Chronic Kidney Disease and Mortality Risk: A Systematic Review
Marcello Tonelli*,,,,
Natasha Wiebe*,
Bruce Culleton||,
Andrew House¶,
Chris Rabbat,
Mei Fok*,
Finlay McAlister*,, and
Amit X. Garg¶,**
Departments of * Medicine, Critical Care, and Public Health Sciences, University of Alberta, and Institute of Health Economics, Edmonton, and || Department of Medicine, University of Calgary, Calgary, Alberta, and Departments of ¶ Medicine and ** Epidemiology and Biostatistics, University of Western Ontario, London, and Department of Medicine, McMaster University, Hamilton, Ontario, Canada
Address correspondence to: Dr. Finlay McAlister, University of Alberta, Department of Medicine, 8440 112 Street, Edmonton, Alberta T6B 2B7, Canada. Phone: 780-407-1399; Fax: 780-407-2680; E-mail: finlay.mcalister{at}ualberta.ca
Received for publication October 18, 2005.
Accepted for publication April 20, 2006.
Current guidelines identify people with chronic kidney disease(CKD) as being at high risk for cardiovascular and all-causemortality. Because as many as 19 million Americans may haveCKD, a comprehensive summary of this risk would be potentiallyuseful for planning public health policy. A systematic reviewof the association between nondialysis-dependent CKDand the risk for all-cause and cardiovascular mortality wasconducted. Patient- and study-related characteristics that influencedthe magnitude of these associations also were investigated.MEDLINE and EMBASE databases were searched, and reference liststhrough December 2004 were consulted. Authors of 10 primarystudies provided additional data. Cohort studies or cohort analysesof randomized, controlled trials that compared mortality betweenthose with and without chronically reduced kidney function wereincluded. Studies were excluded from review when participantswere followed for <1 yr or had ESRD. Two reviewers independentlyextracted data on study setting, quality, participant and renalfunction characteristics, and outcomes. Thirty-nine studiesthat followed a total of 1,371,990 participants were reviewed.The unadjusted relative risk for mortality in participants withreduced kidney function compared with those without ranged from0.94 to 5.0 and was significantly more than 1.0 in 93% of cohorts.Among the 16 studies that provided suitable data, the absoluterisk for death increased exponentially with decreasing renalfunction. Fourteen cohorts described the risk for mortalityfrom reduced kidney function, after adjustment for other establishedrisk factors. Although adjusted relative hazards were consistentlylower than unadjusted relative risks (median reduction 17%),they remained significantly more than 1.0 in 71% of cohorts.This review supports current guidelines that identify individualswith CKD as being at high risk for cardiovascular mortality.Determining which interventions best offset this risk remainsa health priority.
It has been known for many years that ESRD is associated withvery high mortality and accelerated cardiovascular disease (1).Several recent studies suggest that the risk for death is increasedindependently in individuals who have less severe impairmentof kidney function and are not dialysis dependent, comparedwith those who have preserved kidney function (2,3). However,other rigorously conducted studies have found little or no significantincrease in all-cause or cardiovascular mortality in the settingof mild to moderate chronic kidney disease (CKD) (4,5). Evenamong studies that have demonstrated higher mortality ratesin people with CKD, the magnitude of the increased risk hasvaried substantially for reasons that are unclear.
Current guidelines identify individuals with CKD as being athigh risk for cardiovascular disease and other adverse outcomes(6). Because nondialysis-dependent CKD may affect asmany as 19 million Americans (7), a summary of the risk forall-cause and cardiovascular mortality associated with thiscondition potentially would be useful to decision-makers andresearchers. In addition, identification of factors that modifythe strength of the relation between CKD and adverse outcomesmay help to improve the current understanding of how impairedkidney function leads to higher risk.
Despite considerable interest in this topic, previous summariesof the available evidence have been presented predominantlyas narrative reviews, which have widely known limitations (8,9).Therefore, we conducted a systematic review to evaluate theassociation between nondialysis-dependent CKD and therisk for all-cause and cardiovascular mortality. We also soughtto determine patient- and study-related characteristics thatinfluenced the magnitude of these associations.
