Measurement of glomerular filtration rate using endogenous d-serine clearance in living kidney transplant donors and recipients.

BACKGROUND: Endogenous molecules that provide an unbiased and a precise evaluation of kidney function are still necessary. We explored the potential of clearance of d-serine, a rare enantiomer of serine and a biomarker of kidney function, as a measure of glomerular filtration rate (GFR). METHODS: This was a cross-sectional observational study of 200 living kidney transplant donors and recipients enrolled between July 2019 and December 2020 in a single Japanese center, for whom GFR was measured by clearance of inulin (C-in). Clearance of d-serine (C-dSer) was calculated based on blood and urine levels of d-serine, as measured by two-dimensional high-performance liquid chromatography. Analytical performance was assessed by calculating biases. Utilizing data from 129 participants, we developed equations for C-in based on C-dSer and C-cre using a linear regression model, and the performance was validated in 68 participants. FINDINGS: The means of C-in and C-dSer were 66.7 and 55.7 mL/min/1.73 m2 of body surface area, respectively, in the entire cohort. C-dSer underestimated C-in with a proportional bias of 22.0% (95% confidence interval, 14.2-29.8%) and a constant bias of -1.24 (-5.78-3.31), whereas the proportional bias was minor to that of C-cre (34.6% [31.1-38.2%] and 2.47 (-1.18-6.13) for proportional and constant bias, respectively). Combination of C-dSer and C-cre measured C-in with an equation of 0.391 × C-dSer + 0.418 × C-cre + 3.852, which reduced the proportional bias (6.5% [-0.2-13.1%] and -4.30 [-8.87-0.28] for proportional and constant bias, respectively). In the validation dataset, this equation performed well with median absolute residual of 3.5 [2.3-4.8], and high ratio of agreement (ratios of 30% and 15% different from C-in [P30 and P15] of 98.5 [91.4-100] and 89.7 [80.0-95.2], respectively). INTERPRETATION: The smaller proportional bias compared to that of C-cre is an advantage of C-dSer as a measure of C-in. Combinational measurement of d-serine and creatinine, two endogenous molecules, has the potential to serve as a measure of GFR with precision and minor biases and can support important clinical decisions. FUNDING: Japan Society for the Promotion of Science (JSPS, grant number 17H04188), Japan Agency of Medical Research and Development (AMED, JP20gm5010001), Osaka Kidney Bank (OKF19-0010), Shiseido Co., Ltd and KAGAMI Inc.

Identification of Diabetic Nephropathy in Patients Undergoing Kidney Biopsy through Blood and Urinary Profiles of d-Serine.

Background: The diagnosis of diabetic nephropathy (DN), the major cause of ESKD, requires kidney biopsy. d-Serine, present only in trace amounts in humans, is a biomarker for kidney diseases and shows potential to distinguish the origin of kidney diseases, whose diagnoses usually require kidney biopsy. We extended this concept and examined the potential of d-serine in the diagnosis of DN. Methods: We enrolled patients with biopsy sample-proven DN and primary GN (minimal change disease and IgA nephropathy) and participants without kidney disease. A total of 388 participants were included in this study, and d-serine levels in blood and urine were measured using two-dimensional high-performance liquid chromatography, and urinary fractional excretion (FE) of d-serine was calculated. Using data from 259 participants, we developed prediction models for detecting DN by logistic regression analyses, and the models were validated in 129 participants. Results: A d-serine blood level of >2.34 µM demonstrated a high specificity of 83% (95% CI, 70% to 93%) for excluding participants without kidney diseases. In participants with a d-serine blood level >2.34 µM, the threshold of 47% in FE of d-serine provided an optimal threshold for the detection of DN (AUC, 0.85 [95% CI, 0.76 to 0.95]; sensitivity, 79% [95% CI, 61% to 91%]; specificity, 83% [95% CI, 67% to 94%]). This plasma-high and FE-high profile of d-serine in combination with clinical factors (age, sex, eGFR, and albuminuria) correctly predicted DN with a sensitivity of 91% (95% CI, 72% to 99%) and a specificity of 79% (95% CI, 63% to 80%), and outperformed the model based on clinical factors alone in the validation dataset (P<0.02). Conclusions: Analysis of d-serine in blood and urinary excretion is useful in identifying DN in patients undergoing kidney biopsy. Profiling of d-serine in patients with kidney diseases supports the suitable treatment through the auxial diagnosis of the origins of kidney diseases.

