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Changes in dietary protein intake has no effect on serum cystatin C levels independent of the glomerular filtration rate

2010; Elsevier BV; Volume: 79; Issue: 4 Linguagem: Inglês

10.1038/ki.2010.431

1523-1755

Navdeep Tangri, Lesley A. Stevens, Christopher H. Schmid, Yaping Zhang, Gerald J. Beck, Tom Greene, Josef Coresh, Andrew S. Levey,

Biomedical Research and Pathophysiology

Cystatin C is being considered as a replacement for serum creatinine in the estimation of the glomerular filtration rate (GFR); however, its plasma levels might be affected by factors other than the GFR, such as protein intake. We performed a post hoc analysis of the data in the Modification of Diet in Renal Disease study, in which we compared serum creatinine and cystatin C levels in 741 patients with available estimates of protein intake at baseline prior to their randomization to diets containing various amounts of protein, and at 2 years of follow-up in 426 of these patients in whom a cystatin C measurement was available. The 503 patients in study A (GFR 25–55 ml/min per 1.73 m2) had been assigned a low (0.58 g/kg per day) or a usual (1.3 g/kg per day) protein intake, and the 238 participants in study B (GFR 13–24 ml/min per 1.73 m2) were assigned a very low (0.28 g/kg per day) or the low protein intake. In either study group, lowering the dietary protein intake reduced the change in creatinine, but did not have a significant change in cystatin C. Thus, in patients with moderate-to-severe chronic kidney disease, serum cystatin C unlike serum creatinine was not affected by dietary protein intake independent of changes in GFR. Hence, cystatin C may allow more accurate estimates of GFR than creatinine for patients with reduced protein intake. Further study of other non-GFR determinants of cystatin C is needed before the widespread adoption. Cystatin C is being considered as a replacement for serum creatinine in the estimation of the glomerular filtration rate (GFR); however, its plasma levels might be affected by factors other than the GFR, such as protein intake. We performed a post hoc analysis of the data in the Modification of Diet in Renal Disease study, in which we compared serum creatinine and cystatin C levels in 741 patients with available estimates of protein intake at baseline prior to their randomization to diets containing various amounts of protein, and at 2 years of follow-up in 426 of these patients in whom a cystatin C measurement was available. The 503 patients in study A (GFR 25–55 ml/min per 1.73 m2) had been assigned a low (0.58 g/kg per day) or a usual (1.3 g/kg per day) protein intake, and the 238 participants in study B (GFR 13–24 ml/min per 1.73 m2) were assigned a very low (0.28 g/kg per day) or the low protein intake. In either study group, lowering the dietary protein intake reduced the change in creatinine, but did not have a significant change in cystatin C. Thus, in patients with moderate-to-severe chronic kidney disease, serum cystatin C unlike serum creatinine was not affected by dietary protein intake independent of changes in GFR. Hence, cystatin C may allow more accurate estimates of GFR than creatinine for patients with reduced protein intake. Further study of other non-GFR determinants of cystatin C is needed before the widespread adoption. Accurate estimation of the glomerular filtration rate (GFR) is essential for the diagnosis, staging, and management of chronic kidney disease (CKD).1.Levey A.S. Coresh J. Balk E. et al.National Kidney Foundation practice guidelines for chronic kidney disease: evaluation, classification, and stratification.Ann Inter Med. 2003; 139: 137-147Crossref PubMed Scopus (3462) Google Scholar, 2.Stevens L.A. Coresh J. Greene T. et al.Assessing kidney function—measured and estimated glomerular filtration rate.N Engl J Med. 2006; 354: 2473-2483Crossref PubMed Scopus (2161) Google Scholar Serum creatinine is most commonly used to estimate GFR, with current estimating equations taking into account age, sex, race, and weight as non GFR determinants of creatinine.3.Cockcroft D.W. Gault M.H. Prediction of creatinine clearance from serum creatinine.Nephron. 