Validation of a new standardized cystatin C turbidimetric assay: evaluation of the three novel CKD-EPI equations in hypertensive patients

Optimal assessment of the glomerular filtration rate in older chinese patients using the equations of the Berlin Initiative Study

AIM

To evaluate the performances of the various estimated glomerular filtration rate (eGFR) equations of the Chronic Kidney Disease Epidemiology Collaboration, the Berlin Initiative Study (BIS), and the Full Age Spectrum (FAS) in older Chinese.

METHODS

This study enrolled Chinese adults aged ≥ 65 years who underwent GFR measurements (via 99Tcm-DTPA renal dynamic imaging) in our hospital from 2011 to 2022. Using the measured glomerular filtration rate (mGFR) as the reference, we derived the bias, precision, accuracy, and consistency of each equation.

RESULTS

We enrolled 519 participants, comprising 155 with mGFR ≥ 60 mL/min/1.73 m2 and 364 with mGFR < 60 mL/min/1.73 m2. In the total patients, the BIS equation based on creatinine and cystatin C (BIScr-cys) exhibited the lowest bias [median (95% confidence interval): 1.61 (0.77-2.18)], highest precision [interquartile range 11.82 (10.32-13.70)], highest accuracy (P30: 81.12%), and best consistency (95% limit of agreement: 101.5 mL/min/1.73 m2). In the mGFR ≥ 60 mL/min/1.73 m2 subgroup, the BIScr-cys and FAS equation based on creatinine and cystatin C (FAScr-cys) performed better than the other equations; in the mGFR < 60 mL/min/1.73 m2 subgroup, all equations exhibited relatively large deviations from the mGFR. Of all eight equations, the BIScr-cys performed the best.

CONCLUSIONS

Although no equation was fully accurate in the mGFR < 60 mL/min/1.73 m2 subgroup, the BIScr-cys (of the eight equations) assessed the eGFRs of the entire population best. A new equation is urgently required for older Chinese and even East Asians, especially those with moderate-to-severe renal insufficiency.

Performance and Determinants of Serum Creatinine and Cystatin C-Based GFR Estimating Equations in South Asians

INTRODUCTION

The creatinine-based Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) estimated glomerular filtration rate (eGFR) equation was calibrated for the general Pakistan population (eGFRcr-PK) to eliminate bias and improve accuracy. Cystatin C-based CKD-EPI equations (eGFRcys and eGFRcr-cys) have not been assessed in this population, and non-GFR determinants of cystatin C are unknown.

METHODS

We assessed eGFRcys, eGFRcr-cys, and non-GFR determinants of cystatin C in a cross-sectional study of 557 participants (≥40 years of age) from Pakistan. We compared bias (median difference in measured GFR [mGFR] and eGFR), precision (interquartile range [IQR] of differences), accuracy (percentage of eGFR within 30% of mGFR), root mean square error (RMSE), and classification of mGFR <60 ml/min/1.73 m2 (area under the receiver operating characteristic curve [AUC] and net reclassification index [NRI]) among eGFR equations.

RESULTS

We found that eGFRcys underestimated mGFR (bias, 12.7 ml/min/1.73 m2 [95% confidence interval {CI} 10.7-15.2]). eGFRcr-cys did not improve performance over eGFRcr-PK in precision (P = 0.52), accuracy (P = 0.58), or RMSE (P = 0.49). Results were consistent among subgroups by age, sex, smoking, body mass index (BMI), and eGFR. NRI was 7.31% (95% CI 1.52%-13.1%; P < 0.001) for eGFRcr-cys versus eGFRcr-PK, but AUC was not improved (0.92 [95% CI 0.87-0.96] vs. 0.90 [95% CI 0.86-0.95]; P = 0.056). Non-GFR determinants of higher cystatin C included male sex, smoking, higher BMI and total body fat, and lower lean body mass.

CONCLUSION

eGFRcys underestimated mGFR in South Asians and eGFRcr-cys did not offer substantial advantage compared with eGFRcr-PK. Future studies are warranted to better understand the large bias in eGFRcys and non-GFR determinants of cystatin C in South Asians.

Neither creatinine- nor cystatin C-estimated glomerular filtration rate is optimal in oncology patients treated with targeted agents

Background

In the last decade, there has been an increase in the use of anti-angiogenic drugs as treatment for metastatic malignancies. However, use of these targeted therapies could induce both glomerular and tubular damage. Also during targeted therapy, the lysosomal protease cathepsin D is released from the tumour, which is inhibited by the protease inhibitor cystatin C. The aim of this study is to determine if use of cystatin C-estimated glomerular filtration rate (eGFR) is applicable to a patient cohort treated with targeted agents.