A Randomized Trial of Magnesium Oxide and Oral Carbon Adsorbent for Coronary Artery Calcification in Predialysis CKD.

BACKGROUND: Developing strategies for managing coronary artery calcification (CAC) in patients with CKD is an important clinical challenge. Experimental studies have demonstrated that magnesium inhibits vascular calcification, whereas the uremic toxin indoxyl sulfate aggravates it. METHODS: To assess the efficacy of magnesium oxide (MgO) and/or the oral carbon adsorbent AST-120 for slowing CAC progression in CKD, we conducted a 2-year, open-label, randomized, controlled trial, enrolling patients with stage 3-4 CKD with risk factors for CAC (diabetes mellitus, history of cardiovascular disease, high LDL cholesterol, or smoking). Using a two-by-two factorial design, we randomly assigned patients to an MgO group or a control group, and to an AST-120 group or a control group. The primary outcome was percentage change in CAC score. RESULTS: We terminated the study prematurely after an interim analysis with the first 125 enrolled patients (of whom 96 completed the study) showed that the median change in CAC score was significantly smaller for MgO versus control (11.3% versus 39.5%). The proportion of patients with an annualized percentage change in CAC score of ≥15% was also significantly lower for MgO compared with control (23.9% versus 62.0%). However, MgO did not suppress the progression of thoracic aorta calcification. The MgO group's dropout rate was higher than that of the control group (27% versus 17%), primarily due to diarrhea. The percentage change in CAC score did not differ significantly between the AST-120 and control groups. CONCLUSIONS: MgO, but not AST-120, appears to be effective in slowing CAC progression. Larger-scale trials are warranted to confirm these findings.

Assessment of renal function in living kidney donors before and after nephrectomy: A Japanese prospective, observational cohort study.

OBJECTIVE: To investigate the utility of estimated glomerular filtration rate for assessing kidney function in living kidney donors before and after nephrectomy. METHODS: A total of 101 donors underwent inulin clearance measurements before and 1 year after nephrectomy. The mean of three inulin clearance values was used as the measured glomerular filtration rate. Estimated glomerular filtration rate based on serum creatinine and cystatin C levels was calculated using the Japanese estimated glomerular filtration rate equation, Chronic Kidney Disease Epidemiology Collaboration formula and new full age spectrum equation. Age-adjusted chronic kidney disease was defined as glomerular filtration rate <75 mL/min/1.73m2 for donors aged <40 years, <60 mL/min/1.73m2 for donors aged 40-65 years and <45 mL/min/1.73m2 for donors aged >65 years. RESULTS: The postoperative measured glomerular filtration rate <60 mL/min/1.73m2 and age-adjusted chronic kidney disease rate were 36.0% and 27.0%, respectively. In younger donors (aged <50 years), postoperative measured glomerular filtration rate <60 mL/min/1.73m2 and age-adjusted chronic kidney disease rates were 5.3% and 26.3%, respectively. In older donors (aged >70 years), postoperative measured glomerular filtration rate <60 mL/min/1.73m2 and age-adjusted chronic kidney disease rates were 75.0% and 33.3%, respectively. Donor age and measured glomerular filtration rate were significant predictors of postoperative measured glomerular filtration rate. The Japanese estimated glomerular filtration rate equation based on creatinine and cystatin C showed the strongest correlation with measured glomerular filtration rate. However, the Japanese estimated glomerular filtration rate equation based on creatinine overestimated the prevalence of measured glomerular filtration rate <60 mL/min/1.73m2 , whereas the Japanese estimated glomerular filtration rate based on cystatin C underestimated it. CONCLUSIONS: Aged donors might have an increased risk of lower glomerular filtration rate after donor nephrectomy; post-surgery, long-term monitoring of renal function is recommended. Measurement of glomerular filtration rate should be carried out for donors, especially pre-surgery. A more precise glomerular filtration rate equation is required in the future.

Existing creatinine-based equations overestimate glomerular filtration rate in Indians.