1976; 16: 31-41Crossref PubMed Scopus (12499) Google Scholar, 4.Levey A.S. Stevens L.A. Schmid C.H. et al.A new equation to estimate glomerular filtration rate.Ann Inter Med. 2009; 150: 604-612Crossref PubMed Scopus (12894) Google Scholar Cystatin C is being proposed as potentially superior biomarker for GFR estimation.5.Dharnidharka V.R. Kwon C. Stevens G. Serum cystatin C is superior to serum creatinine as a marker of kidney function: a meta-analysis.Am J Kidney Dis. 2002; 40: 221-226Abstract Full Text Full Text PDF PubMed Scopus (1223) Google Scholar Cystatin C is an endogenous 13 kDa protein that is freely filtered at the glomerulus, and then nearly completely reabsorbed and catabolized by proximal tubular epithelial cells with only small amounts excreted in the urine. Cystatin C generation is felt to be constant, thus serum levels are not affected by variables other than kidney function.6.Madero M. Sarnak M.J. Stevens L.A. Serum cystatin C as a marker of glomerular filtration rate.N Engl J Med. 2006; 15: 610-616Google Scholar Therefore, cystatin C is felt to be a promising candidate for replacing creatinine as a biomarker for estimating GFR. More recent studies, however, have found variability in the relationship between cystatin C and measured GFR suggesting the potential for non-GFR determinants of cystatin C.7.Klahr S. Levey A.S. Beck G.J. et al.The effects of dietary protein restriction and blood-pressure control on the progression of chronic renal disease. Modification of Diet in Renal Disease Study Group.N Engl J Med. 1994; 330: 877-884Crossref PubMed Scopus (1909) Google Scholar, 8.Stevens L.A. Levey A.S. Measured GFR as a confirmatory test for estimated GFR.J Am Soc Nephrol. 2009; 20: 2305-2313Crossref PubMed Scopus (359) Google Scholar, 9.Perrone R.D. Madias N.E. Levey A.S. Serum creatinine as an index of renal function: new insights into old concepts.Clin chem. 1992; 38: 1933-1953PubMed Google Scholar Understanding these potential determinants would be important to develop and evaluate GFR estimating equations based on cystatin C and to better understand the relationship between cystatin C and adverse outcomes. To address this question, we analyzed data from the Modification in Diet and Renal Disease (MDRD) study, which was a randomized controlled trial of protein restriction and blood pressure control in patients with CKD stages 3 and 4.7.Klahr S. Levey A.S. Beck G.J. et al.The effects of dietary protein restriction and blood-pressure control on the progression of chronic renal disease. Modification of Diet in Renal Disease Study Group.N Engl J Med. 1994; 330: 877-884Crossref PubMed Scopus (1909) Google Scholar We examined the relationship between dietary protein intake and creatinine and cystatin C levels at baseline after adjustment for measured GFR and GFR measurement error. In addition, we tested the effect of a dietary protein intake prescription on creatinine and cystatin C independent of GFR in a longitudinal analysis comparing randomized groups. Baseline characteristics of the study population are presented in Table 1. Cystatin C measurements were available in 574 out of 585 patients in study A and 251 out of 275 patients in study B at the time of randomization. The mean measured GFR at time of randomization was 38.6 ml/min per 1.73 m2 in study A and 18.5 ml/min per 1.73 m2 in study B. The etiology of kidney disease included polycystic kidney disease (22%), glomerular disease (27%), hypertensive nephrosclerosis (17%), tubulointerstitial diseases (7%), and other or unknown (14%). Only 3.5 % of the patients had diabetic nephropathy, because patients with diabetes requiring insulin were excluded from the MDRD study. Estimated protein intake (EPI) was slightly higher in study A compared with study B.Table 1Baseline characteristics of the study populationStudy A (574)Study B (251)Mean/NaData are presented as mean±s.d. for continuous variables and N/percentage for categorical variables.s.d./%Mean/Ns.d./%Age (years)52.012.250.912.9Female22461.010240.6White529.113.05.2Smoking status Regularly5510.72812.2 Occasionally28154.511148.3 Never13924.25923.5Systolic blood pressure (mm Hg)131.417.5133.117.7Diastolic blood pressure (mm Hg)81.010.080.910.3Etiology of kidney disease Polycystic kidney disease13924.25923.5 Hereditary nephritis and tubulointerstitial disease17530.57429.5 Hypertensive kidney disease10017.43915.5 Diabetic nephropathy173.093.6 Glomerular disease14324.97027.9Laboratory data Serum creatinine (mg/dl)1.90.53.40.9 Serum cystatin C (mg/l)1.90.43.00.5 Measured GFR (ml/min per 1.73 m2)38.68.918.53.4 Proteinuria (g/day)1.01.61.41.7 Serum albumin (g/dl)4.00.34.00.4 Serum CRP (mg/l)0.50.60.40.6 Estimated protein intake (g/kg per day)bProtein intake estimated from urine urea nitrogen excretion rate was available for 741/825 participants.1.10.20.90.2Abbreviations: CRP, C-reactive protein; GFR, glomerular filtration rate.a Data are presented as mean±s.d. for continuous variables and N/percentage for categorical variables.b Protein intake estimated from urine urea nitrogen excretion rate was available for 741/825 participants. Open