Methods

A cohort of 80 patients with various malignancies were continuously recruited and prospectively analysed. Serum and urinary biochemical analytes for renal toxicities were assessed at different time points during treatment. The association between serum cystatin C and cathepsin D was also determined.

Results

A decrease in serum cystatin C concentrations (1.03 versus 0.90 mg/L; P < 0.001), together with an increase in cystatin C-eGFR (71 versus 89 mL/min/1.73 m2; P = 0.002) was observed during therapy, compared with baseline. This decrease in cystatin C concentrations was correlated with cathepsin D (r = 0.307; P < 0.001), which was released from the tumour during targeted therapy. Further analysis demonstrated cathepsin D-mediated proteolysis of cystatin C in serum.

Conclusions

Cystatin C concentrations were decreased during targeted therapy due to cathepsin D-mediated proteolysis. Cystatin C-eGFR is therefore not considered a suitable marker for assessing kidney function in oncology patients, and other techniques to estimate the GFR have to be applied in this patient population.

Clinical benefits of using inulin clearance and cystatin C for determining glomerular filtration rate in HIV-1-infected individuals treated with dolutegravir

INTRODUCTION

Dolutegravir may inhibit creatinine transporters in renal tubules and elevate serum creatinine levels. We investigated the usefulness of glomerular filtration rate (GFR) measured using inulin clearance (Cin), creatinine clearance (Ccr), and estimated GFR based on both serum creatinine (eGFRcre) and serum cystatin C (eGFRcys).

PATIENTS & METHODS

HIV-1-infected Japanese patients with suppressed viremia and whose antiretroviral drug was switched to dolutegravir from other drugs were included (n = 108, Study 1). We compared eGFRcre and eGFRcys at the start and after 48 weeks of dolutegravir administration. For the patients providing consent, we measured Cin and Ccr (n = 15, Study 2). We assessed biases and accuracy and compared Cin with eGFRcre, eGFRcys, and Ccr.

RESULTS

There were no differences in serum cystatin C and eGFRcys between baseline and at 48 weeks. Moreover, eGFRcre was significantly less accurate (within 30% of measured GFR) than both eGFRcys and Ccr (40% accuracy compared to 93% and 93%, respectively). eGFRcys was significantly less biased than eGFRcre and Ccr (p < 0.0001, p = 0.00036, respectively). No significant difference between Cin and eGFRcys was observed. eGFRcys was significantly correlated with Cin (γ = 0.85, p < 0.0001).

CONCLUSIONS

eGFRcys provided the most precise estimate and most closely approximate Cin in HIV-1-infected Japanese patients with suppressed viremia treated with dolutegravir. We demonstrated clinical benefits of inulin clearance and eGFRcys. This is the first study performing inulin clearance for HIV-1-infected individuals and to show data for eGFRcys from a large cohort following a switch to dolutegravir from other antiretroviral agents.

Systematic Review and Metaanalysis Comparing the Bias and Accuracy of the Modification of Diet in Renal Disease and Chronic Kidney Disease Epidemiology Collaboration Equations in Community-Based Populations

BACKGROUND

The majority of patients with chronic kidney disease are diagnosed and monitored in primary care. Glomerular filtration rate (GFR) is a key marker of renal function, but direct measurement is invasive; in routine practice, equations are used for estimated GFR (eGFR) from serum creatinine. We systematically assessed bias and accuracy of commonly used eGFR equations in populations relevant to primary care.

CONTENT

MEDLINE, EMBASE, and the Cochrane Library were searched for studies comparing measured GFR (mGFR) with eGFR in adult populations comparable to primary care and reporting both the Modification of Diet in Renal Disease (MDRD) and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations based on standardized creatinine measurements. We pooled data on mean bias (difference between eGFR and mGFR) and on mean accuracy (proportion of eGFR within 30% of mGFR) using a random-effects inverse-variance weighted metaanalysis. We included 48 studies of 26875 patients that reported data on bias and/or accuracy. Metaanalysis of within-study comparisons in which both formulae were tested on the same patient cohorts using isotope dilution-mass spectrometry-traceable creatinine showed a lower mean bias in eGFR using CKD-EPI of 2.2 mL/min/1.73 m² (95% CI, 1.1-3.2; 30 studies; I² = 74.4%) and a higher mean accuracy of CKD-EPI of 2.7% (1.6-3.8; 47 studies; I² = 55.5%). Metaregression showed that in both equations bias and accuracy favored the CKD-EPI equation at higher mGFR values.

