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Diabetes and the Kidney |









* Merck & Co., Inc., Whitehouse Station, New Jersey;
Baker Heart Research Institute, Melbourne, Victoria, Australia;
Department of Clinical Pharmacology, University Medical Center Groningen, Groningen, The Netherlands;
Service de Nephrologie, Hopital Necker, Paris, France; || University of Illinois at Chicago, Chicago, Illinois; ¶ Department of Medicine, Division of Endocrinology, Washington University, St. Louis, Missouri; ** Department of Medicine, Renal Division, Emory University, Atlanta, Georgia; 
Instituto Di Ricerche Farmacologiche Mario Negri, Laboratories Negri Bergamo, Bergamo, Italy; 
Division of Nephrology, Baylor College of Medicine, Houston, Texas; and 
Department of Medicine, Renal Division, Brigham and Womens Hospital and Harvard Medical School, Boston, Massachusetts
Address correspondence to: Dr. William F. Keane, Merck & Co., Inc., PO Box 4, UG4A-025, West Point, PA 19486-0004. Phone: 267-305-2018; Fax: 267-305-2845; williamf_keane{at}merck.com
| Abstract |
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| Introduction |
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| Materials and Methods |
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Statistical Analyses
Statistical Analysis System version 8 software was used for all analyses. We previously reported the significant independent baseline risk factors for the progression of renal disease to doubling of serum creatinine or the development of ESRD: albuminuria, hypoalbuminemia, increased serum creatinine, and decreased hemoglobin (2). Our purpose in this analysis was to identify the independent risk factors for the development of ESRD alone and to evaluate whether a risk score that is developed from the risk factors has a stronger predictive power than its components for ESRD in the RENAAL study. We also calculated the risk score for the composite end point of ESRD or death using a similar method.
Twenty-nine baseline categorical and continuous variables (including gender and race) were evaluated, and 23 of them had a significant impact on the development of ESRD. Multivariate analysis used a multivariate Cox regression model with backward selection process, with P < 0.01 required for inclusion in final model. The risk score was developed from the linear combination of covariates from the final relative hazard model.
Data presented here are for the pooled losartan and placebo treatment groups. The same risk factors were found in the placebo group, suggesting that the risk predictors for progression of kidney disease were independent of therapy.
To compare the predictive power between the risk score and each of the components, patients were stratified by quartiles for the risk score and each covariate. Quartile range, number of patients who had ESRD per 1000 patient-years of follow-up, and hazard ratios (HR) with respect to the first quartile were determined for combined treatment groups. In addition, Kaplan-Meier curves were generated by the risk score quartile and time-varying risk score quartile for the combined and individual treatment groups.
To validate the risk score, patients were classified further into deciles, and the crude ESRD rates within these categories were calculated. We refer to this as the naïve validation approach. Because of the potential optimistic bias of this approach, we also calculated ESRD rates using the jackknife approach, in which all 1513 patients were classified into deciles on the basis of a score that the patient had no part in developing (4). The distributions of events across quartiles and deciles of the risk score also were examined for gender and race.
The relationship among the four risk factors was explored. Mean and quartile range were calculated for each risk factor by subgroups of proteinuria and serum creatinine. In addition, the correlations among the risk score, albuminuria, serum albumin, serum creatinine, hemoglobin level, and systolic and diastolic BP were tested using Pearson product-moment correlation coefficients. The robustness of the risk score was explored by deriving similar risk score equations using the four risk factors by treatment group and subgroups of dichotomized proteinuria and serum creatinine, gender, and race. The coefficients with 95% confidence intervals were provided.
| Results |
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For comparison of the predictive power of the risk score and its components for ESRD, patients were categorized by the severity of the renal disease into quartiles. As shown in Table 2, the event rate per 1000 patient-years of follow-up increased with increasing albuminuria and serum creatinine and decreasing serum albumin and hemoglobin. Among the four components, baseline albuminuria was the strongest predictor of ESRD, with an event rate per 1000 patient-years of follow-up of 18.7 in the first quartile (albuminuria < 558.0 mg/g [63.1 mg/mmol]) and 227.8 at the last quartile (
2544.5 [287.5]). Even so, the risk score had a better predictive power than albuminuria alone. For the same patients, the event rate per 1000 patient-years of follow-up was only 6.7 in the first quartile (score < 3.0) and 257.2 at the last quartile (score
5.1). Therefore, using the risk score improved predictive power for ESRD versus using albuminuria alone, especially for low-risk patients. The relative predictive power can be assessed using the HR for each covariate: The HR between the fourth and first quartiles of the risk score was 49.0, as compared with the corresponding HR for each component: 14.7 for albuminuria, 9.2 for serum creatinine, 5.5 for hemoglobin, and 10.2 for serum albumin. Therefore, the predictive power of the risk score on ESRD increases at least three-fold as compared with albuminuria alone (Table 2).
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Among the four risk factors, proteinuria and serum creatinine were positively correlated with each other but negatively correlated with serum albumin and hemoglobin. As shown in Table 3, patients who had baseline proteinuria <2000 mg/g (226 mg/mmol) had overlapping quartile ranges of serum creatinine, serum albumin, and hemoglobin with patients who had baseline proteinuria
2000 mg/g (226 mg/mmol). Similar results were seen when patients were stratified by baseline serum creatinine <2 or
2 mg/dl (176.8 µmol/L). None of the risk factors could be reproduced by the other ones. For different subpopulations, similar risk score equations were derived using the four risk factors. As shown in Table 4, when patients were stratified by renal disease severity, either baseline proteinuria <2000 or
2000 mg/g (226 mg/mmol) or baseline serum creatinine <2 or
2 mg/dl (176.8 µmol/L), by gender, white or nonwhite race, and treatment group, the four parameters were almost independent risk factors for each subgroup even though the number of outcomes was reduced dramatically. The coefficients for each risk factor vary somewhat with subgroups, but the 95% confidence intervals overlap each other. In particular, the risk scores are very similar between losartan and placebo treatment groups.
