed that it was appropriate to subclassify ethnicity by race. Descriptive statistics of patient characteristics by Hispanic status were performed for the entire cohort and for each of six combined ethnic-racial subgroups: Hispanic white, Hispanic black, Hispanic other, non-Hispanic white, non Hispanic black, and non-Hispanic other. Categorical variables among groups were compared using ² tests, and means of continuous variables were compared using ANOVA.

The health status of patients at ESRD onset and measures of pre-ESRD care were compared between racial/ethnic subgroups. Multivariate logistic regression was used to explore the association of ethnicity and race (six subgroups) with three routinely measured comorbid indicators as well as a measure of pre-ESRD care. Comorbid indicators were represented by diabetes, coronary artery disease, anemia, and hypoalbuminemia as recorded at ESRD initiation. Severe anemia was defined as a hematocrit <28%, and hypoalbuminemia was defined as a serum albumin <3.5g/dl. These cutoff values were chosen on the basis of their threshold prognostic significance in recent mortality analyses. Furthermore, a hematocrit value of 28% represented the mean for the entire cohort, and a serum albumin value of 3.5g/dl was virtually identical to the mean of all lower limits of normal albumin values recorded on the CMS 2728 form. In addition, the use of erythropoietin before ESRD onset, also captured by the CMS document, served as a surrogate measure of health care during this period. Unadjusted and adjusted odds ratios (OR) were determined in each case with corresponding 95% confidence intervals (CI).

Multivariate Cox regression models explored the relationship of ethnicity-race with mortality risk in new ESRD patients. Patients were followed until death, loss to follow-up, transplantation, or the end of 2 yr from the study start date. Both unadjusted and adjusted hazard ratios were calculated for each ethnic subgroup: Hispanic white, Hispanic black, Hispanic other, non-Hispanic black, and non-Hispanic other, with non-Hispanic white as the referent group. The following variables were adjusted for in the multivariate analyses: age, gender, hypertension, coronary artery disease, cerebrovascular disease, peripheral vascular disease, tobacco use, cardiac arrest or arrhythmia, chronic obstructive pulmonary disease, cancer, alcohol dependence, drug dependence, AIDS, functional status (inability to ambulate or transfer), body mass index (BMI), serum albumin, and hematocrit. Age was modeled as a continuous variable and all others as categorical variables. GFR at initiation of dialysis was estimated using the modified Modification of Diet in Renal Disease equation (13). Diabetic and nondiabetic patients were analyzed in separate models on finding a significant interaction term demonstrating nonproportional hazards. All statistical analyses were carried out using SAS software (V8.0; Cary, NC).

	Results

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Baseline Differences between Hispanic and Non-Hispanic Patients Who Were Initiated on Dialysis
The study cohort consisted of 100,618 patients who were initiated on dialysis between May 1, 1995 and July 31, 1997. Of these, 10,393 (10.3%) were identified as Hispanic. Table 1 illustrates differences in baseline characteristics between Hispanic and non-Hispanic patients. Compared with non-Hispanic ESRD patients, Hispanics were younger (59.2 ± 15.0 versus 61.8 ± 15.3 yr), had larger proportion of whites (87.1 versus 60.7%), and had a higher percentage of patients with diabetic nephropathy (61.3 versus 42.3%). The prevalence of diabetes (either as a cause of ESRD or comorbid condition), hypertension, tobacco use, coronary artery disease or myocardial infarction, congestive heart failure, chronic lung disease, cerebral and peripheral vascular disease, AIDS, and cancer were significantly lower in Hispanic patients compared with non-Hispanic patients. Although serum creatinine values were virtually identical in both groups, the estimated GFR values at ESRD initiation was significantly lower for Hispanic patients compared with non-Hispanic patients (6.8 ± 2.7 versus 7.1 ± 2.8 ml/min). Mean serum albumin (3.1 ± 0.7 versus 3.2 ± 0.7 g/dl) and hematocrit levels (27.6 versus 28.1%) were also significantly lower in the Hispanic group. The percentage of patients who were treated with peritoneal dialysis was significantly lower in Hispanic compared with non-Hispanic patients (10.4 versus 13.3%). The demographic and clinical characteristics of patients who were excluded from this analysis as a result of missing or incomplete data were nearly identical to those of the study population.