Search Strategy
Two reviewers searched Medline (1969 to 2004) and EMBASE (1988to 2004) and the reference lists of primary studies, reviewarticles, and clinical practice guidelines independently andin duplicate. The search strategies were created with the assistanceof a research librarian and were not restricted to the Englishlanguage (Appendixes A and B). Any study that was deemed potentiallyrelevant by one or more reviewers was retrieved for inspection.
Study Selection
The search strategy and data extraction were defined by a prospectiveprotocol. Studies were eligible for inclusion when they studiedadults and contained data that permitted comparison of long-term(>1 yr) cardiovascular mortality or all-cause mortality betweenthose with and without kidney disease (defined by abnormal GFR,creatinine clearance [CrCl], or serum creatinine [SCr] levels).Studies were excluded when they were published in abstract only,included data from patients with acute renal failure (or includedpeople who were at high risk for acute renal failure, e.g.,those with acute myocardial infarction), or included patientswho had ESRD and were on dialysis or received a renal transplant.Finally, meta-analyses, case-control studies, studies with <100subjects, studies without a contemporaneous control group drawnfrom the same population, and studies that did not report theoutcomes of interest were excluded.
Validity Assessment and Data Extraction
We assessed criteria that can influence study validity (10).A study was described as retrospective when the initiation ofthe study occurred after both the exposures and the outcomesof interest occurred (11). When multiple publications from thesame group were found, the studies were reviewed carefully toensure that no data were analyzed in duplicate. Data were extractedinto a database and checked for accuracy by a second reviewer.When data were reported in strata, the data were extracted asseparate cohorts. The following data were extracted from eachincluded study: design, definition of CKD, age, gender, baselinerenal function, clinical subpopulation (heart failure, knowncardiovascular disease, hypertension, general population, andother), cardiovascular risk factors other than CKD (hypertension,smoking, diabetes, BP, LDL cholesterol, and body mass index),proteinuria, medication use (angiotensin-converting enzyme inhibitors,aspirin, blockers, and statins), and mortality (all-cause andcardiovascular). When included studies did not report data thatpermitted construction of a two-by-two table (mortality by CKDstatus), additional information was requested from the authors.
Definitions of Kidney Function and Kidney Disease
When possible, we used estimates of kidney function that werederived from estimating equations that are based on SCr, age,and/or race and gender rather than SCr alone. We defined CKDas present when estimated CrCl or GFR was <60 ml/min (withor without standardization to body surface area) or when SCrwas >120 µmol/L (>1.3 mg/dl) when data from estimatingequations were unavailable. Although not all studies presentedresults in this form, we used values as close as possible tothese levels of kidney function. Although CrCl is conceptuallydifferent from GFR, it is commonly used as an estimate of GFR(6); therefore, CrCl was used interchangeably with GFR to assessthe primary outcome. Similarly, estimates of GFR and/or CrClthat were normalized to body surface area were pooled with thosethat were not.
Outcome Measures
We considered the outcomes of all-cause death and cardiovasculardeath. For the latter, we used the definitions that were usedby the authors of the primary studies; these typically includeddeath from coronary, noncoronary cardiac, and cerebrovascularcauses. Overall summary estimates of the relation between CKDand these outcomes are not presented because of the extensiveclinical and methodologic heterogeneity. However, pooled estimatesare presented for the clinical subpopulations (heart failure,known cardiovascular disease, hypertension, general population,and other).
