D-Asparagine is an Ideal Endogenous Molecule for Measuring the Glomerular Filtration Rate.

Introduction: An ideal endogenous molecule for measuring glomerular filtration rate (GFR) is still unknown. However, a rare enantiomer of serine, d-serine, is useful in GFR measurement. This study explored the potential of other d-amino acids for kidney function assessment. Methods: This was a cross-sectional observational study of 207 living kidney transplant donors and recipients, for whom GFR was measured using clearance of inulin (C-in). Associations between levels of d-amino acids and GFR were analyzed using multivariate factor analysis. Fractional excretion (FE), a ratio of the clearance of a substance to C-in as a standard molecule, was calculated to monitor the excretion ratio after glomerular filtration. Dissociation from an ideal FE of 100% was assessed as a bias. Proportional bias against C-in was calculated using Deming regression. Results: Multivariate analysis identified the blood level of d-asparagine to reflect GFR. Means of blood d-asparagine and clearance of d-asparagine (C-d-Asn) were 0.21 µM and 65.0 ml/min per 1.73 m2, respectively. Inulin-based FE (FEin) of d-asparagine was 98.67% (95% confidence interval [CI]: 96.43-100.90%) and less biased than those of known GFR markers, such as FEin of creatinine (147.93 [145.39-150.46]; P < 0.001) and d-serine (84.84 [83.22-86.46]; P < 0.001). A proportional bias of C-d-Asn to C-in was -7.8% (95% CI, -14.5 to -0.6%), which was minor compared to those of clearance of creatinine (-34.5% [-37.9 to -31.0%]) and d-serine (21.2% [13.9-28.9]). Conclusion: D-Asparagine acts similar to inulin in the kidney. Therefore, d-asparagine is an ideal endogenous molecule that can be used for GFR measurement.

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.

Dynamics of D-serine reflected the recovery course of a patient with rapidly progressive glomerulonephritis.

We experienced a case of a 36-year-old female with rapidly progressive glomerulonephritis (RPGN) due to anti-neutrophil cytoplasmic antibody (ANCA)-associated nephritis and systemic lupus erythematosus (SLE) nephritis. Chiral amino acid metabolomics revealed a prominent profile of D-serine in this patient. At the fulminant period of RPGN, the level of plasma D-serine, a potential biomarker in CKD that reflects actual glomerular filtration ratio (GFR), was extremely high. On the other hand, urinary fractional excretion (FE) of D-serine, which was usually much higher than that of L-isoform, was 0% in this patient. These abnormal D-serine profiles normalized in response to the intensive treatment. Normalizations of blood D-serine levels were in parallel with those of blood creatinine levels and potentially reflect the recovery of GFR. FE of D-serine increased transiently before the normalization of D-serine profile, suggesting that kidney promotes urinary excretion of D-serine for the normalization of plasma D-serine level. These unexplored clinical features of D-serine well reflected the clinical course of this patient. Blood D-serine level can also serve as a biomarker in acute kidney injury (AKI) or RPGN, and, in combination with FE of D-serine, may render the clinical practitioners to judge the efficacy of intensive treatments.

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.

D-Serine reflects kidney function and diseases.

D-Amino acids, long-term undetected enantiomers of L-amino acids, are now emerging as potential biomarkers, especially for kidney diseases. Management of chronic kidney disease (CKD), a global problem with its high prevalence and poor prognosis, is currently unsatisfactory due to the difficulty in estimating kidney function and in early detection of diseases. We now show that intra-body dynamics of D-serine reflect kidney function and diseases. The blood level of D-serine correlated well with the actual glomerular filtration ratio, a key kidney function. This correlation was compatible with those of conventional kidney markers, and blood level of D-serine was relatively unaffected by such clinical factors as body size. The balance between excretion and reabsorption of amino acids by the kidney was controlled with chiral selectivity, and the reabsorption of D-serine was sensitive to the presence of CKD. The combination of blood level and urinary dynamics of D-serine effectively distinguished CKD from non-CKD. These lines of evidence provide new insights into the enantioselective amino acid dynamics in the human body that reflect disease pathophysiology. D-Serine may serve as a vital biomarker that suppress CKD onset through the precise assessment of kidney function and the diagnosis of CKD.

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.

Proteinuria-associated renal magnesium wasting leads to hypomagnesemia: a common electrolyte abnormality in chronic kidney disease.

