Abstract
Black renal transplant recipients experience shorter graft survival than white recipients, but no published data describe the graft outcomes among black Canadian recipients. Here, we analyzed data from the Canadian national renal replacement therapy registry, which included 20,243 incident dialysis patients (3% black, 97% white), 5036 of whom received a renal transplant during the study period. Black patients were significantly less likely to receive a renal transplant (deceased and living-donor combined) when compared with white patients (hazard ratio 0.59; 95% confidence interval 0.51 to 0.69; P < 0.0001). Among patients who underwent a renal transplant, there was no significant difference in the likelihood of graft failure between black and white patients, even after adjustment for comorbidities and socioeconomic status; black patients, however, had significantly lower posttransplantation mortality compared with white patients (hazard ratio 0.49; 95% confidence interval 0.28 to 0.88; P = 0.02). In conclusion, graft outcomes between black and white Canadian renal transplant patients are similar. Because this differs from the experience reported from the United States, further direct comparisons between the two populations is warranted.
Since the first renal transplant more than 50 yr ago, there has been significant improvement in both patient and graft survival.1–4 Despite this, disparities continue to exist in access to renal transplantation and posttransplantation outcomes for certain minority groups. In the United States, renal allograft survival is significantly lower in black than in white patients,5–10 which might be due to higher immunologic risk, poorer medication adherence, or decreased access to pre- and posttransplantation care.8,11–23 Whether black Canadian transplant recipients also have decreased transplant survival is unclear because there are no published data describing transplant outcomes among this patient group. A recent study comparing outcomes in Canada and the United States demonstrated that American transplant recipients had an increased risk for mortality. In the subgroup of black recipients, the higher risk for death seemed to be magnified in American versus Canadian black transplant recipients.24
It has been suggested that black Canadians are comparable to black Americans, because the ancestry of both populations lies in the slave trade of people who migrated from sub-Saharan Africa to the Caribbean; however, unlike black Americans, black Canadians are entitled to free access to renal transplantation and follow-up care at all kidney transplant centers. Therefore a detailed analysis of transplant outcomes in black Canadians may provide useful insights into the factors contributing to the inferior transplant outcomes among black Americans. We used national data from the Canadian Organ Replacement Register (CORR) to compare the likelihood of transplantation as well as patient and graft survival in renal transplant recipients among black and white Canadians treated for kidney failure in Canada.
RESULTS
There were 26,098 incident dialysis patients in this study, representing a random sample of approximately 75% of all patients initiating dialysis in Canada during the study period. Of these, 3459 (13%) were of neither black nor white race and 2396 (9%) were of unknown race and were therefore excluded. All analyses were performed on the remaining 20,243 individuals, 3% were black of whom of whom and 97% of whom were white (Table 1). By study end, 12,409 (61%) died before receiving a transplant and 214 (1%) were lost to follow-up. Black patients were significantly more likely than white patients to have diabetic nephropathy listed as their primary renal disease (37 versus 27%). Among comorbid conditions, coronary disease was significantly higher among white compared with black patients (33 versus 24%), as was current smoking status (15 versus 6%). Black patients were significantly more likely to be in the lowest quintile of socioeconomic stats (SES; 38 versus 23%).
Table 1.