table in a new tab Abbreviations: CRP, C-reactive protein; GFR, glomerular filtration rate. In all, 84 patients (10%) did not have 24-h urine collections available at the final baseline visit for determination of EPI, and were therefore excluded from the cross-sectional analysis. A higher proportion of men and patients from study A were missing EPI at baseline. Patients with missing EPI also had slightly lower values for creatinine and cystatin C and higher measured GFR (Appendix A, Table A1). Over the follow-up period of 2 years, 23 patients died and 86 went on to develop kidney failure. Cystatin C measurements were available in 426 patients at 2 years, with 290 patients missing cystatin C measurements at follow-up. These patients had no statistically significant differences in age, etiology of kidney disease, and proteinuria, from the group with available cystatin C measurements. Small differences in measured GFR and EPI were again observed (Appendix A, Table A2). The cross-sectional associations relating EPI to serum creatinine and cystatin C are presented in Table 2. EPI was more strongly associated with serum creatinine than cystatin C at baseline after adjustment for GFR, GFR measurement error, age, sex, and race; a 0.2 g/kg per day higher EPI was associated with a 2.4 (0.6)% higher serum creatinine and a 0.9 (0.6)% higher cystatin C. The association for creatinine, but not cystatin C, was statistically significant.Table 2Association of estimated protein intake with creatinine and cystatin C at baselineaProtein intake is estimated from urine urea nitrogen, cystatin, and creatinine are log transformed.Not adjustedAdjusted for GFRAdjusted for GFR measurement error 0.015Adjusted for GFR measurement error (0.015), age, sex, and racebInterpretation: after adjustment for GFR, age, race and sex, and GFR measurement error, a 0.2 g/kg per day increase in baseline protein intake is associated with a 2.4 (0.6)% higher baseline serum creatinine and a 0.9 (0.6)% higher baseline serum cystatin C.NCoeff (s.d.)P-valueCoeff (s.d.)P-valueCoeff (s.d.)P-valueCoeff (s.d.)P-valueSerum creatinine (%)741-0.141 (0.010)<0.001-0.006 (0.008)0.450.012 (0.008)0.1450.024 (0.006)<0.001Serum cystatin C (%)741-0.110 (0.009)<0.001-0.007 (0.006)0.260.022 (0.006)<0.0010.009 (0.006)0.13Abbreviations: Coeff, coefficient; GFR, glomerular filtration rate.a Protein intake is estimated from urine urea nitrogen, cystatin, and creatinine are log transformed.b Interpretation: after adjustment for GFR, age, race and sex, and GFR measurement error, a 0.2 g/kg per day increase in baseline protein intake is associated with a 2.4 (0.6)% higher baseline serum creatinine and a 0.9 (0.6)% higher baseline serum cystatin C. Open table in a new tab Abbreviations: Coeff, coefficient; GFR, glomerular filtration rate. A longitudinal analysis of randomized groups is presented in Table 3 and Figure 1. The change in measured GFR from baseline to year 2 was identical in the usual and low protein diet groups (-0.3 ml/min per 1.73 m2 in study A and -0.2 ml/min per 1.73 m2 in study B). In study A, the change in serum creatinine was lower (-0.22 (-0.36, -0.08) mg/dl) in the low protein intake arm compared with the usual protein intake arm. Consequently, the change in the creatinine-based GFR estimate was higher (2.2 (0.6, 3.9) ml/min per 1.73 m2) in the low protein intake arm compared with the usual protein intake arm. In study B, the changes in serum creatinine levels and the creatinine-based GFR estimate did not differ significantly between the low and very low protein diet (-0.28 (-0.82, 0.21) mg/dl and 0.8 (-1.0, 2.6) ml/min per 1.73 m2, respectively). Changes in the serum cystatin C concentration and the cystatin-based GFR estimate did not differ between randomized groups in either study.Table 3Effect of prescribed dietary protein on change in measured GFR, creatinine, and cystatin C independent of GFR, and estimated GFR using creatinine and cystatin CaDifferences between randomized groups (change in low protein diet group minus change in usual protein diet group in study A; very low minus low protein diet group). Change is defined as 24-month baseline. Changes in creatinine and cystatin C, and eGFR measurements are adjusted for change in measured GFR.Study A (n=302)Study B (n=124)ΔGFR (ml/min per 1.73 m2)-0.3 (-2.1, 1.6)-0.2 (-1.9, 1.4)ΔSerum creatinine (mg/dl)-0.22 (-0.36, -0.08)-0.28 (-0.82, 0.21)ΔeGFRcr (ml/min per 1.73 m2)+2.2 (0.6, 3.9)+ 0.8 (-1.0, 2.6)ΔSerum cystatin C (mg/l)0.02 (-0.08, 0.13)0.10 (-0.15, 0.26)ΔeGFRcys (ml/min per 1.73 m2)-0.4 (-2.1, 0.9)-0.3 (-1.5, 2.2)Abbreviations: eGFRcr, creatinine-based glomerular filtration rate