SUMMARY

Both equations underestimated mGFR, but CKD-EPI gave more accurate estimates of GFR.

Estimating glomerular filtration rate by serum creatinine or/and cystatin C equations: An analysis of multi-centre Chinese subjects

AIM

Various equations based on serum creatinine or/and cystatin C, required further validation in a Chinese population. We compared the performance of six Chinese equations (Mascr, Peiscr, Macys, Fengcys, Mascr-cys and Fengscr-cys) with the CKD-EPI equations in multi-centre Chinese subjects and evaluated their applicability in clinical practice.

METHODS

A total of 1522 adult patients from four different hospitals of China were enrolled in the study. (99m) Tc-DTPA renal dynamic imaging was used as the reference GFR (rGFR), and serum creatinine and cystatin C were measured by standardized assays. An optimal score system was implemented in the study.

RESULTS

The average rGFR of recruited subjects was 67.30±28.89 mL/min per 1.73m(2) . All estimated GFR (eGFR) correlated well with rGFR. In accordance with Bland-Altman analysis, the Fengscr-cys equations achieved optimal overall performance (score 14 vs 0-6), with least bias (median difference, -0.57 mL/min per 1.73m(2) ; median absolute difference, 8.83 mL/min per 1.73m(2) ), best precision (17.99 mL/min per 1.73m(2) ), highest accuracy (percentage of eGFR within 15%, 30% and 50% of the rGFR (P15 , P30 and P50 ; 49.7%, 78.7% and 91.8%, respectively); root-mean-square-error (RMSE, 16.28)). The Fengcys equation, a typical cystatin C based equation, was another well-behaved formula with an impressive performance. The Ma equations performed much poorer than the CKD-EPI equations. Consistent results can be observed in the GFR- /age- and sex-specific subgroups, while none equation yielded ideal accuracy in GFR<60 mL/min per 1.73 m(2) subgroup.

CONCLUSION

The Fengscr-cys equation appeared to achieve the best performance for GFR estimation in overall Chinese adult patients. However, further research is warranted to improve the accuracy of available equations in GFR less than 60 mL/min per 1.73 m(2) individuals.

Accurate Quantification of Disease Markers in Human Serum Using Iron Oxide Nanoparticle-linked Immunosorbent Assay

Accurate and reliable quantification of biomarkers in the blood is essential in disease screening and diagnosis. Here we describe an iron oxide nanoparticle (IONP)-linked immunosorbent assay (ILISA) for detecting biomolecules in human serum. Sandwich ILISA was optimized for the detection of four important serological markers, IgA, IgG, IgM, and C-reactive protein (CRP), and assessed with normal sera, simulated disease-state sera and the serum samples from patients infected with West Nile virus (WNV) or human herpes virus (HHV). Our study shows that using the detection assay formulated with 18.8 nm wüstite nanocrystals, ILISA can achieve sub-picomolar detection sensitivity, and all four markers can be accurately quantified over a large dynamic range. In addition, ILISA is not susceptible to variations in operating procedures and shows better linearity and higher stability compared with ELISA, which facilitates its integration into detection methods suitable for point of care. Our results demonstrate that ILISA is a simple and versatile nanoplatform for highly sensitive and reliable detection of serological biomarkers in biomedical research and clinical applications.

Cystatin C standardization decreases assay variation and improves assessment of glomerular filtration rate

BACKGROUND

Cystatin C is increasingly used in glomerular filtration rate (GFR) estimation equations. The dependence of cystatin C results upon the analytical method has been a major source of controversy.

METHODS

Cystatin C was measured with non-standardized turbidimetric Roche Generation 1 and standardized nephelometric Siemens assays in 3666 and additionally with standardized Roche Generation 2 and Siemens in 567 blood samples of the Berlin Initiative Study. Cystatin C-based GFR was assessed with CKD-EPIcys (Chronic Kidney Disease Epidemiology) and CAPA (Caucasian, Asian, Pediatric, Adult) equations and the impact of the assays on GFR estimation was determined. Equation performance compared to measured GFR was evaluated.

RESULTS

Concordance of Roche Gen2 and Siemens was high with median difference of 0.003 ± 0.13 mg/L (limits of agreement: -0.12 to 0.12) and Passing Bablok correlation was essentially perfect. Roche Gen1 assay showed worse concordance with Siemens: median difference was 0.08 ± 0.13 mg/L (limits of agreement: -0.18 to 0.34) and correlation was inferior. Mean difference (± SD) of estimated GFRCKD-EPIcys was 0 ± 4 mL/min/1.73 m(2) for Gen2 and Siemens compared to -5 ± 8 with Gen1. Performance of GFR estimating equations was not influenced by the choice of Siemens or Gen2 assays.