BACKGROUND: Accurate estimation of glomerular filtration rate (GFR) is important for diagnosis and risk stratification in chronic kidney disease and for selection of living donors. Ethnic differences have required correction factors in the originally developed creatinine-based GFR estimation equations for populations around the world. Existing equations have not been validated in the vegetarian Indian population. We examined the performance of creatinine and cystatin-based GFR estimating equations in Indians. METHODS: GFR was measured by urinary clearance of inulin. Serum creatinine was measured using IDMS-traceable Jaffe's and enzymatic assays, and cystatin C by colloidal gold immunoassay. Dietary protein intake was calculated by measuring urinary nitrogen appearance. Bias, precision and accuracy were calculated for the eGFR equations. RESULTS: A total of 130 participants (63 healthy kidney donors and 67 with CKD) were studied. About 50% were vegetarians, and the remainder ate meat 3.8 times every month. The average creatinine excretion were 14.7 mg/kg/day (95% CI: 13.5 to 15.9 mg/kg/day) and 12.4 mg/kg/day (95% CI: 11.2 to 13.6 mg/kg/day) in males and females, respectively. The average daily protein intake was 46.1 g/day (95% CI: 43.2 to 48.8 g/day). The mean mGFR in the study population was 51.66 ± 31.68 ml/min/1.73m2. All creatinine-based eGFR equations overestimated GFR (p < 0.01 for each creatinine based eGFR equation). However, eGFR by CKD-EPICys was not significantly different from mGFR (p = 0.38). The CKD-EPICys exhibited lowest bias [mean bias: -3.53 ± 14.70 ml/min/1.73m2 (95% CI: -0.608 to -0.98)] and highest accuracy (P30: 74.6%). The GFR in the healthy population was 79.44 ± 20.19 (range: 41.90-134.50) ml/min/1.73m2. CONCLUSION: Existing creatinine-based GFR estimating equations overestimate GFR in Indians. An appropriately powered study is needed to develop either a correction factor or a new equation for accurate assessment of kidney function in the Indian population.

Association between Density of Coronary Artery Calcification and Serum Magnesium Levels among Patients with Chronic Kidney Disease.

BACKGROUND: The Agatston score, commonly used to quantify coronary artery calcification (CAC), is determined by the plaque area and density. Despite an excellent predictability of the Agatston score for cardiovascular events, the density of CAC has never been studied in patients with pre-dialysis chronic kidney disease (CKD). This study aimed to analyze the CAC density and its association with serum mineral levels in CKD. METHODS: We enrolled patients with pre-dialysis CKD who had diabetes mellitus, prior cardiovascular disease history, elevated low-density lipoprotein cholesterol levels, or smoking history. The average CAC density was calculated by dividing the Agatston score by the total area of CAC. RESULTS: The mean estimated glomerular filtration rate (eGFR) of 109 enrolled patients was 35.7 mL/min/1.73 m2. The correlation of the Agatston score with density was much weaker than that with the total area (R2 = 0.19, P < 0.001; and R2 = 0.99, P < 0.001, respectively). Multivariate analyses showed that serum magnesium level was inversely associated with the density, but not with the total area, after adjustment for demographics and clinical factors related to malnutrition-inflammation-atherosclerosis syndrome and mineral and bone disorders including fibroblast growth factor 23 (P = 0.006). This inverse association was pronounced among patients with higher serum phosphate levels (P for interaction = 0.02). CONCLUSION: CAC density was inversely associated with serum magnesium levels, particularly in patients with higher serum phosphate levels.

Utility of Cystatin C for Estimating Glomerular Filtration Rate in Patients With Muscular Dystrophy.

Emerging concerns regarding heart failure, arrhythmia, and sudden death in patients with muscular dystrophy are of significant clinical importance. On the other hand, little attention has been paid to renal dysfunction because these patients have low serum creatinine levels. Serum cystatin C, unaffected by muscle quantity, is a potentially superior marker for estimating renal function. Here, we present cases with muscular dystrophy in which estimated glomerular filtration rate (GFR) by cystatin C (eGFRcys) provided good agreement with simultaneously measured GFR by inulin renal clearance (differences less than 20%). Sudden death with acute heart failure occurred in a patient with underlying renal dysfunction and elevated BNP. Neurologists and cardiologists should evaluate renal function using GFR with cystatin C in patients with muscular dystrophy.

[Clinical Importance of GFR-Estimating Equations (eGFRcreat and eGFRcys)].