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| Discussion |
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Albuminuria is a critical baseline risk predictor for ESRD in this and other studies of patients with diabetic nephropathy (6,7). Level of albuminuria is the most important clinical marker for future renal events, and UACR should be monitored in all patients with diabetic nephropathy. In RENAAL, there was a nearly linear relationship between progression to ESRD and albuminuria at baseline and on treatment (6). However, the effect of treatment with losartan on albuminuria was responsible for only 50% of the effect of losartan on ESRD (6).
The value of the RENAAL ESRD risk score is that it demonstrates that factors in addition to albuminuria predict ESRD: hypoalbuminemia, increased serum creatinine level, and decreased hemoglobin level. Although the four factors were correlated with each other, one still worked beyond the correlation and could not be fully predicted by the others. The inclusion of these factors improved the risk prediction for progression of nephropathy to ESRD from 50% with consideration of albuminuria alone to >80% when the risk score was used. Hypoalbuminemia worsens as renal function deteriorates (8) and may be related to albuminuria, inflammation, and nutrition. Low serum albumin at initiation of dialysis has been associated with increased morbidity and mortality in dialysis patients (9,10). As expected, the incidence of ESRD increased with increasing serum creatinine (e.g., 40.5% in the serum creatinine 2.1 to 3.6 mg/dl [185.6 to 318.2 µmol/L] tertile versus 7.3% in the 0.9 to 1.6 mg/dl [79.6 to 141.4 µmol/L] tertile) (11). Anemia is common in diabetic kidney disease and contributes to cardiovascular morbidity and mortality in patients with nephropathy (12,13). Anemia also was identified as an independent predictor for progression to ESRD in this study (14). Even mild anemia (<13.8 g/dl [138 g/L]) was associated with increased risk. After adjustment for known risk factors for ESRD, the average increase in relative risk was 11% for each 1 g/dl (10 g/L) decrease in hemoglobin concentration. The contribution of each factor on ESRD was robust regardless of whether patients were at low or high risk, as defined by either baseline proteinuria < or
2000 mg/g (226 mg/mmol) or baseline serum creatinine < or
2 mg/dl (176.8 µmol/L), also regardless of gender, white/nonwhite race, and treatment group. The four risk predictors for progression of kidney disease in the risk score were independent of therapy.
Control of hypertension is of primary importance in patients with diabetic nephropathy. Current guidelines state that the BP goal in patients with diabetes is
130/80 mmHg. Because BP was treated aggressively and equivalent levels were achieved in both treatment groups in the RENAAL study, this may have resulted in our inability to identify BP as an independent risk factor in multivariate analyses. However, it is possiblebut unlikelythat the lack of correlation may have been because hypertension was not associated with risk for ESRD.
Glycemic control is important in this population, and HbA1c entered the model only when the risk score for the composite end point of ESRD or death was calculated. This is consistent with observations of other large observational studies that demonstrated a role for glycemic control in cardiovascular disease but not kidney disease (15).
Identifying covariates that are highly associated with outcome plays an important role in clinical trial design. In the International Conference on Harmonization guidelines, investigators are advised to identify the covariates that may influence the primary outcome and to prespecify the method to be used to account for them to compensate for any imbalance between groups (16). The risk score that combines the identified important covariates helps to avoid the use of multiple covariates to adjust for the treatment effect in statistical analyses. The homogeneity of the treatment effect across severity of disease can be explored by using subgroup analysis of the risk score.
An important limitation of the risk score validation via examination of ESRD events by quartiles and deciles was that few events occurred in the lower quartiles and deciles. An additional important limitation was that this analysis used clinical trial data, and the same data from which score was derived to validate score; however, the outcome of the application of the jackknife procedure causes us to be optimistic about the broad applicability of this score. Because the risk score was derived from the Cox model using RENAAL baseline data, the absolute risk score may not be interpretable. The difference in risk score between patients suggests relative risks on the development of ESRD.
The RENAAL risk score for ESRD emphasizes the importance of identification of level of albuminuria, hypoalbuminemia, increased serum creatinine, and decreased hemoglobin level to predict the development of ESRD in patients with type 2 diabetes and nephropathy. Although albuminuria is a very strong predictor for ESRD, the contribution of serum albumin, serum creatinine, and hemoglobin level further enhances the prediction of ESRD. Future trials with a similar patient population and outcome measures as those of the RENAAL study should consider adjusting analyses for baseline risk factors.
| Acknowledgments |
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Preliminary results of these analyses were presented as a poster at the annual meeting of the American Society of Nephrology, November 1, 2002, Philadelphia, PA.
W.F.K., D.d.Z., J.-P.G., J.M., W.E.M., G.R., R.T., and B.M.B. were paid by Merck for their services as clinical investigators in this study and have served as consultants to the company. W.F.K., Z.Z., P.A.L., S.S., and S.M.S. are or have been employees of Merck and may own stock or hold stock options in the company.
| Footnotes |
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Received October 20, 2005. Accepted March 29, 2006.
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