Statistical Analyses
Because of the differing definitions of kidney function andCKD between studies, we decided to dichotomize kidney function,using a GFR threshold as close as possible to 60 ml/min as mentionedabove. We calculated unadjusted risk ratios (RR) from the rawdata to facilitate comparisons between studies. These latterresults then were pooled using a random-effects model (12).For comparison, we also extracted adjusted hazard ratios fromCox regression analyses (which was the analysis that most studiesreported) when available. To facilitate comparisons with unadjustedresults, we included in our analysis only adjusted hazard ratiosthat were based on dichotomization of GFR, meaning that we didnot report adjusted estimates from studies that categorizedGFR into more than two categories (2,1318) or treatedGFR as a continuous variable (1925). Reasons for diversityin study results were explored using meta-regression. The associationbetween the following factors and outcomes was considered atthe study level: mean age; proportion of patients who were female;average duration of follow-up; population (hypertensive, heartfailure, cardiovascular disease, or from the general population);proportion of patients who were hypertensive or current smokersor had diabetes or a family history of coronary disease; meanbaseline renal function, BP, total cholesterol, body mass index,proteinuria, and C-reactive protein; and medication use. Medicationuse, proteinuria, and C-reactive protein were not analyzed furtherbecause too few studies reported these data. Methodologic factors(10) (e.g., study design) and the definition of CKD also wereexplored as potential explanatory variables in meta-regression.For each meta-regression, only studies for which the factorof interest was available were included in the analysis. Themodels were fit using the step-wise selection method. Statisticalheterogeneity was assessed using the I2 statistic (26,27) andthe 2 test for heterogeneity. I2 is the percentage of variancethat is due to between-study variance. A permutation test wasapplied to all fitted models to guard against spurious findings(28). Publication bias was not assessed because of the extensivestatistical heterogeneity, since covariates that lead to heterogeneityof the relation between exposure and outcome also may lead tofunnel plot asymmetry (29).
Data from the studies that presented results for subgroups ofpatients with renal insufficiency (e.g., GFR 30 to 59.9 and15 to 29.9 ml/min) were included in additional analyses thatexamined the graded relation between severity of renal dysfunctionand mortality. In these analyses, the level of renal dysfunctionin each subgroup was categorized by substituting the midpointof the range for interval data and adding or subtracting 15ml/min from bounded data. Therefore, in these analyses, eachstudy might contribute one estimate of risk (mortality associatedwith GFR <60 ml/min) or multiple estimates of risk (mortalityassociated with 30 to 59.9 and 15 to 29.9 ml/min, respectively).We used a random intercept logistic regression model (30) tocontrol for the possibility of within-study nonindependence,with mortality as the dependent variable and the index of renalfunction (estimated GFR in ml/min) as the independent variable.Confidence intervals (CI) for the best fit lines were calculatedusing the parameter estimates and the estimated covariance structurefrom the fitted model. Statistical analyses were performed withStata 8.2 (31), S-Plus 7.0 (32), and Review Manager 4.2.7 (33).Statistical significance was set at P < 0.05 for all analyses.This report was structured in accordance with published guidelines(34) and was approved by the institutional review board at theUniversity of Alberta.
Study Flow and Characteristics
Our search retrieved 3114 citations. Of these, 232 full-textarticles were retrieved, and 39 studies with a total of 1,371,990patients (25,1325,3556) met our criteriafor review (Table 1). Reasons for study exclusion are presentedin Figure 1: Six studies were duplicate publications, and fivestudies (5761) did not report or subsequently providedata in a usable format.
Figure 1. Flow diagram of studies that were considered for inclusion.
Seventeen of the 39 studies were nested cohort analyses of randomized,controlled trials. Of the remaining 22 cohort studies, sevenwere retrospective. Details of the 39 studies are outlined inTable 1. The sample selection process and the prognostic variablewere described adequately in 100% of the studies. As well, most(97%) described the inclusion and exclusion criteria adequately.Other quality items such as the description of clinical anddemographic characteristics, the description of outcomes, andadjustment for important prognostic factors were adequate in74 to 90% of the studies. In all studies, the definition ofCKD seemed to be based on a single measurement of SCr. One study(50) reported re-analyzing the baseline SCr measurement whenthe value exceeded 5.0 mg/dl, although repeat blood samplingseems not to have been performed. The number of patients whowere lost to follow-up was reported in 67% of the studies. Ofthese, 81% of studies lost <10% of patients during theirfollow-up. Median follow-up for the included studies was 4.5yr (range 0.8 to 16).