Demographics and clinical characteristics in dialysis patientsa
| Parameter | Black | White | P |
|---|---|---|---|
| n (%) | 699 (3) | 19,544 (97) | |
| Age (yr; median [IQR]) | 55 (42 to 66) | 64 (50 to 73) | <0.0001b |
| Male, n (%) | 378 (54) | 11,895 (61) | 0.0003 |
| Cause of ESRD, n (%) | |||
| diabetic nephropathy | 260 (37) | 5342 (27) | <0.0001 |
| glomerulonephritis | 110 (16) | 3352 (17) | 0.3300 |
| hypertensive/ischemic renal disease | 150 (21) | 4038 (21) | 0.6100 |
| polycystic kidney disease | 27 (4) | 1169 (6) | 0.0200 |
| other | 152 (22) | 5643 (29) | <0.0001 |
| Comorbidity, n (%) | |||
| diabetesc | 42 (6) | 1322 (7) | 0.4300 |
| coronary diseased | 166 (24) | 6514 (33) | <0.0001 |
| chronic heart failure | 229 (33) | 5524 (28) | 0.0100 |
| stroke or TIA | 64 (9) | 2041 (10) | 0.2700 |
| chronic lung disease | 25 (4) | 2127 (11) | <0.0001 |
| peripheral vascular disease | 91 (13) | 3589 (18) | 0.0003 |
| malignancy | 36 (5) | 1852 (9) | 0.0001 |
| Current smoker, n (%) | 40 (6) | 2895 (15) | <0.0001 |
| Initial peritoneal dialysis modality, n (%) | 245 (35) | 5502 (28) | 0.0001 |
| Lowest quintile of SES, n (%) | 263 (38) | 4407 (23) | <0.0001 |
| Rural, n (%) | 16 (2) | 4220 (22) | <0.0001 |
| Geographic region, n (%) | |||
| Atlantic Canada | 29 (4) | 1990 (10) | <0.0001 |
| Quebec | 152 (22) | 5250 (27) | 0.0030 |
| Ontario | 458 (66) | 7502 (38) | <0.0001 |
| Manitoba | 7 (1) | 765 (4) | 0.0001 |
| Saskatchewan | 3 (0) | 697 (4) | <0.0001 |
| Alberta | 34 (5) | 1471 (8) | 0.0080 |
| British Columbia | 16 (2) | 1869 (10) | <0.0001 |
| Year of dialysis initiation, n (%) | |||
| 1990 to 1992 | 115 (16) | 4017 (21) | 0.0080 |
| 1993 to 1994 | 111 (16) | 3242 (17) | 0.6200 |
| 1995 to 1996 | 124 (18) | 3638 (19) | 0.5600 |
| 1997 to 1998 | 175 (25) | 4170 (21) | 0.0200 |
| 1999 to 2000 | 174 (25) | 4477 (23) | 0.2200 |
IQR, interquartile range; TIA, transient ischemic attack.
Kruskal-Wallis test; all others χ2 test.
In participants for whom the primary cause of ESRD was not diabetic nephropathy.
Includes angina, previous myocardial infarction, or previous coronary revascularization.
During the study period, 5036 individuals underwent kidney transplantation. Table 2 presents the relative rates of renal transplantation by black versus white race. After adjustment, black patients were significantly less likely to receive a renal transplant (deceased and living donor combined) when compared with white patients (hazard ratio [HR] 0.59; 95% confidence interval [CI] 0.51 to 0.69; P < 0.0001). The difference between black and white patients seemed to be less pronounced for deceased as compared with living donors. The interaction between race and age and between race and diabetes status was significant (P < 0.0001 and P = 0.005, respectively), suggesting that it would be appropriate to present the rate of transplantation after stratification by age and diabetes status.
Table 2.
Likelihood of transplantation by race
| Parameter | Any Transplant
|
Deceased-Donor Transplant Only
|
Living-Donor Transplant Only
|
||||||
|---|---|---|---|---|---|---|---|---|---|
| n (%) | Age Adjusted (HR [95% CI]) | Fully Adjusted (HR [95% CI])a | n (%) | Age Adjusted (HR [95% CI]) | Fully Adjusted (HR [95% CI])a | n (%) | Age Adjusted (HR [95% CI]) | Fully Adjusted (HR [95% CI])a | |
| Black | 195 (28) | 0.53 (0.46 to 0.61) | 0.59 (0.51 to 0.69) | 161 (23) | 0.58 (0.50 to 0.68) | 0.70 (0.59 to 0.82) | 34 (5) | 0.37 (0.26 to 0.52) | 0.35 (0.25 to 0.50) |
| White | 4841 (25) | 1.0 | 1.0 | 3576 (18) | 1.0 | 1.0 | 1265 (6) | 1.0 | 1.0 |
| P | <0.0001 | <0.0001 | <0.0001 | <0.0001 | <0.0001 | <0.0001 | |||
Adjusted for age, gender, primary cause of ESRD, year of diagnosis, comorbidities (diabetes, coronary heart disease, hypertension, chronic heart failure, stroke or TIA, chronic lung disease, serious medical illness, peripheral vascular disease, malignancy), smoking status, initial dialysis modality, SES, geographic region, and rural status.