Finally, we compared the likelihood of having a low serum albumin at ESRD onset among groups. Compared with non-Hispanic whites, only Hispanic whites were more likely to be hypoalbuminemic at ESRD onset (OR, 1.22; 95% CI, 1.16 to 1.28) within the Hispanic groups. However, non-Hispanic blacks and non-Hispanic others had significantly greater likelihood of hypoalbuminemia (non-Hispanic blacks: OR, 1.16; 95% CI, 1.12 to 1.20; non-Hispanic others: OR, 1.26; 95% CI, 1.18 to 1.35) compared with non-Hispanic whites.

Survival Differences between Hispanic and Non-Hispanic Patients during Follow-Up
Of the 100,618 incident patients, 25% patients died during the follow-up period. Adjusted Cox survival curves were estimated for non-Hispanic and Hispanic patients and are shown in Figures 1 and 2. Overall survival was significantly poorer for non-Hispanics compared with Hispanics in both diabetic (Figure 1) and nondiabetic (Figure 2) cohorts. For diabetic patients, the survival curves begin to diverge early on and continue to diverge thereafter, with significantly poorer survival for non-Hispanic compared with Hispanic patients. For those without diabetes, the pattern was similar, although less impressive, with divergence of the survival curves occurring after 6 mo of follow-up.

View larger version (14K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 1. Adjusted Cox survival curves for new diabetic ESRD patients by Hispanic ethnicity. All models adjusted for age at study start, gender, race, hypertension, coronary artery disease, cardiac arrest/arrhythmia, peripheral vascular and cerebrovascular disease, congestive heart failure, tobacco use, chronic lung disease, AIDS, neoplasm, alcohol dependence, body mass index, serum albumin, hematocrit, estimated GFR, functional status (inability to walk and transfer), and pre-ESRD erythropoietin use.

View larger version (13K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 2. Adjusted Cox survival curves for new nondiabetic ESRD patients by Hispanic ethnicity. All models adjusted for age at study start, gender, race, hypertension, coronary artery disease, cardiac arrest/arrhythmia, peripheral vascular and cerebrovascular disease, congestive heart failure, tobacco use, chronic lung disease, AIDS, neoplasm, alcohol dependence, body mass index, serum albumin, hematocrit, estimated GFR, functional status (inability to walk and transfer), and pre-ESRD erythropoietin use.

	Discussion

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In this study, we describe the mortality risk patterns of an incident dialysis cohort in the United States with contemporaneous consideration of ethnicity and race. Overall, mortality risks for Hispanic patients were significantly lower than that of non-Hispanic patients, although this pattern was not consistent across all subgroups. Among those with diabetes, the survival advantage of Hispanic white and black patients over non-Hispanic whites did not extend to the Hispanic other category after adjustment. Among those without diabetes, neither patients who were categorized as Hispanic black or Hispanic other achieved a survival advantage over non-Hispanic whites. This study highlights significant variation in mortality risks among Hispanic subgroups initiating dialysis therapy in the United States, a finding that was not explained by adjustment for differences in case mix. Moreover, it demonstrates an important effect modification of race on the ethnicity–survival relationship, underscoring the need for simultaneous inclusion of these indicators in future analyses when evaluating clinical outcomes.