Relation between CKD and All-Cause Death
Forty-one cohorts from 37 studies that included 1,366,126 patientsreported or provided all-cause mortality by CKD status. Theunadjusted relative risk for mortality in patients with CKDcompared with those without CKD ranged from 0.94 to 5.0 andwas significantly more than 1.0 in 38 (93%) of 41 cohorts. TheI2 statistic was >99% (Q = 6907, P < 0.001), indicatingstatistical heterogeneity between studies, and visual inspectionof the forest plots demonstrated qualitative heterogeneity,suggesting that presenting a pooled estimate of the overallrisk associated with CKD would be inappropriate. Within clinicalsubpopulations, statistical heterogeneity remained, but qualitativeheterogeneity was much reduced, suggesting that pooling wouldbe informative. Therefore, pooled relative risks in varioussubpopulations are presented in Figure 2.
Figure 2. All-cause mortality in chronic kidney disease (CKD).
We performed additional analyses to determine factors that mightpartially explain this heterogeneity. Twenty-four variableswere explored in meta-regression. In several meta-regressionmodels, studies with the following characteristics tended tofind a higher relative risk for mortality associated with CKD:Lower risk patients (younger mean age, higher proportion ofwomen, less hypertension at baseline, and subjects from thegeneral population rather than those with known heart failureor cardiovascular disease), poorer quality studies (retrospectivestudies, shorter follow-up duration, and studies not reportingloss to follow-up), studies with a representative sample ofparticipants (acceptable external validity), and cohort studies(as opposed to nested cohort analyses of randomized trials).In the final multivariable meta-regression model (31 cohorts),the predicted relative risk for death associated with CKD wassignificantly higher in retrospective trials, cohorts with youngeraverage participant age, and those in which patients had lowerbaseline prevalence of known cardiovascular disease (all P <0.05). Adjustment for these three covariates only marginallyreduced I2 from >99 to 88% (Q statistic reduced from 6787to 227). All permutation tests were significant, supportingthese variables as contributors to the heterogeneity in therisk associated with CKD as reported by the primary studies.
To examine the graded relation between severity of CKD and mortality,we calculated multiple risks per study for various severitiesof renal dysfunction (expressed as the frequency of death withineach stratum of renal dysfunction). Sixteen prospective studiesstratified results by estimated GFR at baseline and so wereavailable for this analysis. We found an exponential relationbetween the severity of renal dysfunction and the risk for all-causemortality (Figure 3). For example, the predicted risk for deathduring the median follow-up of 4.9 yr was 12% (95% CI 8 to 19)when CKD was defined as estimated GFR = 80 ml/min, 17% (95%CI 11 to 25) for GFR = 60 ml/min, and 25% (95% CI 17 to 35)for GFR = 40 ml/min. Phrased in relative terms, the estimatedrelative odds for death associated with GFR of 80, 60, and 40ml/min were 1.9, 2.6, and 4.4, respectively (all compared witha GFR of 100 ml/min).
Figure 3. All-cause mortality in CKD by baseline renal function. Risk for all-cause mortality in 42 cohorts according to baseline estimated GFR (ml/min). Risks are expressed as proportions (e.g., 0.6 = 60%). The area of each circle (i.e., data point) is proportional to the sample size of each cohort. The center line models the estimated risk for baseline GFR from the unadjusted analysis. Dotted lines represent the 95% confidence intervals. Median follow-up is 4.9 yr (range 0.8 to 14 yr).
Twenty-six analyses (from 23 studies) performed and reportedCox regression models using kidney function as an independentvariable and all-cause mortality as the dependent variable,adjusted for potential confounders. In total, >100 differentcovariates were considered for inclusion in the individual studies.Of these, only four variables (age, BP or hypertension, diabetes,and gender) were considered in at least half of these analyses.The adjusted hazard ratios for 13 of 14 analyses that treatedCKD as a dichotomous variable (Figure 4) all were lower in magnitudethan the unadjusted relative risks (median reduction 17% lower;range 27% increase to 52% lower), consistent with the observationthat CKD frequently coexists with other risk factors for death,such as diabetes and hypertension. Ten of the 14 adjusted estimateswere statistically significant.