Evaluation of the renal function is fundamental for the diagnosis and treatment of kidney diseases. It is also important for adjustment of the doses of drugs that are excreted by the kidney. The incidence of contrast-induced nephropathy is high in subjects with a low GFR. Assessment of the renal function is required prior to contrast medium injections. Renal inulin clearance with continuous venous injection is the gold standard for measuring GFR. However, the method is time-consuming. The recent Japanese CKD guide and KDIGO guidelines for CKD management recommended the use of the estimated GFR based on serum creatinine (eGFRcreat) or serum cystatin C (eGFRcys). Because the serum creatinine level is affected by the muscle mass, eGFRcreat is under- or overestimated in subjects with a high or low muscle mass, respectively. The serum cystatin C concentration is less influenced by the muscle mass. Assessment of the renal function by eGFRcys may be useful in subjects with a low or high muscle mass. Recently, it was reported that the association between eGFRcys and the risk of all-cause mortality was much closer compared with eGFRcreat. eGFRcys may be useful for detecting a high risk of complications in a general population and in subjects with CKD.

Effects of serum albumin and glycated albumin levels on performance of the Japanese GFR equation based on serum cystatin C.

BACKGROUND: We reported that both serum albumin (Alb) and glycated albumin (GA) levels influenced the performance of the Japanese GFR equation based on serum creatinine. In the present study, we studied the effects of both markers on the estimated GFR by Japanese GFR equation based on serum cystatin C (Eq-cys). METHODS: 715 Japanese subjects were included. GFR was measured by inulin renal clearance (Cin). Correlations between estimated GFR by Eq-cys (eGFRcys) and Cin were evaluated in subjects stratified by GA (GA-1: GA ≤ 16.3 % and GA-2: GA ≥ 16.4 %) and Alb levels (Alb-1: Alb ≤ 3.5 g/dl and Alb-2: Alb ≥ 3.6 g/dl). RESULTS: Correlation coefficients between eGFRcys and Cin were 0.863, 0.919, 0.948 and 0.974 in GA-1, GA-2, Alb-1 and Alb-2, respectively. Slopes (95 % confidential interval) of the regression lines with zero intercepts were 1.014 (0.993-1.035), 0.989 (0.944-1.033), 1.019 (0.970-1.068) and 1.011 (0.990-1.031), respectively. The slopes were not significantly different from 1.0, suggesting that Eq-cys performed well in subjects at wide spectrum of GA and Alb levels. Single regression analysis of GA on eGFRcys/GFR was not significant in total subjects and subjects with GA > 12.4 %. Single regression analysis of Alb on eGFRcys/GFR was significant in total subjects, but the correlation coefficient was very low (r = 0.08, p = 0.03). Multiple regression analysis showed that Alb and GA were not significantly associated with eGFRcys/Cin in subjects with GA > 12.4 %. These results suggested that both parameters were not important factors affecting the performance of Eq-cys. CONCLUSION: Performance of Eq-cys was well irrespective of GA and Alb levels. Both parameters were not important factors affecting the estimated GFR by Eq-cys.

Serum albumin, but not glycated albumin was a potent factor affecting the performance of GFR equation based on serum creatinine.

BACKGROUND: Recently, Tsuda et al. reported that high HbA1C or high glycated albumin (GA) level is a major factor in overestimation of GFR by Japanese GFR equation based on serum creatinine (Eq-cr). They developed a modified equation of Eq-cr (M-Eq-cr) using GA or HbA1c. Therefore, effect of GA levels on the estimated GFR (eGFR) by Eq-cr was evaluated in Japanese subjects. We validated the accuracy of the modified equation using GA by Tsuda et al. (M-Eq-cr) and new equations that we developed in the present study. METHODS: Seven hundred and fifteen Japanese subjects were included. GFR was measured by inulin renal clearance (Cin). The subjects were divided into two groups by upper limit of the GA reference range (GA-1: GA < 16.3 % and GA-2: GA > 16.4 %). Factors affecting the ratio of eGFR to Cin (eGFR/Cin) were evaluated using multivariate analysis. New equations based on creatinine and albumin (Eq-cr-alb) and based on creatinine, albumin and GA were developed from development dataset (382 subjects). Performances of the equations were validated in validation dataset (333 subjects). RESULTS: Correlation coefficients between eGFR by Eq-cr and Cin were 0.839 and 0.914 in GA-1 and GA-2, respectively. Slopes (95 % confidential interval) of the regression lines with zero intercepts were 1.013 (0.991 to 1.036) and 0.997 (0.951 to 1.043), respectively. Both slopes were not significantly different from 1.0. Biases were -2.3 ± 19.0 and 0.2 ± 11.7 ml/min/1.73 m(2), respectively. Accuracy (p30; percentage of subjects within 30 % of Cin) (95 % CI) were 78 % (75, 81) and 71 % (62, 78), respectively. There was no significant difference in bias and accuracy between the two groups, indicating a reasonable accuracy of Eq-cr in GA-1 and GA-2. Multiple regression analysis showed that lower serum albumin and higher GA were associated with higher eGFR/Cin. Albumin was a more potent factor affecting eGFR/Cin than GA. M-Eq-cr significantly underestimated GFR and had significantly larger bias compared with Eq-cr in subjects with GA > 20 %, suggesting that the modification of Eq-cr using GA by Tsuda et al. was too much compensation in our subjects. Precisions of Eq-cr-alb were significantly better compared with Eq-cr. CONCLUSION: Eq-cr has a reasonable accuracy in GA-1 and GA-2. Lower serum albumin and higher GA were significantly associated with higher eGFR/Cin. The former was a more potent factor affecting eGFR/Cin. Eq-cr-alb showed better performance compared with Eq-cr. M-Eq-cr using GA showed too much compensation and did not improve the accuracy of the equation in our subjects.