The overall survival advantage of Hispanics over non-Hispanics in this study might well be attributed to lower comorbidity burden at ESRD onset. The prognostic impact of each of these comorbid conditions has been demonstrated consistently in previous mortality analyses (14,15). When these differences were accounted for in the adjusted analysis, the lower mortality risk for Hispanics remained. A novel and more perplexing observation from this study is the significant variation in mortality rates among Hispanic subgroups. Hispanic whites experienced the lowest mortality rates among all Hispanic groups, a finding that was present in those both with and without diabetes. Lower mortality risks were also observed for Hispanic blacks, but these were seen only among those with diabetes. Finally, no mortality advantage was seen for Hispanic others, who had similar risk for death to those of non-Hispanic whites. This was surprising as cardiovascular comorbidity levels were substantially lower and BMI levels were significantly higher in the Hispanic other category compared with the remaining Hispanic groups—findings that favored increased survival (16–18). Although differences in case mix may have contributed to these findings, it should also be noted that the lack of survival advantage among the Hispanic other category may reflect the heterogeneity of this group encompassing people with diverse genetic backgrounds.

Differences in access to and delivery of health care between Hispanic and non-Hispanic cohorts as well as among Hispanic subgroups before ESRD start could also have contributed to these mortality differences. Several major indicators of pre-ESRD care, including hematocrit level at ESRD initiation, serum albumin concentration, and prescribed erythropoietin use before initiation of dialysis, were significantly lower among Hispanics compared with non-Hispanics. These findings concur with the observations of Kausz et al. (5), who, in a similar study, found that Hispanics were 47% more likely to be initiated late on dialysis (GFR <5 ml/min at initiation of dialysis) compared with whites. Moreover, many of these components of pre-ESRD care have strong prognostic significance in survival analysis (19–22). Despite adjusting for several pre-ESRD care indicators, the overall mortality advantage of Hispanics over non-Hispanics remained. Similarly, in the subgroup comparisons, the higher mortality rates among the Hispanic other category persisted, having taken into consideration baseline differences in pre-ESRD care among groups. It might be argued that the higher relative mortality risks among the Hispanic other group might reflect unequal delivery of pre-ESRD care. The low rates of pre-ESRD erythropoietin use and the low levels of residual renal function at dialysis initiation among patients in the Hispanic other category are certainly suggestive of this. Nevertheless, the adjusted analysis also confirmed higher-than-expected death rates in the Hispanic other category, suggesting that factors other than those measured contributed to these differences.

Although several studies have compared the mortality risks of Hispanics and non-Hispanics in the general population, few have compared survival differences in ESRD cohorts (23–25). Those published have been limited in generalizability as a result of center effect or selection bias (23,24). To our knowledge, only one other study has explored the mortality risks of Hispanic patients who undergo dialysis therapy. Frankenfield et al. (4), in an analysis of data from the ESRD Clinical Performance Measures Project, found significantly lower mortality among Hispanic compared with non-Hispanic patients who were treated with HD after 1 yr of follow-up. Although this study had several strengths—large sample size and nationally representative—it also had a number of limitations that could potentially confound mortality analysis that is unique to this population. First, their analyses were based on a cross-sectional sample of "prevalent" dialysis patients. The survival bias of prevalent ESRD cohorts is a well-described limitation in mortality analyses of this nature. Second, analyses were restricted to HD patients, thereby reducing its applicability to those who were treated with peritoneal dialysis. Furthermore, the availability of clinical and laboratory data on comorbid conditions recorded at ESRD onset and surrogate markers of pre-ESRD care were lacking. Our study, in contrast, overcame many of these deficits. The inclusion of an incident dialysis population that is nationally representative; the adjustment for a broad list of sociodemographic, comorbid, and pre-ESRD care indicators; and finally the extension of the mortality comparisons to Hispanic subgroups are signal strengths of our approach.

