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Kidney Epidemiology and Cost Center, Departments of Internal Medicine, Biostatistics, and Epidemiology, The University of Michigan, Ann Arbor, Michigan.
Correspondence to: Dr. Friedrich K. Port, University of Michigan, KECC, 315 W. Huron, Ste. 240, Ann Arbor, MI. 48103. Phone: 734-998-6611; Fax: 734-998-6620; E-mail: portb{at}umich.edu
| Abstract |
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| Introduction |
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We have previously shown that the correlation between lower dialysis dose and higher mortality is stronger when body size is considered simultaneously through strata or statistical adjustment. Furthermore, both low dose of dialysis (Kt/V) and small body size were found to be independent risk factors for mortality in chronic HD patients (9). We hypothesized that the threshold level beyond which there is no mortality benefit might differ among patient groups of different body size (15). The optimal level of HD dose for end-stage renal disease (ESRD) patients clearly continues to be a major question that deserves monitoring and investigation.
This study asks two specific questions for recent years of 1998 through 2000: (1) Does the dose-mortality relationship show diminishing benefit within the current dosing range; and (2) Does the pattern vary by body mass index (BMI)? This study is based on all Medicare ESRD patients treated with HD in the United States with consideration of body size and adjusting for a wide spectrum of variables and minimizing the potential effect of residual renal function.
| Materials and Methods |
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Statistical Analyses
Cox proportional hazards models were used to analyze the time from study start date, i.e. the end of the 15th month of ESRD for each patient, to death (censored after 24 mo or transplantation). The analyses were adjusted for patient age, race, gender, diabetes, incidence year, and 18 comorbid conditions listed in Tables 1 and 2. These measures were collected at the start of the study, except for comorbid conditions, which were collected at the start of ESRD. Results from Cox models are presented as relative mortality risks and as death rates adjusted to patients with overall average characteristics.
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| Results |
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The analyses by TBW groups showed similar results except at the highest tertile. The large TBW group showed no downward trend for URR >75% compared with the URR group of 70 to 75% (RR, 1.05; P = 0.55). In all groups, 70 to 75% had significantly lower RR than the URR group of 65 to 70%. This significant difference continued in the next higher URR category (>75%) for the tertile with the lowest TBW.
The analyses performed on the basis of BMI were each significantly more predictive of mortality than the analyses performed on the basis of TBW (P < 0.0001). There was little difference between weight and BMI in predicting mortality. Other sensitivity analyses without adjustment for comorbid conditions also showed similar results to those reported here. When using BMI groups in quartiles, the results were similar. The highest BMI quartile showed the lowest risk for the URR >75% group as compared with the 70 to 75% group (RR, 0.92; P = 0.27).
Additionally, adjusting for the same demographic and comorbid conditions plus body weight and height revealed a steeper gradient of RR by URR ranges similar to the BMI adjustment as shown in the second to the rightmost column of Table 3. In this analysis, the URR >75% group showed a 14% lower mortality risk compared with the 70 to 75% group (RR, 0.86; P < 0.0001). The analysis suggests that weight/height is significantly (P < 0.0001) more predictive of survival than is BMI (weight/height2).
| Discussion |
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There are several differences between the present study and these earlier observational studies. As the dose of dialysis has clearly increased in the United States during the 1990s (17), there are many more patients receiving the higher dose ranges. Some of this apparent increase may have been due to a recent trend toward drawing postdialysis blood earlier, e.g., 15 to 30 s after the end of dialysis as recommended by DOQI (11). This could partially explain the reduction in the fraction of patients with URR <65% as suggested by Lowrie et al. (18). This trend allowed a better assessment of mortality RR for patients treated in higher dose ranges (i.e., URR >75%). Earlier studies may also have suffered from the large variability in the timing for the postdialysis blood draw (19). Such variability tends to bias the results toward the null, i.e., toward a flatter correlation. Newer dialyzers may make it easier to achieve such higher doses of dialysis.
The present study simultaneously considered body size (by BMI or an estimate of TBW) and URR. Disregarding body size leads to a weaker (less steep) correlation of dose with mortality RR, as is clearly demonstrated in Table 3. The inverse association of BMI and mortality has been previously shown (2022) and is confirmed here. At any level of dose (URR), a lower BMI is associated with higher mortality (Figure 1), corroborating the importance of nutritional status in addition to dialysis dose (9). This has also been recognized by DOQI guidelines on nutrition (23). There is a confounding due to the tendency of large patients to receive a smaller than average URR and smaller patients a higher URR (24). To overcome this confounding, we studied three body-size groups separately. Additionally, this study adjusts for a large number of comorbid conditions, which had previously been accomplished only by Held et al. (5). When we limited the adjustments by excluding comorbidity indicators (data not shown), the correlation between dose and mortality RR became flatter and may be incorrectly interpreted to suggest a much smaller benefit from higher URR. This observation may explain why Owen et al. (6), Chertow et al. (16), and Szczech et al. (8) showed a lack of survival advantages at higher URR levels. Their analyses had no comorbidity adjusters except for demographics and laboratory values. Similar to our findings, Szczech et al. (8) showed that the median URR and the threshold for mortality benefit have increased over the past few years. Furthermore, a recent analysis from Lowrie et al. (25) showed the lowest mortality risk for the highest URR when adjustments included height and weight. The present study is interesting because it describes very recent years and has a very large sample size. With this large sample size, statistical significance is less important than the magnitude of the observation. Indeed, the differences in adjusted 1-yr death rates are of large magnitude and clinical importance. For every 5% higher URR, the mortality risk was on average 17 to 19% lower. Even between the highest two URR categories, >75% compared with 70 to 75%, the relative mortality risk was 9 to 16% lower. This translates for a dialysis unit having 20 deaths per year at a URR of 70 to 75% to having 17 to 18 deaths per year at a URR >75%.