Generation of a new cystatin C-based estimating equation for glomerular filtration rate by use of 7 assays standardized to the international calibrator.

BACKGROUND: Many different cystatin C-based equations exist for estimating glomerular filtration rate. Major reasons for this are the previous lack of an international cystatin C calibrator and the nonequivalence of results from different cystatin C assays. METHODS: Use of the recently introduced certified reference material, ERM-DA471/IFCC, and further work to achieve high agreement and equivalence of 7 commercially available cystatin C assays allowed a substantial decrease of the CV of the assays, as defined by their performance in an external quality assessment for clinical laboratory investigations. By use of 2 of these assays and a population of 4690 subjects, with large subpopulations of children and Asian and Caucasian adults, with their GFR determined by either renal or plasma inulin clearance or plasma iohexol clearance, we attempted to produce a virtually assay-independent simple cystatin C-based equation for estimation of GFR. RESULTS: We developed a simple cystatin C-based equation for estimation of GFR comprising only 2 variables, cystatin C concentration and age. No terms for race and sex are required for optimal diagnostic performance. The equation, [Formula: see text] is also biologically oriented, with 1 term for the theoretical renal clearance of small molecules and 1 constant for extrarenal clearance of cystatin C. CONCLUSIONS: A virtually assay-independent simple cystatin C-based and biologically oriented equation for estimation of GFR, without terms for sex and race, was produced.

[New topics regarding equations for GFR estimation based on serum creatinine and cystatin C].

Japanese GFR equations and CKD-EPI equations based on standardized serum creatinine and standardized cystatin C are recommended in recent Japanese CKD guides and KDIGO guidelines for CKD management, respectively. CKD-EPIcreat overestimates GFR in Japanese subjects, probably due to the difference in muscle mass between Japanese and Caucasians. Unlike CKD-EPIcreat, CKD-EPIcys performs well in Japanese subjects, indicating the advantages of using cystatin C as a GFR marker. KDIGO guidelines suggest measuring eGFRcys in adults with eGFRcreat of 45-59 ml/min/1.73 m2 who do not have markers of kidney damage if confirmation of CKD is required. Creatinine is excreted by glomerular filtration, but also secreted by the tubules. Alteration of the tubular secretion of creatinine may influence the performance of GFR equations based on serum creatinine. Multivariate analysis showed that GFR and serum albumin levels were independent parameters affecting the fractional excretion of creatinine (FE-Cr). Alteration of FE-Cr according to the serum albumin levels may be one of the reasons for the bias of GFR equations based on serum creatinine. Low GFR is a risk factor for all-cause and cardiovascular mortality in a general population. However, the relationship between eGFR and the hazard risk of events is different depending on whether cystatin C or creatinine is used to calculate eGFR. The association between eGFRcys and the hazard risk is much stronger compared with eGFRcreat. Cystatin C may be a useful alternative to creatinine for detecting a high risk of complications in a general population and subjects with CKD.

Lower serum albumin level is associated with higher fractional excretion of creatinine.