For patients aged <40 yr, black patients had a 51% lower rate of transplantation as compared with white patients (HR 0.49; 95% CI 0.38 to 0.62; P < 0.0001), whereas those in the 50.0- to 59.9-yr age group had a 41% lower rate than comparably aged white patients (HR 0.59; 95% CI 0.42 to 0.82; P = 0.002). Among those aged ≥60 yr, the rate of transplantation was nonsignificantly higher for black than for white patients (HR 1.47; 95% CI 0.97 to 2.23; P = 0.07. Among those with diabetes, there was no significant difference in the rate of renal transplantation for black compared with white patients (HR 0.90; 95% CI 0.66 to 1.22; P = 0.50); however, among those without diabetes, black patients had a significantly lower rate of transplantation compared with white patients (HR 0.57; 95% CI 0.48 to 0.67; P < 0.001).
Table 3 compares the characteristics of black and white transplant recipients during the study period. Of these 5036 patients, 4841 (96%) were white and 195 (4%) were black. Overall, differences between black and white transplant recipients were similar to those in the general dialysis population. For example, black renal transplant patients were slightly younger, more often lived in lower income urban communities, and were significantly more likely than white patients to have hypertensive/ischemic nephropathy as their primary renal diagnosis.
Table 3.
Demographics and clinical characteristics in transplant recipients
| Parameter | Black | White | P |
|---|---|---|---|
| n (%) | 195 (4) | 4841 (96) | |
| Age (yr; median [IQR]) | 43 (30 to 52) | 45 (35 to 55) | 0.0400a |
| Male | 129 (66) | 3104 (64) | 0.5600 |
| Cause of ESRD, n (%) | |||
| diabetic nephropathy | 38 (19) | 984 (20) | 0.7800 |
| glomerulonephritis | 55 (28) | 1540 (32) | 0.2900 |
| hypertensive/ischemic renal disease | 43 (22) | 343 (7) | <0.0001 |
| polycystic kidney disease | 11 (6) | 636 (13) | 0.0020 |
| other | 48 (25) | 1338 (28) | 0.3500 |
| Comorbidity, n (%)b | |||
| diabetesc | 7 (4) | 116 (2) | 0.2900 |
| coronary diseased | 22 (11) | 439 (9) | 0.2900 |
| chronic heart failure | 30 (15) | 459 (9) | 0.0060 |
| stroke or TIA | 9 (5) | 120 (2) | 0.0600 |
| chronic lung disease | 2 (1) | 120 (2) | 0.2000 |
| peripheral vascular disease | 6 (3) | 176 (4) | 0.6800 |
| malignancy | 4 (2) | 125 (3) | 0.6500 |
| Current smoker, n (%) | 12 (6) | 748 (15) | 0.0004 |
| Initial peritoneal dialysis modality, n (%) | 61 (31) | 1655 (34) | 0.400 |
| Lowest quintile of SES | 70 (36) | 961 (20) | <0.0001 |
| Rural | 4 (2) | 1032 (21) | <0.0001 |
| Geographic region, n (%) | |||
| Atlantic Canada | 8 (4) | 482 (10) | 0.0070 |
| Quebec | 40 (21) | 1182 (24) | 0.2100 |
| Ontario | 115 (59) | 1747 (36) | <0.0001 |
| Manitoba | 2 (1) | 145 (3) | 0.1100 |
| Saskatchewan | 1 (1) | 175 (4) | 0.0200 |
| Alberta | 18 (9) | 519 (11) | 0.5100 |
| British Columbia | 11 (6) | 591 (12) | 0.0060 |
| Year of dialysis initiation, n (%) | |||
| 1990 to 1992 | 46 (24) | 1217 (25) | 0.6200 |
| 1993 to 1994 | 35 (18) | 914 (19) | 0.7400 |
| 1995 to 1996 | 44 (23) | 920 (19) | 0.2200 |
| 1997 to 1998 | 38 (19) | 962 (20) | 0.8900 |
| 1999 to 2000 | 32 (16) | 828 (17) | 0.8000 |
Kruskal-Wallis test; all others χ2 test.
