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Acute Renal Failure |



* Division of Nephrology, Department of Medicine, University of California San Francisco, San Francisco, California;
Division of Nephrology, Department of Medicine, Maine Medical Center, Portland, Maine;
Division of Nephrology, Department of Medicine, Cleveland Clinic Foundation, Cleveland, Ohio;
Division of Nephrology, Department of Medicine, Vanderbilt University, Nashville, Tennessee; and || Division of Nephrology, Department of Medicine, University of California San Diego, San Diego, California
Address correspondence to: Dr. Glenn M. Chertow, University of California San Francisco, Department of Medicine Research, UCSF Laurel Heights Suite 430, 3333 California Street, San Francisco, CA 94118-1211. Phone: 415-476-2173; Fax: 415-476-9531; E-mail: chertowg{at}medicine.ucsf.edu
| Abstract |
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76 mg/dl in the low degree of azotemia group [n = 122] versus BUN >76 mg/dl in the high degree of azotemia group [n = 121]). Standard Kaplan-Meier product limit estimates, proportional hazards (Cox) regression methods, and a propensity score approach were used to account for selection effects. Crude survival rates were slightly lower for patients who started dialysis at higher BUN concentrations, despite a lesser burden of organ system failure. Adjusted for age, hepatic failure, sepsis, thrombocytopenia, and serum creatinine and stratified by site and initial dialysis modality, the relative risk for death that was associated with initiation of dialysis at a higher BUN was 1.85 (95% confidence interval 1.16 to 2.96). Further adjustment for the propensity score did not materially alter the association (relative risk 1.97; 95% confidence interval 1.21 to 3.20). Among critically ill patients with AKI, initiation of dialysis at higher BUN concentrations was associated with an increased risk for death. Although the results could reflect residual confounding by severity of illness, they provide a rationale for prospective testing of alternative dialysis initiation strategies in critically ill patients with severe AKI. | Introduction |
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The Program to Improve Care in Acute Renal Disease (PICARD) is an observational study from five academic medical centers (University of California San Diego, Cleveland Clinic Foundation, Maine Medical Center, Vanderbilt University, and University of California San Francisco [UCSF]) that aimed to identify demographic, process of care, and clinical factors that were associated with favorable and adverse outcomes after AKI among ICU patients (3). For this study, we focused our inquiry on the subpopulation of patients who had AKI and required dialysis and examined the association of timing of initiation of dialysis with mortality. We hypothesized that the timing of initiation of dialysis would vary among and within sites and that delayed dialysis initiation would be associated with increased mortality rates.
| Materials and Methods |
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0.5 mg/dl and baseline serum creatinine <1.5 mg/dl or an increase in serum creatinine
1.0 mg/dl and baseline serum creatinine
1.5 mg/dl and <5.0 mg/dl, as described previously (13). Patients with a baseline serum creatinine
5.0 mg/dl were not considered for study inclusion. A detailed description of PICARD inclusion and exclusion criteria, data elements, data collection, and management strategies are described elsewhere (3). Patients who were contacted by study personnel and who signed (or whose proxy signed) informed consent were enrolled in the study cohort. The reason for nonenrollment was determined for patients who did not sign informed consent, although no additional data were collected for privacy considerations (14). The Committees on Human Research at each participating clinical site approved the study protocol and informed consent. The modality and the intensity of dialysis and other co-interventions were determined by the treating physician with no influence from study personnel. A total of 398 (64%) of the 618 enrolled patients received dialysis during their ICU stay. To give patients in our analysis an equal "opportunity" to receive dialysis with a low and high degree of azotemia, we excluded individuals with an estimated GFR (eGFR) of <30 ml/min per 1.73 m2 at the time of hospital admission, reflecting National Kidney Foundation Kidney Disease Outcomes Quality Initiative (K/DOQI) stage IV chronic kidney disease or significant/evolving AKI.