BACKGROUND: Creatinine clearance (Ccr) overestimates glomerular filtration rate (GFR) due to the tubular secretion of creatinine. It is known that fractional excretion of creatinine (FE-Cr) increases with decreasing GFR. Association of serum albumin level with the tubular secretion of creatinine was also reported previously. Alteration of FE-Cr may affect the performance of GFR estimating equations based on serum creatinine. Therefore, we analyzed the factors influencing FE-Cr and compared the performance of GFR equations in subjects stratified by serum albumin levels. METHODS: Seven hundred and fifty-seven Japanese subjects were included. GFR was measured by inulin renal clearance. GFR and Ccr were measured simultaneously. FE-Cr was calculated as the ratio of Ccr to GFR. Multivariate analysis was performed to evaluate the factors influencing FE-Cr. Age, gender, GFR, body mass index (BMI), body weight, height and serum albumin level were analyzed as the parameters. Estimated GFR was calculated by Japanese GFR equations based on serum creatinine (Eq-cr), serum cystatin C (Eq-cys) and 5 variables including serum albumin (Eq-5var). RESULTS: FE-Cr in subjects with serum albumin <3.0, 3.0-3.9 and ≥4.0 g/dl were 1.63 ± 0.48, 1.53 ± 0.55, and 1.40 ± 0.36, respectively. FE-Cr in subjects with serum albumin <3.0 or 3.0-3.9 g/dl were significantly higher than the value in subjects with serum albumin ≥4.0 g/dl. Multivariate analysis showed that GFR (p < 0.0001) and serum albumin level (p = 0.004) were independent parameters affecting FE-Cr. Biases of Eq-cr, Eq-cys and Eq-5var in subjects with serum albumin <3.0 g/dl were -9.5 ± 17.5, -0.7 ± 17.1 and -0.6 ± 14.8 ml/min/1.73 m(2), respectively. Eq-cr significantly overestimated GFR compared with Eq-cys or Eq-5var. Biases in subjects with serum albumin ≥4.0 g/dl were 6.4 ± 18.8, 2.0 ± 18.1 and 3.0 ± 18.3 ml/min/1.73 m(2), respectively. Eq-cr significantly underestimated GFR compared with Eq-cys or Eq-5var. CONCLUSION: GFR and serum albumin level were independent parameters affecting FE-Cr. Alteration of FE-Cr according to the serum albumin levels may be one of the reasons of the bias of GFR equation based on serum creatinine.

[Development of evaluation of kidney function and classification of chronic kidney disease (CKD)--including CKD clinical practice guide 2012].

Chronic kidney disease (CKD) Clinical Practice Guide 2012 for Japanese (CKD guide 2012) was released to update CKD guide 2009. Classification of CKD was altered according to the classification of KDIGO 2012 CKD Clinical Practice Guideline, which was based on cause, glomerular filtration rate (GFR) categories, and albuminuria categories. Because evaluation of albuminuria is not common for most CKD subjects in Japan, proteinuria categories comparable to the albuminuria categories were added to CKD Guide 2012. A GFR equation based on serum creatinine is recommended and has been used for evaluation of renal function. In CKD Guide 2012, a GFR equation based on serum cystatin C was also included. Serum cystatin C is a new GFR marker. Recently, measurement of cystatin C was standardized using international certified reference material ERM-DA471/IFCC. A new GFR equation based on standardized serum cystatin C was developed for Japanese. Estimated GFR based on serum creatinine (eGFRcreat) is influenced by not only renal function but also muscle mass. It might be overestimated in subjects with low muscle mass, such as muscle wasting diseases, and underestimated in those with high muscle mass, such as athletes. Estimated GFR based on serum cystatin C (eGFRcys) is little-influenced by muscle mass. If eGFRcreat is less accurate, additional evaluation of eGFRcys is useful. Generally, the average value of eGFRcreat and eGFRcys(eGFRaverage) is most accurate.

GFR estimation using standardized serum cystatin C in Japan.