Comorbidity data collected at dialysis initiation.
In participants for whom the primary cause of ESRD was not diabetic nephropathy.
Includes angina, previous myocardial infarction, or previous coronary revascularization.
Table 4 compares transplant outcomes between black and white patients. The median follow-up time among transplant recipients was 5.2 yr; 857 (15%) experienced graft failure, death occurred in 641 (13%) of patients, and 14 (0.3%) were lost to follow-up. In this analysis, there was no significant difference in the adjusted rate of graft failure between black and white patients (HR 1.16; 95% CI 0.82 to 1.64; P = 0.40) or the adjusted rate of graft failure or death (HR 0.90; 95% CI 0.67 to 1.21; P = 0.48; Table 4). After adjustment, black transplant recipients had significantly lower posttransplantation mortality compared with white patients (HR 0.49; 95% CI 0.28 to 0.88; P = 0.02). The bootstrapped 95% CI were similar (graft failure 0.82 to 1.67; graft failure or death 0.67 to 1.23; death 0.28 to 0.94).
Table 4.
Likelihood of graft failure or death by race
| Parameter
|
|||
|---|---|---|---|
| Black | White | P | |
| Graft Failure or Death | |||
| n (%) | 48 (25) | 1450 (30) | |
| Age Adjusted (HR [95% CI]) | 0.930 (0.700 to 1.240) | 1.000 | 0.63 |
| Fully Adjusted (HR [95% CI])a | 0.900 (0.670 to 1.210) | 1.000 | 0.48 |
| Graft Failure | |||
| n (%) | 36 (18) | 821 (17) | |
| Age Adjusted (HR [95% CI]) | 1.170 (0.840 to 1.640) | 1.000 | 0.35 |
| Fully Adjusted (HR [95% CI])a | 1.160 (0.820 to 1.640) | 1.000 | 0.40 |
| Death | |||
| n (%) | 12 (6) | 629 (13) | |
| Age Adjusted (HR [95% CI]) | 0.560 (0.320 to 0.999) | 1.000 | 0.05 |
| Fully Adjusted (HR [95% CI])a | 0.490 (0.280 to 0.880) | 1.000 | 0.02 |
Adjusted for age, gender, primary cause of ESRD, year of diagnosis, comorbidities (diabetes, coronary heart disease, hypertension, chronic heart failure, stroke or TIA, chronic lung disease, serious medical illness, peripheral vascular disease, malignancy), smoking status, initial dialysis modality, SES, geographic region, rural status, and time on dialysis (<1.0, 1.0 to 1.9, ≥2.0 yr).