CONCLUSIONS

Standardization of Roche Gen2 assay improved accuracy of cystatin C measurement compared to Siemens. It suggests only negligible method bias and results in equal performance of both assays when estimating GFR indicating that successful calibration has led to major progress in cystatin C analysis.

Performance of cystatin C- and creatinine-based estimated glomerular filtration rate equations depends on patient characteristics

BACKGROUND

The Kidney Disease Improving Global Outcomes (KDIGO) guideline recommends use of a cystatin C-based estimated glomerular filtration rate (eGFR) to confirm creatinine-based eGFR between 45 and 59 mL · min(-1) · (1.73 m(2))(-1). Prior studies have demonstrated that comorbidities such as solid-organ transplant strongly influence the relationship between measured GFR, creatinine, and cystatin C. Our objective was to evaluate the performance of cystatin C-based eGFR equations compared with creatinine-based eGFR and measured GFR across different clinical presentations.

METHODS

We compared the performance of the CKD-EPI 2009 creatinine-based estimated GFR equation (eGFRCr) and the newer CKD-EPI 2012 cystatin C-based equations (eGFRCys and eGFRCr-Cys) with measured GFR (iothalamate renal clearance) across defined patient populations. Patients (n = 1652) were categorized as transplant recipients (n = 568 kidney; n = 319 other organ), known chronic kidney disease (CKD) patients (n = 618), or potential kidney donors (n = 147).

RESULTS

eGFRCr-Cys showed the most consistent performance across different clinical populations. Among potential kidney donors without CKD [stage 2 or higher; eGFR >60 mL · min(-1) · (1.73 m(2))(-1)], eGFRCys and eGFRCr-Cys demonstrated significantly less bias than eGFRCr; however, all 3 equations substantially underestimated GFR when eGFR was <60 mL · min(-1) · (1.73 m(2))(-1). Among transplant recipients with CKD stage 3B or greater [eGFR <45 mL · min(-1) · (1.73 m(2))(-1)], eGFRCys was significantly more biased than eGFRCr. No clear differences in eGFR bias between equations were observed among known CKD patients regardless of eGFR range or in any patient group with a GFR between 45 and 59 mL · min(-1) · (1.73 m(2))(-1).

CONCLUSIONS

The performance of eGFR equations depends on patient characteristics that are readily apparent on presentation. Among the 3 CKD-EPI equations, eGFRCr-Cys performed most consistently across the studied patient populations.

Comparison of estimated GFR equations based on serum cystatin C alone and in combination with serum creatinine in patients with coronary artery disease

OBJECTIVE

Several new equations (GFRCKD-EPI-cr, GFRCKD-EPI-CysC, GFRCKD-EPI Cr+CysC) are used for the calculation of estimated GFR (eGFR) to evaluate renal function. These equations explicitly demonstrate the association of coronary artery disease (CAD) and severe renal impairment cases. However, these equations are considered insufficient to explain the relation with normal or mildly impaired eGFR and CAD. Our hypothesis was to indicate the inversely proportional relationship of eGFR values, calculated by the different equations, with the presence of CAD in patients with normal or mildly impaired renal function.

METHODS

Eighty-eight patients who underwent elective coronary angiographic intervention were enrolled into the study. The study population was divided into two groups based on angiographic documents: patients with normal coronary arteries (CAD-) and patients with CAD (CAD +). These patients were stable and decided to implement angiography for the purpose of suspicion about CAD and control. Since it is thought that eGFR equations based on creatinine are inadequate to determine chronic kidney disease (CKD) and overestimate CKD diagnosis, cystatin C-based equations are considered an alternative. Due to the potential effects of inflammatory events of the markers used in equations, patients with diabetes mellitus, severe CKD, and inflammatory bowel disease were excluded from the study.

RESULTS

The average age of all participants was 51.93±9.31 (32-65 years); 80.7% (n=71) was male. A statistical difference was found between the CAD (-) group and the CAD (+) group in terms of the variables of age (45.46±8.48 vs. 54.95±8.11, p<0.001), gender (67.9% vs. 86.7%, male, p=0.037), cystatin C values (1.37±0.34 vs. 0.85±0.39, p<0.001), and GFR equations defined by the Chronic Kidney Disease Epidemiology: GFRCKD-EPI-cr (85.86±14.20 vs. 79.45±10.25, p=0.018), GFRCKD-EPI-CysC (58.61±21.87 vs. 100.82±32.00, p<0.001), and GFRCKD-EPI Cr+CysC (68.29±13.49 vs. 90.75±18.34, p<0.001). After adjustment of the variables in multiple regression analyses, only age (OR, 1.199; 95% CI, 1.077 to 1.335, p=0.001), gender (OR, 8.252; 95% CI, 0.223 to 55.659, p=0.030), and the GFRCKD-EPI-CysC equation (OR, 1.059; 95% CI, 1.028 to 1.090, p<0.001) were detected as predictors for presence of CAD.