BACKGROUND: Recently, the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) developed glomerular filtration rate (GFR)-estimating equations based on standardized serum cystatin C (CKD-EPI(cys)) and standardized serum creatinine plus standardized serum cystatin C (CKD-EPI(cr-cys)). We developed new GFR-estimating equations based on standardized cystatin C for a Japanese population and compared their accuracy with the CKD-EPI equations. STUDY DESIGN: Accuracy of diagnostic test study. SETTING & PARTICIPANTS: 413 (development data set) and 350 individuals (validation data set). INDEX TEST: CKD-EPI(cys); CKD-EPI(cr-cys); modifications to CKD-EPI(cys) and CKD-EPI(cr-cys) using Japanese coefficients; and newly developed Japanese eGFR equations based on standardized serum cystatin C (Eq(cys)), cystatin C with a nonrenal factor reflecting hypothesized extrarenal elimination (Eq(cys+nonrenal)), and creatinine in combination with cystatin C (Eq(cr-cys)). Standardized cystatin C values were determined by a colloidal gold immunoassay traceable to the international certified reference material ERM-DA471/IFCC. REFERENCE TEST: Measured GFR by inulin renal clearance. RESULTS: In a development data set, we calculated Japanese coefficients for CKD-EPI(cys) and CKD-EPI(cr-cys) of 0.977 (95% CI, 0.853-1.002) and 0.908 (95% CI, 0.889-0.928), respectively. In a validation data set, we compared CKD-EPI(cys), Eq(cys), and Eq(cys+nonrenal) with each other. Bias and accuracy were not significantly different among the 3 equations. The precision of CKD-EPI(cys) was significantly better than for Eq(cys) (P = 0.007) and not significantly different from Eq(cys+nonrenal) (P = 0.6). We then compared 0.908 × CKD-EPI(cr-cys), Eq(cr-cys), and Eq(average) (the average value of Eq(cr) [previous Japanese equation based on standardized serum creatinine] and Eq(cys+nonrenal)) with each other in the validation data set. Bias and accuracy were not significantly different among the 3 equations. The precision of 0.908 × CKD-EPI(cr-cys) was significantly better than for Eq(cr-cys) (P = 0.004) and not significantly different from Eq(average) (P = 0.06). LIMITATIONS: Limited number of participants with measured GFR >90 mL/min/1.73 m(2). Extrarenal elimination of cystatin C was not measured. CONCLUSIONS: CKD-EPI(cys) performed well in Japanese individuals, suggesting that equations based on serum cystatin C could be used in patients with different races without modification. Accounting for extrarenal elimination of cystatin C may improve the performance of estimating equations.

Performance of GFR equations in Japanese subjects.

BACKGROUND: Japanese GFR equations based on serum creatinine (Scr) (Eq(cr)), serum cystatin C (Scys) (Eq(cys)) and average value of Eq(cr) and Eq(cys) (Eq(average)), and coefficient-modified CKD-EPI equations based on Scr (CKD-EPI(cr)), Scys (CKD-EPI(cys)) and Scys in combination with Scr (CKD-EPI(cr-cys)) are now available for Japanese subjects. Performance of these equations has not been well evaluated in subjects stratified by GFR. Therefore, the bias, precision and accuracy of the GFR equations were compared in Japanese subjects stratified by measured GFR. METHODS: Three hundred fifty Japanese subjects were included for validation. These subjects were stratified by measured GFR (0-29, 30-59, 60-89, 90-119 ml/min/1.73 m(2) and total). Japanese equations (Eq(cr), Eq(cys) and Eq(average)) were compared with coefficient-modified CKD-EPI equations (0.813 × CKD-EPI(cr), CKD-EPI(cys) and 0.908 × CKD-EPI(cr-cys)), respectively. GFR was measured by inulin renal clearance. Standardized Scr was measured by enzymatic method. Standardized Scys was measured by colloidal gold immunoassay. RESULTS: Bias and accuracy were not significantly different between Japanese GFR equations and coefficient-modified CKD-EPI equations in all mGFR ranges. The precision of Eq(cr) was significantly better in GFR 0-29 ml/min/1.73 m(2) and significantly worse in GFR 60-89 and GFR 90-119 ml/min/1.73 m(2) compared with 0.813 × CKD-EPI(cr). The precision of Eq(cys) was significantly better in GFR 30-59 and GFR 60-89 ml/min/1.73 m(2) compared with CKD-EPI(cys). The precision of Eq(average) was significantly better in GFR 30-59 ml/min/1.73 m(2) and significantly worse in GFR 90-119 ml/min/1.73 m(2) compared with 0.908 × CKD-EPI(cr-cys). CONCLUSION: Japanese GFR equations performed well in subjects with GFR under 60 ml/min/1.73 m(2) compared with the coefficient-modified CKD-EPI equations.

Performance of serum cystatin C versus serum creatinine as a marker of glomerular filtration rate as measured by inulin renal clearance.