estimate; eGFRcys, cystatin-based glomerular filtration rate estimate; GFR, glomerular filtration rate.All values are reported as mean and 95% confidence interval. Values highlighted in boldface font are statistically significant with P<0.01.a Differences between randomized groups (change in low protein diet group minus change in usual protein diet group in study A; very low minus low protein diet group). Change is defined as 24-month baseline. Changes in creatinine and cystatin C, and eGFR measurements are adjusted for change in measured GFR. Open table in a new tab Abbreviations: eGFRcr, creatinine-based glomerular filtration rate estimate; eGFRcys, cystatin-based glomerular filtration rate estimate; GFR, glomerular filtration rate. All values are reported as mean and 95% confidence interval. Values highlighted in boldface font are statistically significant with P<0.01. A comparison of the performance of the MDRD study equation and the CKD–EPI cystatin C 2008 equation at baseline and at 2 years for the treatment groups is presented in Table 4. When computing estimated GFR (eGFR) using serum creatinine, in study A, the difference in bias between the low and usual protein diet groups was not significantly different at baseline, (0.23 (-1.61, 1.15) ml/min per 1.73 m2) but was significantly greater in the low protein diet group at follow-up (-2.77 (-4.08, -1.48) ml/min per 1.73 m2), reflecting a greater overestimation of measured GFR in the low protein diet group. In study B the results were qualitatively similar, although the difference at follow-up was not statistically significant. The relative change in bias, compared with mean baseline mGFR, was 8.01% in study A and 7.44% in study B. In contrast, when computing eGFR using serum cystatin C, there was no difference in bias between randomized groups at either baseline or follow-up in study A or study B, and no change over time (<1 ml/min per 1.73 m2 and <2.5%).Table 4Effect of prescribed dietary protein on bias in GFR estimation using the MDRD study equation and the CKD–EPI cystatin C 2008 study equationaDifferences in mean bias between randomized groups (bias in low protein diet group minus bias in usual protein diet group in Study A; very low minus low protein diet group in Study B). Bias is defined as measured-estimated GFR.Equation performanceTimingStudy A (n=302)Study B (n=124)ΔBias eGFRcr (ml/min per 1.73 m2)Baseline-0.23 (-1.61, 1.15)-0.35 (-1.75, 1.10)ΔBias eGFRcr (ml/min per 1.73 m2)Follow-up-2.77 (-4.08, -1.48)-1.37 (-2.88, 0.15)ΔBias eGFRcys (ml/min per 1.73 m2)Baseline-0.60 (-2.20, 0.99)-0.50 (-0.75, 1.76)ΔBias eGFRcys (ml/min per 1.73 m2)Follow-up-0.31 (-1.75, 1.13)-0.02 (-1.70, 1.75)Abbreviations: CKD, chronic kidney disease; EPI, estimated protein intake; eGFRcr, creatinine-based glomerular filtration rate estimate; eGFRcys, cystatin-based glomerular filtration rate estimate; GFR, glomerular filtration rate; MDRD, Modification in Diet and Renal Disease.All values are reported as mean and 95% confidence interval. Values highlighted in boldface are statistically significant with P<0.01. P for eGFRcr at follow-up in Study B is 0.08.a Differences in mean bias between randomized groups (bias in low protein diet group minus bias in usual protein diet group in Study A; very low minus low protein diet group in Study B). Bias is defined as measured-estimated GFR. Open table in a new tab Abbreviations: CKD, chronic kidney disease; EPI, estimated protein intake; eGFRcr, creatinine-based glomerular filtration rate estimate; eGFRcys, cystatin-based glomerular filtration rate estimate; GFR, glomerular filtration rate; MDRD, Modification in Diet and Renal Disease. All values are reported as mean and 95% confidence interval. Values highlighted in boldface are statistically significant with P 61 years) versus low (≤92 mm Hg for age ≤60 years, and ≤98 mm Hg for age >61 years) blood pressure goals in a 2 × 2 factorial design.7.Klahr S. Levey A.S. Beck G.J. et al.The effects of dietary protein restriction and blood-pressure control on the progression of chronic renal disease. Modification of Diet in Renal Disease Study Group.N Engl J Med. 1994; 330: 877-884Crossref PubMed Scopus (1909) Google Scholar For this report, participants in both blood pressure groups are combined for all analyses. GFR was measured as four period urinary clearance of 125I-iothalamate. Samples were assayed for cystatin C with a particle-enhanced immunonephelometric assay (N Latex Cystatin C, Dade Behring, IL) in samples stored at –80° C. The inter- and intraassay coefficients of variation for cystatin C were 3.2–4.4 and 2.0–3.0%, respectively. Stability in serum stored at -80 °C has been demonstrated.22.Erlandsen E.J. Randers E. Kristensen J.H. Reference intervals for serum cystatin C and serum creatinine in adults.Clin Chem Lab Med. 1998; 36: 393-397Crossref PubMed Scopus (75) Google Scholar, 23.Erlandsen E.J. Randers E. Kristensen J.H. Evaluation of the Dade Behring N latex cystatin C assay on the Dade Behring nephelometer II system.Scand J Clin Lab Invest. 