A "survival advantage" for Hispanics has been observed in the general population, as well as in relation to specific disease (25–28). This advantage is observed despite a lower socioeconomic status, higher poverty rates, less education, and less health insurance, all factors that are potentially associated with higher mortality and morbidity rates. The reasons for this so called "Hispanic paradox" are poorly understood, but several hypotheses have been postulated. They include greater family support systems, more favorable health behaviors, genetic factors, and the "salmon-bias" theory, which postulates lower mortality rates among Hispanics in the United States on the basis that older Hispanics return to their native birthplace to die and are as a result not included in the US mortality statistics (29). Several possible explanations could account for the overall lower mortality rates among Hispanics despite their adverse risk profile documented at ESRD initiation. First, factors that are associated with a Hispanic survival benefit in the general population may be operating in patients who are treated with ESRD. Second, reduced access to transplantation for Hispanics and lower rates of renal transplantation for those who are wait-listed could explain the "apparent" survival advantage of Hispanics over non-Hispanics who remain in the dialysis pool (30–35). Transplantation was a censoring event in the current analysis ensuring that any observed survival advantage was attributed only to characteristics that occurred before or during the dialysis period. Finally, the contribution of genetic polymorphisms toward a Hispanic survival advantage cannot be ignored. A lower prevalence of complex trait diseases such as coronary artery disease as a result of reduced gene expression may exert considerable protection in Hispanic populations (36).

This study is not without limitations. First, the possibility of misclassification of ethnicity status should not be ignored. The designation of Hispanic ethnicity is typically carried out by personnel at the dialysis unit. We submit that assignment of ethnic status is not based on self-designation or through a systematic assessment of a patient’s background and may result in misclassification. However, the assignment of ethnicity is usually performed by members of the dialysis team who have interacted with the patient and their caregivers on several health-related, social, and financial issues, for which appropriate classification of ethnic status may be required. As such, we believe that a serious misclassification is unlikely, and, if it occurred, it is likely to be random in nature. Second, differential underreporting of comorbid conditions on the CMS 2728 form may bias mortality comparisons among groups and lead to biased estimates (37). For this reason, we repeated the regression models adjusting only for objective measures of comorbid status, including serum albumin, hematocrit, serum creatinine at ESRD initiation (an indirect measure of nutrition), BMI, and residual renal function, and found similar results. Third, the CMS document, while capturing the presence of comorbid disease, does not capture disease severity, which may have differed between Hispanics and non-Hispanics as well as among Hispanic subgroups. Although this is a potential concern, it should be noted that recent comparisons of different comorbidity instruments have demonstrated no incremental benefit of those that use disease severity -grading systems over those that do not (38).

In conclusion, the current study demonstrates that the overall survival of new ESRD patients of Hispanic ethnicity is substantially greater than that of non-Hispanics, with a 17% lower adjusted mortality risk among those without diabetes and a 30% lower adjusted mortality risk among those with diabetes. However, this survival advantage is not consistent across all Hispanic subgroups, with Hispanic whites and Hispanic blacks experiencing the lowest and Hispanic others experiencing the highest mortality rates. Although the mortality risks for Hispanic whites and Hispanic blacks were significantly lower than those of non-Hispanic Whites, they were almost equal to those of non-Hispanic blacks and other non-Hispanic groups. These differences in mortality outcomes cannot be explained easily by differences in baseline comorbidity profiles among groups or differences in transplantation rates during follow-up. Consequently, greater efforts should be invested in evaluating the contribution of access to dialysis therapy and, once on dialysis, access to transplant waiting list as possible causes for these differences. Moreover, the contribution of genetic polymorphisms to these disparate outcomes requires further exploration. Finally, given the strong effect modification of race on the Hispanic–survival relationship, we suggest that both race and ethnicity be considered as contemporaneous predictor variables in future mortality analyses. The disparity in mortality outcomes across Hispanic subgroups as demonstrated in this study should urge health care providers and policy makers to improve accessibility and delivery of medical care to all minority groups with chronic kidney disease.

	Acknowledgments

 
A.G.S. is supported by a National Scientist Development Grant (0335317N) from the American Heart Association.

A portion of this work was presented in part at the annual meeting of the American Society of Nephrology, San Diego, California, November 2003, and has been published in abstract form (J Am Soc Nephrol 14: 715A, 2003).

	Footnotes

 
Published online ahead of print. Publication date available at www.jasn.org.

The data reported here were supplied by the United States Renal Data System. The interpretation and reporting of these data are the responsibility of the authors and in no way should be seen as the official policy or interpretation of the United States government.

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