INTRODUCTION: Serum cystatin C was recently proposed as an alternative marker of glomerular filtration rate (GFR), with a suggested better performance than creatinine. However, detailed studies are limited. We evaluated the performance of cystatin C as a GFR marker. METHODS: GFR was measured by inulin clearance in 763 Japanese subjects. Factors other than GFR influencing serum cystatin C or serum creatinine were analyzed by multivariate analyses. RESULTS: After adjustment for GFR, the value of serum creatinine was 25.2% lower in females than males, and decreased by 5.2% for every 20 years of age. Serum cystatin C was 8.2% lower in females, and did not change significantly with aging. Creatinine but not cystatin C was significantly affected by body weight, height and body mass index after adjustment for GFR, gender and age. The correlation coefficient between GFR and 1/cystatin C was significantly higher than that of 1/creatinine in total subjects (0.866 and 0.810, respectively, p < 0.001). Unlike serum creatinine, serum cystatin C did not increase in association with the reduction of GFR in subjects with very low GFR. The regression line of 1/cystatin C against GFR showed a significantly negative intercept of about -8 ml/min/1.73 m(2). CONCLUSION: The performance of serum cystatin C was not good in the subjects with very low GFR. Non-renal elimination of cystatin C may contribute to the result. The correlation between reciprocal cystatin C and GFR suggested its superiority in predicting GFR compared to creatinine in subjects with normal and mildly reduced GFR.

Allogenic fetal membrane-derived mesenchymal stem cells contribute to renal repair in experimental glomerulonephritis.

Mesenchymal stem cells (MSC) have been reported to be an attractive therapeutic cell source for the treatment of renal diseases. Recently, we reported that transplantation of allogenic fetal membrane-derived MSC (FM-MSC), which are available noninvasively in large amounts, had a therapeutic effect on a hindlimb ischemia model (Ishikane S, Ohnishi S, Yamahara K, Sada M, Harada K, Mishima K, Iwasaki K, Fujiwara M, Kitamura S, Nagaya N, Ikeda T. Stem Cells 26: 2625-2633, 2008). Here, we investigated whether allogenic FM-MSC administration could ameliorate renal injury in experimental glomerulonephritis. Lewis rats with anti-Thy1 nephritis intravenously received FM-MSC obtained from major histocompatibility complex-mismatched ACI rats (FM-MSC group) or a PBS (PBS group). Nephritic rats exhibited an increased urinary protein excretion in the PBS group, whereas the FM-MSC group rats had a significantly lower level of increase (P < 0.05 vs. PBS group). FM-MSC transplantation significantly reduced activated mesangial cell (MC) proliferation, glomerular monocyte/macrophage infiltration, mesangial matrix accumulation, as well as the glomerular expression of inflammatory or extracellular matrix-related genes including TNF-α, monocyte chemoattractant protein 1 (MCP-1), type I collagen, TGF-ß, type 1 plasminogen activator inhibitor (PAI-1) (P < 0.05 vs. PBS group). In vitro, FM-MSC-derived conditioned medium significantly attenuated the expression of TNF-α and MCP-1 in rat MC through a prostaglandin E(2)-dependent mechanism. These data suggest that transplanted FM-MSC contributed to the healing process in injured kidney tissue by producing paracrine factors. Our results indicate that allogenic FM-MSC transplantation is a potent therapeutic strategy for the treatment of acute glomerulonephritis.

Comparison of a simple and a standard method for inulin renal clearance.

BACKGROUND: The standard method of renal inulin clearance consists of three sets of 30-min clearances. We previously proposed a simple method with a single urine collection for 1 h and two blood samples. In this study, we compared the two methods. METHODS: The study involved 112 individuals. Three sets of 30-min urine sample collections were started 45 min after inulin infusion , and serum concentrations were measured at the midpoint (60, 90, 120 min) of each clearance period. The mean of the three (Cin-ST) or average of the first two (Cin-ST2) clearances was used for the standard method. Clearance calculated by the simple method (Cin-S) combined the first two collections and the mean of serum concentrations at the beginning (45 min) and end (105 min) of the clearance period. Clearance was also calculated by estimated area under the plasma concentration curve from 45 to 105 min (Cin-A) as a more reliable value. RESULTS: Cin-S correlated highly with Cin-ST (r = 0.992). Bland-Altman plot indicated that Cin-S was lower than Cin-ST at the same rate in all glomerular filtration rate (GFR) ranges. Total Cin-S of all patients was significantly lower (5.9%, 4.8%, and 3.6%) than Cin-ST, Cin-ST2, and Cin-A, respectively. Cin-ST2 was 1.3% higher than Cin-A. The change in serum inulin concentration by time from 45 to 105 min was not linear but concave. This led to the underestimation of clearance by the simple method. CONCLUSION: The simple method of renal inulin clearance gives slightly lower results than the standard method. The difference was small, indicating the simple method is accurate enough for use in clinical practice.