BACKGROUND: Hypomagnesemia (Hypo-Mg) predicts mortality and chronic kidney disease (CKD) progression. However, in CKD, its prevalence, kidney-intrinsic risk factors, and the effectiveness of oral magnesium (Mg) therapy on serum Mg levels is uncertain. METHODS: In a cross-sectional study enrolling pre-dialysis outpatients with CKD, the prevalence of electrolyte abnormalities (Mg, sodium, potassium, calcium and phosphorus) was compared. In an open-label randomized controlled trial (RCT), we randomly assigned CKD patients to either the magnesium oxide (MgO) or control arm. The outcome was serum Mg levels at 1 year. RESULTS: In 5126 patients, Hypo-Mg was the most common electrolyte abnormality (14.7%) with similar prevalence across stages of CKD. Positive proteinuria was a risk factor of Hypo-Mg (odds ratio 2.2; 95% confidence interval 1.2-4.0). However, stratifying the analyses by diabetes mellitus (DM), it was not significant in DM (Pinteraction = 0.04). We enrolled 114 patients in the RCT. Baseline analyses showed that higher proteinuria was associated with higher fractional excretion of Mg. This relationship between proteinuria and renal Mg wasting was mediated by urinary tubular markers in mediation analyses. In the MgO arm, higher proteinuria or tubular markers predicted a significantly lower 1-year increase in serum Mg. In patients with a urinary protein-to-creatinine ratio (uPCR) <0.3 g/gCre, serum Mg at 1 year was 2.4 and 2.0 mg/dL in the MgO and control arms, respectively (P < 0.001), with no significant between-group difference in patients whose uPCR was ≥0.3 g/gCre (Pinteraction=0.001). CONCLUSIONS: Proteinuria leads to renal Mg wasting through tubular injuries, which explains the high prevalence of Hypo-Mg in CKD.

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.

Is the new GFR equation using inulin clearance a more accurate method for Asian patients?

Recently, a new glomerular filtration rate (GFR) equation for the Japanese population was proposed using measured inulin clearance. To expand its applicability to other Asian populations, we performed a comparative study in the Korean population. Inulin clearance was measured in 166 patients from seven participating medical centers in Korea. Patient's sera and urine were collected, and baseline clinical characteristics were measured to provide an estimated GFR (eGFR) by the Japanese GFR equation using inulin clearance (Japanese-GFR equation), the Modification of Diet in Renal Disease (MDRD) study equation, and the Chronic Kidney Disease - Epidemiology Collaboration (CKD-EPI) equation. We compared the results to determine which equation best estimated the measured GFR (mGFR). Accuracy (95% CI) within 30% of mGFR by the Japanese-GFR equation, the CKD-EPI equation and the MDRD study equation were 66 (58 - 72), 51 (43 - 58), and 55 (47 - 62)%, respectively. Bias (mGFR minus eGFR) were 3.4 ± 22.4, -12.0 ± 22.1, and -9.7 ± 23.8 mL/min/1.73 m2, respectively. The accuracy of the Japanese-GFR equation was significantly better than MDRD study equation in subjects with mGFR < 60 mL/min/1.73 m2 and in total subjects. The bias of the Japanese-GFR equation was significantly smaller compared with other two equations in total subjects. The Japanese-GFR equation has a higher accuracy with less bias than the other equations in estimating GFR in Korean populations. Further studies are required to determine if the current Japanese-GFR equation could represent the standard eGFR for other Asian populations.

[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.

Longitudinal Study of the Decline in Renal Function in Healthy Subjects.

BACKGROUND: Chronic kidney disease is an important concern in preventive medicine, but the rate of decline in renal function in healthy population is not well defined. The purpose of this study was to determine reference values for the estimated glomerular filtration rate (eGFR) and rate of decline of eGFR in healthy subjects and to evaluate factors associated with this decline using a large cohort in Japan. METHODS: Retrospective cross-sectional and longitudinal studies were performed with healthy subjects aged ≥18 years old who received a medical checkup. Reference values for eGFR were obtained using a nonparametric method and those for decline of eGFR were calculated by mixed model analysis. Relationships of eGFR decline rate with baseline variables were examined using a linear least-squares method. RESULTS: In the cross-sectional study, reference values for eGFR were obtained by gender and age in 72,521 healthy subjects. The mean (±SD) eGFR was 83.7±14.7 ml/min/1.73 m2. In the longitudinal study, reference values for eGFR decline rate were obtained by gender, age, and renal stage in 45,586 healthy subjects. In the same renal stage, there was little difference in the rate of decline regardless of age. The decline in eGFR depended on the renal stage and was strongly related to baseline eGFR, with a faster decline with a higher baseline eGFR and a slower decline with a lower baseline eGFR. The mean (±SD) eGFR decline rate was ‒1.07±0.42 ml/min/1.73 m2/year (‒1.29±0.41%/year) in subjects with a mean eGFR of 81.5±11.6 ml/min/1.73 m2. CONCLUSIONS: The present study clarified for the first time the reference values for the rate of eGFR decline stratified by gender, age, and renal stage in healthy subjects. The rate of eGFR decline depended mainly on baseline eGFR, but not on age, with a slower decline with a lower baseline eGFR.

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.