DISCUSSION
As in multiple other countries, black patients treated with dialysis in Canada waited longer for a kidney transplant after initiation of dialysis and were less likely to undergo transplantation than white patients, especially for transplants from a living donor; however, black transplant recipients had similar graft survival and lower all-cause mortality compared with white patients, a finding that is strikingly different from that reported in American studies.2–5,18,25,26
Black Americans have lower access to renal transplantation than white Americans, probably because of lower referral rates for transplantation assessment, which may themselves be due to physician attitudes about poor adherence and lower graft success rates among black patients.17,22,27,28 Numerous studies from the United States also show inferior graft outcomes among black transplant recipients. Data from the United Network for Organ Sharing registry showed that between 1995 and 1999, the 5-yr graft survival rates for deceased donor transplants was 70 and 55%, respectively, for white and black patients. Among living donor transplants, the graft survival rates were 80 and 64%, respectively, for white and black patients. These differences were observed even in HLA-identical sibling transplants, which provided the best results for people of both races but led to markedly superior graft half-lives in white patients (42 versus 27 yr).5
In theory, the lack of disparity in graft failure rates between black and white patients in Canada versus the United States might depend on both macro-level factors (health care system and population) and micro-level factors (biologic and other determinants) that affect the transplant recipient. It is possible, although less plausible, that black Americans and the Canadian black population are “biologically” different as a result of historical patterns of immigration. This biologic argument has been used to explain differences in outcomes among black and white Americans for decades, but recent studies have shown that it likely plays a minor role in influencing transplant outcomes.11,13,14,19,25,29,30
A second possible explanation for differences in graft failure in black Americans versus black Canadians is that relative levels of comorbidity may differ between the two populations. For example, a large proportion of black Americans have hypertension listed as their primary renal diagnosis,1 whereas this was not the case in our study of the Canadian dialysis population during 1990 through 2000. In our analysis, black patients had significantly higher rates of diabetes as a primary renal diagnosis, but white dialysis patients were significantly older than black patients, with a higher prevalence of comorbid diseases including coronary disease, smoking, and peripheral vascular disease. This likely reflects that a higher proportion of white patients undergo transplantation in Canada and are therefore likely to have more comorbidity. Because we did not perform a direct comparison of Canadian and American data, we are unable to compare the level of comorbidity between either white or black patient groups in either of the two countries.
The third (and, in our opinion, the most probable) potential explanation for our findings may relate to differential access to health services. All Canadians who receive government assistance (welfare) are eligible for immunosuppressive medications through provincial drug programs. Recipients who are employed and have private insurance may receive coverage for the majority of their medications through a blend of private and public drug coverage. In contrast, the poor seem to have substantially reduced access to immunosuppressive drugs in the United States. Under the current US system, the cost of immunosuppressive medication is reimbursed in the first 3 to 5 yr after renal transplantation, and much of the graft loss among lower income recipients occurs after reimbursement ends.29 Because black Americans are more likely to be uninsured compared with white Americans, this policy may contribute to their higher risk for graft loss28,31; however, graft survival was lower in black than white patients even among US veterans, all of whom are eligible for medication coverage through the Veterans Administration medical system,32 and the explanation for this finding remains unclear.
We also found that black transplant recipients had 51% lower mortality after transplantation than white recipients, which is also out of keeping with published US data.33 The reasons for this are unclear but may again be related to differential access to pre- and posttransplantation care for comorbid disease or differences in the relative burden of comorbidity between the two black populations. Alternatively, because black Canadian patients are less likely than seemingly comparable white patients to receive a kidney transplant, their lower posttransplantation mortality relative to white patients may be due to selection bias. No recent studies have examined characteristics of black versus white Canadian renal transplant patients; however, the lower number of black renal transplant patients in Canada suggests that perhaps there is a disparity in referral or placement on the waiting list for black versus white patients. This may have resulted in only relatively healthier black patients’ receiving a renal transplant. This disparity in the total number of black Canadians’ receiving renal transplantation is concerning and supports previously published studies that showed decreased access to renal transplantation among other minority patient groups in Canada34,35 and warrants further investigation into possible causes.