The sensitivity analyses using tertiles of V as a measure of patient body size suggest that these results hold regardless of whether one uses the Kt/V or the Kt metric to measure the dose of dialysis. Declines in mortality were seen for all tertiles of V as the URR increased. Increases in URR correspond to increases in Kt when V is held (nearly) constant in each tertile of V. Thus, the increases in URR within volume groups correspond to increases in Kt. Given the popularity of URR over Kt/V measurements and the high correlation of URR with Kt/V in population studies, this study suggests that both higher BMI and higher URR are strongly associated with lower mortality risk. We present the results on the basis of BMI rather than those with weight or volume because BMI was more predictive of mortality than was either weight or volume. In addition, we analyzed weight and height as separate predictors of mortality. The analysis suggests that weight/height may be a better predictor of mortality than BMI (weight/height2).
Potential Limitations
First, as this is an observational study, only correlation can be shown, but causation cannot be directly proven (26). A randomized trial, the ongoing HEMO trial, was designed to ascertain a cause-and-effect relationship. Even this large trial may provide limited answers to our specific questions because it is not designed to evaluate differences in mortality for patient body-size subgroups, as shown in our results. A recent study by the authors evaluated dialysis dose as a dialysis unit practice pattern and used the dialysis unit mortality as the outcome. This approach reduces the potential bias of sicker patients receiving a lower dose (e.g., because of hypotensive episodes or poor blood flow). The practice pattern of having a larger fraction of patients receiving a URR <65% was significantly associated with a significantly higher mortality risk (27). This was true even when adjusting for the percent of patients with a central venous vascular access.
Second, the comorbidity measures were obtained 15 mo before the start of study for each patient, so additional comorbid conditions may have developed during this interval. This would have led to a misclassification bias, which would have likely reduced the observed effect of comorbidities on outcome. It is noteworthy, however, that adjustment for comorbidity did correlate significantly with mortality risk in the present study. Furthermore, the association of comorbid conditions from the HCFA Medical Evidence Form with mortality RR was similar to that previously noted in USRDS Special Studies (28), where comorbidity indicators were abstracted without such delays at study start. A similar bias "toward the null," i.e., to a less steep mortality by dose relationship, may be introduced by "noise" or imprecise measurement of URR. The technique of drawing the postdialysis BUN sample has been variable in the past (19). However, the DOQI guidelines may already have contributed to a more standardized sampling technique and to a reduction in the noise for the URR measure.
We explored potential explanations for the relatively steep and consistent correlation of high dose and low mortality risk through additional analyses. We hypothesize that a high URR is more likely achieved with high-flux membranes and that the low mortality RR is due to the improved middle molecule clearance (29) rather than the small molecule, URR. We were able to adjust for the frequency of high-flux dialyzer use at each patients dialysis unit. This adjustment did not modify the results, thus providing no support for this hypothesis. Second, we hypothesized that central venous catheters may be associated with a lower dialysis dose and be independently associated with an increased mortality risk (30). This mechanism could explain the steep correlation of URR and mortality RR at lower ranges of URR. Using again the percent use of central venous catheters at the patients dialysis unit as a proxy, we again found virtually no modification of the overall correlation despite the fact that this proxy measure for catheters was significantly associated with mortality RR.
The trends in recent years show a remarkable increase in average dialysis dose (URR) in the United States according to annual national random samples (17,31). The USRDS Case Mix Adequacy Study (5) had shown a substantially lower dose in 1990 with an average URR of only 60.1%. The reduction in mortality during the 1990s can be explained to a large extent by the steady increase in dialysis dose (17,31). What is yet to be determined is the "optimal dose" of HD, which is the level above which no improvement in outcomes can be observed. The present study suggests that the delivered dose for most patients is substantially below this "optimum," at least after 15 mo of ESRD when most patients have lost their residual renal function. Data for URR >75% could not be subdivided further in the present data collection by URR category. Future studies are needed to evaluate the risk by URR for subgroups above URR of 75%.
We conclude that the dose of dialysis is a strong predictor of patient mortality through the highest ranges of URR recorded (i.e., URR >75). The relationship of better outcomes with higher dialysis dose is strong in all body-size groups. Comparison of mortality risk for the URR >75% with the URR 70 to 75% groups was significant and large except in the overweight patient group that is underrepresented in the highest dose category. Thus, this study suggests that further reductions in mortality might be achieved by increasing the dose of HD beyond the DOQI guidelines (> 65%) and that the target be revised to above 70% URR or above 75% URR.
| Acknowledgments |
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| References |
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