Determination of Degree of Azotemia
Of the 250 PICARD participants who received dialysis during their ICU stay and were admitted to the hospital with an eGFR >30 ml/min per 1.73 m2, the BUN on the day of dialysis initiation was available in 243 (97%). The median BUN on the day of dialysis initiation was 76 mg/dl. We considered patients whose BUN was >76 mg/dl (n = 121; mean BUN 114.8 ± 28.5 mg/dl) at dialysis initiation as having started dialysis with a high degree of azotemia and patients whose BUN was
76 mg/dl (n = 122; mean BUN 47.4 ± 17.9 mg/dl) as having started dialysis with a relatively low degree of azotemia. We used the median BUN rather than a predetermined absolute value (e.g., 100 mg/dl) to allow for sufficient sample size and to provide reasonably wide separation between the two comparison groups.
Statistical Analyses
Continuous variables were expressed as mean ± SD or median and interquartile range and compared using t test, the Wilcoxon rank sum test, or the Kruskal-Wallis test, where appropriate. Categorical variables were expressed as proportions and compared with the Cochran-Mantel-Haenszel
2 test or Fisher exact test. We examined the time to death within 60 d of ICU admission using the Kaplan-Meier product limit estimate and compared survival curves with the log-rank test.
We created a propensity score using dialysis initiation at a high BUN as the dependent variable. Using multiple logistic regression, we considered as candidate variables all demographic, clinical, and laboratory factors that were associated with the timing of dialysis initiation on univariate analysis. We retained all variables with P < 0.20 in the propensity score. Discrimination of the propensity score model was assessed using the area under the receiver operating characteristic curve, with higher values indicating better discrimination. Calibration was assessed using the Hosmer-Lemeshow goodness-of-fit test. The Hosmer-Lemeshow test compares model performance (observed versus expected) across deciles of risk to test whether the model is biased (i.e., performs differentially at the extremes of risk). An NS value for the Hosmer-Lemeshow
2 suggests an absence of such bias.
Cox proportional hazards regression was used to determine the associations of timing of dialysis initiation and other covariates, stratified by site and modality (continuous renal replacement therapy versus intermittent hemodialysis). Survival was measured from the first day that the patient met the criteria for AKI. We included as covariates factors that were associated with mortality on the day of dialysis initiation (15). Hazard ratios and 95% confidence intervals (CI) were calculated from model parameter coefficients and SE, respectively. Plots of log (log [survival rate]) against log (survival time) were performed to establish the validity of the proportionality assumption. We fitted models adjusted for covariates only, the propensity score only, and a combination of covariates plus the propensity score.
Two-tailed P < 0.05 were considered significant. Statistical analyses were conducted using SAS 9.1 (SAS Institute, Cary, NC).
| Results |
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Independent Predictors of Dialysis Initiation with High BUN
Independent predictors of dialysis initiation with a high BUN included a history of chronic obstructive pulmonary disease (odds ratio [OR] 2.78; 95% CI 1.20 to 6.49) and higher serum creatinine (OR 1.43; 95% CI 1.21 to 1.69 per mg/dl). Tachycardia was associated with a lower likelihood of dialysis initiation at a high BUN (OR 0.89; 95% CI 0.77 to 1.04 per 10 beats/min). Patients with higher plasma bicarbonate concentrations (OR 1.05; 95% CI 0.99 to 1.10 per mmol/L) were more likely to start dialysis with a high BUN, as were patients who did not have a pulmonary artery catheter in place at the time dialysis was initiated (OR 1.59; 95% CI 0.85 to 2.99). The last three variables were included in the propensity score equation on the basis of the more liberal P value criterion (P < 0.2) but were not significantly (P < 0.05) associated with modality assignment after adjustment for the other variables noted above. Other variables that were associated with BUN at dialysis initiation on univariate screening (including gender and hematologic and liver failure) were considered but removed from the multivariable model on the basis of the P > 0.2 criterion. The area under the models receiver operating characteristic curve was 0.75, indicating good discrimination in determining the timing of initiation of dialysis, and the model was well calibrated (Hosmer-Lemeshow
2, P = 0.17).