Our argument that renal transplant outcomes are influenced by medical systems rather than biology is supported by recently published data from other countries that have medical systems that are similar in structure and funding to Canada's. A recent study from France showed comparable renal transplant outcomes for both white patients and patients of African descent.36 Although less comparable to Canada, Brazil has also been shown to have similar graft survival among black versus white patients in the recipients of living-donor kidneys.37
There are a number of limitations to our study. The use of registry data has inherent shortcomings, including limited availability of clinical variables collected by the Canadian Renal Replacement Registry, the renal database we used for our analysis. Important transplant variables such as HLA typing, cold/warm ischemic time, and donor characteristics would allow for a more in-depth analysis but are not currently available through this national registry. For instance, racial differences in HLA types may have influenced graft survival in white renal transplant recipients, as compared with those who were black. Although we cannot exclude the possibility that cold/warm ischemic time and clinical donor characteristics systematically differed by race, we believe that this is unlikely. Although CORR has a comprehensive collection of prospectively collected data from all Canadian transplant centers, like most registries, it has no formal validation process. The CORR data collection manual indicates that patient racial identity should be coded through self-reported ethnicity, although some (10%) patients had missing data for race. For this to have influenced our results, patients with missing data on race would have had to be systematically different in terms of both race and outcomes compared with those with complete data. We carried out a sensitivity analysis of missing or unknown race data and found no difference between substitution of black or white patients. Similarly, our findings could have been confounded by unmeasured comorbidity or socioeconomic factors. Although we believe that residual confounding is unlikely to have changed our conclusions, the possibilities that missing or incorrect information was partially responsible for our findings should be considered. In addition, we were unable to obtain comorbidities at the time of renal transplantation but had to rely on comorbidity data collected when the patients were on dialysis. Third, we did not have information on the date of placement on the waiting list for transplantation, which would have allowed us to examine whether longer waiting time influenced graft or patient outcomes, and little is known about access to placement on the waiting list for transplantation among Canadian ethnic minorities. This might be especially relevant for black patients, given that patients from ethnic minorities have longer average waiting times in multiple countries such as Australia (Aboriginals) and the United States (black and Native Americans).38–40 Fourth, we did not have data on the cause of death or graft failure, which would have helped us to speculate as to why our findings differed from those in the United States. Fifth, data regarding donor and immunologic characteristics (e.g., HLA type, cold/warm ischemia time) were not available and would have strengthened our analyses significantly. Sixth, we did not include data on preemptive transplantation; however, this type of transplantation was infrequently performed in Canada during the study period, and thus we believe that this omission is unlikely to have affected our conclusions. Finally, although we studied a large and representative sample from a national database, the number of black patients in our study was relatively small (n = 195). We investigated the stability of our time to graft failure and/or death models using the bootstrap method (with 5000 iterations), which did not change our findings. For example, the 95% CI for the fully adjusted time to death bootstrap model was 0.28 to 0.94 (versus 0.28 to 0.88 from our original model).41 Although our findings suggest that the risk for graft failure is similar in black and white Canadians, the low number of black patients treated in Canada during the study period means that we cannot exclude a clinically relevant increased risk for graft failure in this population.
Our data suggest that Canada, unlike the United States, has similar renal transplant outcomes for both black and white renal transplant patients. Our results are consistent with the hypothesis that these between-country differences may be due to differences in health care delivery and raises potentially important questions about whether better access to health services for black Americans would improve outcomes after kidney transplantation in this population.
CONCISE METHODS
Data Collection
CORR is a population-based, national registry that includes all reported cases of renal replacement therapy (RRT) as well as each patient's demographic and baseline clinical data, as collected by the treating center. Patients are followed with respect to treatment, transplantation data, and mortality. To comply with CORR privacy regulations, the data set used for this analysis was a random sample of 75% of all patients initiating dialysis in Canada for the stated calendar period.
Patient Population
We studied all patients who initiated RRT between January 1, 1990, and December 31, 2000, and were aged ≥18 yr at the date of RRT initiation. Race subgroups as recorded in CORR included East Asian; black; white; Indian subcontinent; Aboriginal; and other, consisting of Mid-Eastern/Arabians, Pacific Islanders, and patients of unknown race. For our analysis, we compared black and white patient groups only and excluded all other groups. SES was estimated using the neighborhood income per person equivalent, which is an estimate of household income that is adjusted for the size of the household on the basis of data provided by the 1996 Canadian census.40
Study Outcomes
The main outcome measure for all patients was adjusted rate of receiving a renal transplant. For all patients who received a primary renal transplant, we also considered additional outcome measures: (1) Adjusted rate of graft failure, (2) adjusted rate of death, and (3) adjusted rate of graft failure or death.