1999; 59: 1-8Crossref PubMed Google Scholar Serum creatinine assays were calibrated to standardized serum creatinine values at the Cleveland Clinic Research Laboratory. The results of the calibration procedures have been previously described. GFR estimates using serum creatinine and cystatin C were calculated using the MDRD study equation and the CKD–EPI cystatin equation 2008, respectively.24.Stevens L.A. Manzi J. Levey A.S. et al.Impact of creatinine calibration on performance of GFR estimating equations in a pooled individual patient database.Am J Kidney Dis. 2007; 50: 21-35Abstract Full Text Full Text PDF PubMed Scopus (167) Google Scholar, 25.Levey A.S. Bosch J.P. Lewis J.B. et al.A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group.Ann Inter Med. 1999; 130: 461-470Crossref PubMed Scopus (12277) Google Scholar Descriptive statistics are reported as percentages for categorical data, and mean and standard deviation for normally distributed continuous data. Continuous variables were transformed so as to create a linear relationship with log-transformed cystatin C and creatinine in bivariate analyses. Sex and race were expressed as binary factors indicating presence or absence of female sex and black race, respectively. Differences between groups were tested using the χ2-test, Student t-test, and the Mann–Whitney test as appropriate. The relationships of cystatin C and creatinine with the predictor variables were investigated by first performing separate linear regressions to relate log-transformed cystatin C and creatinine to EPI at the final baseline visit after controlling for age, sex, and log-transformed GFR. An increment of 0.2 g/kg per day for protein intake was used for its clinical applicability (14 g/day for a 70 kg person) and to maintain consistency with previous work.10.Effects of diet and antihypertensive therapy on creatinine clearance and serum creatinine concentration in the Modification of Diet in Renal Disease Study.J Am Soc Nephrol. 1996; 7: 556-566PubMed Google Scholar We repeated these analyses using errors-in-variables regression analysis to incorporate measurement error in GFR into these models. A measurement error variance of 0.015 was assumed for log-transformed GFR based on the analyses of longitudinal variability in log-transformed baseline GFR.26.Li L. Greene T. Varying coefficients model with measurement error.Biometrics. 2008; 64: 519-526Crossref PubMed Scopus (26) Google Scholar GFR measurements were spaced an average of approximately 3 months apart in the MDRD study. The effects of the dietary protein intervention on the change in measured and eGFR, and serum levels of creatinine and cystatin C from baseline to 2 years was examined in study A and study B. For the serum levels of creatinine and cystatin C, the change was estimated using analysis of covariance, with the model adjusting for baseline serum levels of the filtration markers, baseline, and follow-up GFR; and indicator variables for randomized diet group, respectively. In this analysis, patients were analyzed according to their randomized group assignment, irrespective of achieved protein intake during follow-up. The performance of the MDRD study and CKD–EPI cystatin C 2008 equations was evaluated at baseline and after the 2 year follow-up in both the usual and low protein diet groups in study A and low and very low protein diet groups in study B. The mean difference between measured and eGFR is defined as bias. The mean difference in bias between randomized groups was compared for both studies at baseline and after 2 years. The mean change over time in the mean difference in bias for the two equations was compared using unpaired t-tests. All the authors declared no competing interests. The authors acknowledge the participants of the MDRD study, and collaborators in the CKD–EPI collaboration. NT is supported by the KRESCENT post-doctoral fellowship award, a joint initiative of the Kidney Foundation of Canada, the Canadian Institute of Health Research and the Canadian Society of Nephrology. This work is also supported by the NIDDK grants UO1 DK 053869, UO1 DK 067651, and UO1 DK 35073. Abbreviations: CRP, C-reactive protein; GFR, glomerular filtration rate. Abbreviations: CRP, C-reactive protein; GFR, glomerular filtration rate.