Timing of Dialysis Initiation and Mortality
Crude survival rates tended to be slightly lower for patients who initiated dialysis at a higher starting BUN, despite a reduced burden of organ system failure (survival at 14 and 28 d 0.80 and 0.65 for "low" BUN versus 0.75 and 0.59 for "high" BUN; log rank P = 0.09). Adjusted for age, hepatic failure, sepsis, thrombocytopenia, and serum creatinine and stratified by site and initial dialysis modality, the relative risk (RR) for death associated with dialysis initiation with a higher degree of azotemia was 1.85 (95% CI 1.16 to 2.96). Adjustment for the propensity score alone (RR 2.07; 95% CI 1.30 to 3.29) or for the covariates plus the propensity score (RR 1.97; 95% CI 1.21 to 3.20) did not materially alter the association between higher BUN and mortality.
| Discussion |
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Two additional studies (11,12) examined the timing of initiation of dialysis for AKI after cardiac surgery and demonstrated a benefit to earlier initiation of dialysis. These studies used a definition of AKI to justify the provision of early dialysis of a urine output <100 ml during the first 8 h after bypass surgery regardless of solute clearance. Demirkilic et al. (11) studied a total of 61 patients; the overall mortality rate in those who were treated with early dialysis was 24%, compared with 56% in control subjects (P = 0.016). Elahi et al. (12) reported the results of their analysis of 64 patients with postbypass AKI; the overall mortality rate in those who were treated with early dialysis was 22%, compared with 43% in control subjects (P < 0.05). However, the relevance of these data to nonpostoperative patients or to patients with nonoliguric AKI is unclear.
The analyses on dialysis initiation from PICARD extend those from previously published reports by including a large study sample from five geographically and ethnically diverse clinical sites, adjusting for confounding using multivariable analysis and by considering selection effects with propensity scores. The results are consistent with or without adjustment for key covariates as well as the likelihood of being prescribed dialysis with a high or low degree of azotemia.
There are several important limitations to this study. Even with adjustment for confounding and selection effects, patients with higher BUN concentrations at the start of dialysis may be different from other patients in ways for which we could not adjust. We attempted to capture these differences with stratification by site and initial dialysis modality and the consideration of several process-of-care variables (e.g., use or nonuse of pulmonary artery catheter) in the development of the propensity score. Despite our efforts, data on several important exposures were not available. For instance, we collected information on whether patients received any nutrition support before starting dialysis but were unable to capture data on cumulative protein or amino acid intake, which could have influenced the BUN concentration independent of kidney function. We eliminated patients who were admitted with an eGFR of <30 ml/min per 1.73 m2 to give all patients a relatively equal chance to initiate dialysis at the lower BUN level. To avoid lead time bias, we measured day 60 survival in all patients from the first day that they met criteria for AKI, rather than from the first day of dialysis (15). Those with lower BUN concentrations at dialysis initiation may have had relative volume overload (i.e., a larger volume of distribution of urea), which is associated with increased morbidity in the critically ill (17). Although we could have considered other parameters, such as the serum creatinine or urine output, alone or in combination with BUN, these, too, would have been arbitrary and subject to criticism. Virtually all of the previously published literature in this area has focused on BUN, and in the critical care setting, the BUN drives dialysis practice more so than other laboratory or clinical parameters. Finally, although the studys focus was on the timing of initiation of dialysis, other aspects of dialysis care, including frequency and dose, were not assigned uniformly and may have confounded the results.
| Conclusion |
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| Acknowledgments |
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We acknowledge Drs. Stephen Hulley, Joachim Ix, and Dennis Osmond and the Roadmap K12 scholars/faculty at University of California San Francisco, for helpful comments and discussion.
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See the related editorial, "Can We Rely on Blood Urea Nitrogen as a Biomarker to Determine When to Initiate Dialysis?" on pages 903904.
Received April 28, 2006. Accepted May 24, 2006.
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