Statistical Analysis
Differences between groups were compared using χ2 tests for categorical variables and Kruskal-Wallis tests for all continuous variables. To compare races for time-to-event outcomes, we used the Cox proportional hazards model. For analyses with time to kidney transplantation as the dependent variable, patients were followed from dialysis initiation until they either received a kidney transplant or were censored (at death; loss to follow-up; or end of study, December 31, 2004). Among patients receiving a transplant, we used posttransplantation mortality as the dependent variable; censoring events were loss to follow-up and end of study only. Analyses with graft survival as the dependent variable considered both death-censored graft loss and the composite of death or graft loss.
Analyses were adjusted for factors potentially associated with the response of interest: Age (<40.0, 40.0 to 49.9, 50.0 to 59.9, ≥60.0 yr), gender, primary renal disease (diabetic nephropathy, glomerulonephritis, hypertensive/ischemic renal disease, polycystic kidney disease, and other renal diseases), calendar period of transplant (1990 to 1992, 1993 to 1994, 1995 to 1996, 1997 to 1998, 1999 to 2000), comorbidities present at the start of RRT (malignancy, diabetes, coronary disease, chronic heart failure, stroke or transient ischemic attach, chronic lung disease, peripheral vascular disease, hypertension, other serious illnesses), geographic region, current smoking status and SES. These comorbidity data are collected in CORR at the time of dialysis initiation and then updated annually thereafter. Follow-up comorbidity data are currently collected at the time of transplantation, but these data were added to the registry in 1999 and thus were not available for the entire period of our study. The early years were not collected by all centers. We therefore relied on the data that are collected at the time of dialysis initiation to describe comorbidity in transplant recipients.
Analyses evaluating posttransplantation outcomes adjusted for the previously listed covariates in addition to donor type (deceased versus living) and the duration of dialysis treatment before transplantation (<1.0 yr, 1 to 1.9 yr, ≥2.0 yr). Because the number of graft failures and/or deaths was small in the black transplant recipient population, we tested the robustness of our findings using the bootstrap method (with 5000 iterations). We report the 95% CI.42
We quantified the effect of race by the covariate-adjusted HR, with white patients chosen as the reference category. We dealt with missing comorbidity data (representing approximately 10% of all information on comorbidity) by assuming that the characteristic was absent (results did not differ when analyses were repeated after deletion of all patients with missing data). In addition, we assessed the potential for informative censoring (because those who die early are less likely to have received a kidney transplant) by assuming that those who died would not have received a transplant if they had survived until the end of the study (i.e., patients who died were assigned a date of last follow-up of December 31, 2004). Cross-product interaction terms were used to determine whether race was modified by age, gender, diabetes, or SES.
The proportional hazard assumption was checked and satisfied by examination of plots of the log-negative-log of the within-group survivorship functions versus log time as well as comparing Kaplan-Meier (observed) with Cox (expected) survival curves. Statistical significance was defined as P < 0.05. All analyses were carried out using SAS 8.2 (SAS Institute, Cary, NC). The study was approved by the institutional review board at Queen's University (Kingston, ON, Canada).
DISCLOSURES
None.
Acknowledgments
K.Y. is supported by a Biomedical Scholarship from the Kidney Foundation of Canada. B.H. and M.T. are supported by New Investigator awards from the Canadian Institutes of Health Research and Population Health Investigator awards from the Alberta Heritage Foundation for Medical Research. Dr. Yeates had full access to all of the data in the study and takes full responsibility for the integrity of the data and the accuracy of the data analysis.
The authors thank the Canadian Organ Replacement register (CORR) of the Canadian Institute for Health Information. CORR is fully supported by grants from the federal, provincial, and territorial Ministries of Health. CORR would not exist without the voluntary participation of the Canadian Society of Nephrology, the Canadian Society of Transplantation, and the Canadian Association of Nephrology Nurses and Technologists, along with their constituent members at the dialysis and transplant centers across the country. The Kidney Foundation of Canada also supports CORR by active participation on the board of directors and with the financial support for the publication of the annual reports.
Published online ahead of print. Publication date available at www.jasn.org.
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