Figure 4. All-cause mortality in CKD using adjusted hazard ratios.
Relation between CKD and Cardiovascular Death
Fourteen studies that included a total of 100,064 participantsreported cardiovascular mortality. The unadjusted relative riskfor mortality was qualitatively increased in patients with renalinsufficiency and ranged from 1.4 to 3.7 (Figure 5). There wassubstantial heterogeneity between the studies; results werenot mathematically pooled (Q = 127, P < 0.001; I2 90%). Meta-regressionfound that the risk associated with CKD was significantly higherin cohorts with lower mean age. After adjustment for mean age,we found that I2 reduced from 91 to 74% (Q reduced from 127to 43, all P < 0.001), and the permutation test was statisticallysignificant. No further covariates were statistically significant.Using a mean age of 50 yr, our model predicted a relative riskof 3.4 (95% CI 2.1 to 5.5) with renal insufficiency, whereaswith a mean age of 70 yr, the relative risk was 1.5 (95% CI0.96 to 2.3). Five of the seven adjusted hazard ratios for cardiovascularmortality were statistically significant. Four of five adjustedestimates were lower in magnitude than the unadjusted relativerisks (median reduction 20% lower; range 13% increase to 29%lower).
To our knowledge, this is the first systematic review to examinethe relation between nondialysis-dependent CKD and mortalityin multiple patient populations. The point estimate for theunadjusted relative risk for all-cause death associated withCKD exceeded 1.0 for 40 of 41 cohorts and was statisticallysignificant in 38 (93%) of 41. The absolute risk for death increasedexponentially with decreasing renal function in these studies.The magnitude of the unadjusted increase in risk ranged fromapproximately 38% to >1100%, reflecting considerable between-studyheterogeneity. However it is important to emphasize that thisheterogeneity was quantitative, not qualitative. Therefore,although the point estimates varied across studies, CKD wasassociated consistently with increased all-cause and cardiovascularmortality in a wide range of clinical populations. In the 14studies that adjusted these risks for other patient factorssuch as diabetes and hypertension, the risks were substantiallylower than unadjusted risks. This reflects that CKD often coexistswith other factors that are associated with adverse outcomes.The increased risk for all-cause mortality largely was drivenby cardiovascular deaths, which constituted 58% of the deathsin the 13 studies that reported both cardiovascular and all-causedeaths.
The magnitude of the increased risk associated with CKD wasgreater with more advanced kidney dysfunction, suggesting thatdifferent definitions of CKD between studies may have explainedsome of the observed heterogeneity. Although differences incomorbidities between studies did not seem to explain all thevariation in the quantitative estimates of increased risk associatedwith CKD, this conclusion should be interpreted with cautiongiven that most studies did not account for all potential confounders.Multivariable meta-regression suggested that the relative riskfor death associated with CKD was significantly higher in lowerrisk populations, such as those with younger participants orwith a lower prevalence of cardiovascular disease. These findingscontrast with previous suggestions that the risk as a resultof CKD might be limited to those with preexisting vascular disease(62) but are consistent with data from the United States RenalData System indicating that the relative risk for cardiovasculardeath in dialysis patients (versus the general population) isgreater in younger patients (1). From a population health perspective,this might suggest that screening for CKD should focus on younger,healthier populations to maximize the impact of preventive strategies.Unfortunately, this benefit would need to be balanced againstthe much lower likelihood of detecting CKD in such populations.
Although the mechanisms by which CKD might mediate increasedrisk for death is unknown, there are several possibilities.First, CKD often coexists with other cardiovascular risk factors,including dyslipidemia, hypertension, smoking, and diabetes(63), and these risk factors are associated with substantialincreases in absolute risk in the general population (6467).Second, patients with evidence of renal disease are less likelyto receive proven efficacious therapies (23,6870). Inaddition, it is possible that therapies that are proved to beefficacious in trials with few CKD patients might be less efficaciousor more toxic in the setting of CKD. Third, impaired kidneyfunction may be a marker merely for severity of vascular disease,including atherosclerosis that is not yet clinically evident(62,71). Finally, impaired kidney function is associated withmarkers of inflammation and other putative risk factors forcardiovascular events (7275), which might contributedirectly to adverse outcomes.
As with all systematic reviews, the strength of our conclusionsis influenced by the quality of the constituent studies, andanalysis of aggregate (study-level) data may have concealedsubgroup effects that would have been apparent if subject-leveldata were available. We assessed study quality using a standardinstrument with 10 domains (10) and found that between 67 and100% of studies met at least eight of the 10 quality criteria.Most studies were not conducted in a sample that was broadlyrepresentative of the population that they sought to investigate,and only a minority reported reasons for losses to follow-up.Because our primary interest was in CKD, we excluded resultsfrom studies in which acute renal failure seemed likely, suchas those that were conducted in participants with acute coronarysyndromes. However, all studies in this report defined CKD onthe basis of a single measurement of kidney function; therefore,some individuals may have been misclassified with respect toCKD status. This suggests that the true relation between CKDand death is stronger than that suggested by our results (becausemisclassification would bias our results toward the null forfinding any association [76]). Future studies should classifyindividuals on the basis of multiple measurements of kidneyfunction, preferably with temporal separation to confirm chronicity.In addition, it is plausible that more rapid rates of kidneyfunction loss are associated with adverse outcomes, and futurestudies should test this hypothesis. Finally, because proteinuriais strongly associated with the risk for cardiovascular events(a relation that is independent of reduced GFR) (77), futurestudies should report the association between CKD and risk instrata that are defined by urinary protein excretion.
Nondialysis-dependent CKD was associated with an increasedrisk for all-cause and cardiovascular death in the majorityof studies. This finding remained after within-study adjustmentfor potential confounders and was consistent across studiesdespite variations in design, study populations, event ratesin control subjects, and definitions of CKD. This review supportscurrent guidelines that identify individuals with CKD as beingat high risk for cardiovascular disease and other adverse outcomesand supports calls for more intensive intervention in patientswith CKD to prevent adverse outcomes (78). Therapeutic strategiesthat have been shown to prevent cardiovascular events in patientswith CKD include aggressive BP control, statins, and angiotensin-convertingenzyme inhibitors/angiotensin receptor blockers (79). The firststep in addressing any care gap is identifying and quantifyingthe magnitude of the problem; our systematic review achievesthis goal by highlighting the consistency of the evidence aboutthe hazards that are associated with CKD. The challenge thatclinicians now face is to search for CKD and manage it aggressively.The challenge that researchers now face is to evaluate novelmeans of detecting CKD (80) and to expand the therapeutic armamentariumfor patients with CKD.
This study was supported by the Alberta Kidney Disease Network.M.T. and F.M. were supported by Population Health InvestigatorAwards from the Alberta Heritage Foundation for Medical Researchand New Investigator Awards from the Canadian Institutes forHealth Research. M.F. was supported by a summer studentshipfrom the Canadian Institutes for Health Research. A.X.G. wassupported by a Clinician Scientist Award from the Canadian Institutesfor Health Research.
We thank the following authors for sending us additional data:Graziella Bruno, Rory Collins, Justin Ezekowitz, Alan Go, JoseMarie Moreno Planas, Giovanni Pulignano, Jorg Muntwyler, GiuseppeSchillaci, Michael Shlipak, and Susanna Sihvonen. We also thankKris Cramer for data checking and Alex Stewart and Julie Patersonfor text retrieval.
Footnotes
Published online ahead of print. Publication date availableat www.jasn.org.
This study links an increased tendency for uric acid stone formationto type II diabetes and postulates that the association relatesto the reduced urine pH seen in type II diabetics with insulinresistance. In the companion issue of CJASN, Musso et al. presenta more comprehensive review of all the established renal manifestationsof insulin resistance in metabolic syndrome and type II diabetes(pages 616622).
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