CONCLUSION

GFR equations based on cystatin C or combined with creatinine may have superiority to GFR equations based on creatinine alone in CAD patients. However, the impact of different variables on the GFRCKD-EPI-CysC equation should not be ignored in specific groups, such as CAD.

Calibration and precision of serum creatinine and plasma cystatin C measurement: impact on the estimation of glomerular filtration rate

Serum creatinine (SCr) is the main variable for estimating glomerular filtration rate (GFR). Due to inter-assay differences, the prevalence of chronic kidney disease (CKD) varies according to the assay used, and calibration standardization is necessary. For SCr, isotope dilution mass spectrometry (IDMS) is the gold standard. Systematic differences are observed between Jaffe and enzymatic methods. Manufacturers subtract 0.30 mg/dl from Jaffe results to match enzymatic results ('compensated Jaffe method'). The analytical performance of enzymatic methods is superior to that of Jaffe methods. In the original Modification of Diet in Renal Disease (MDRD) equation, SCr was measured by a Jaffe Beckman assay, which was later recalibrated. A limitation of this equation was an underestimation of GFR in the high range. The Chronic Kidney Disease Epidemiology (CKD-EPI) consortium proposed an equation using calibrated and IDMS traceable SCr. The gain in performance was due to improving the bias whereas the precision was comparable. The CKD-EPI equation performs better at high GFR levels (GFR >60 ml/min/1.73 m(2)). Analytical limitations have led to the recommendation to give a grade (>60 ml/min/1.73 m(2)) rather than an absolute value with the MDRD equation. By using both enzymatic and calibrated methods, this cutoff-grade could be increased to 90 ml/min/1.73 m(2) (with MDRD) and 120 ml/min/1.73 m(2) (with CKD-EPI). The superiority of the CKD-EPI equation over MDRD is analytical, but the precision gain is limited. IDMS traceable enzymatic methods have been used in the development of the Lund-Malmö (in CKD populations) and Berlin Initiative Study equations (in the elderly). The analytical errors for cystatin C are grossly comparable to issues found with SCr. Standardization is available since 2011. A reference method for cystatin C is still lacking. Equations based on standardized cystatin C or cystatin C and creatinine have been proposed. The better performance of these equations (especially the combined CKD-EPI equation) has been demonstrated.

Comparisons between the 2012 new CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equations and other four approved equations

OBJECTIVE

The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) reported two equations in 2012: one based on cystatin C concentration (CKD-EPI2012cys) and the other using both serum creatinine and cystatin C concentrations (CKD-EPI2012Scr-cys). We compared the adaptability of new formulae with other four equations.

METHODS

Participants (n = 788; median age, 54 [range, 19-94] years) were recruited from the First Affiliated Hospital of Nanjing Medical University. The reference glomerular filtration rate (rGFR) was measured by a (99)mTc-DTPA renal dynamic imaging method, and the estimated glomerular filtration rate (eGFR) was calculated separately by the Chinese adapted Modification of Diet in Renal Disease equation (C-MDRD), MacIsaac, Ma, serum creatinine-based CKD-EPI equation (CKD-EPI2009Scr), CKD-EPI2012cys and CKD-EPI2012Scr-cys equations. We compared the performance of six equations with rGFR.

RESULTS

Median rGFR was 76.35 (interquartile range, 59.03-92.50) mL/min/1.73 m(2). Compared with CKD-EPI2009Scr, CKD-EPI2012Scr-cys formula had better diagnostic value with larger area under the receiver operating characteristic curve (ROC(AUC), 0.879, p = 0.006), especially in young participants (ROC(AUC), 0.883, p = 0.005). CKD-EPI2012cys equation did not perform better than other available equations. Accuracy (the proportion of eGFR within 30% of rGFR [P30]) of the CKD-EPI2012Scr-cys equation (77.03%) was inferior only to MacIsaac equation (80.2%) in the entire participants, but performed best in young participants with normal or mildly-injured GFR. Neither of the two new CKD-EPI equations achieved any ideal P30 in the elderly participants with moderately-severely injured GFR. Linear regression analysis demonstrated a consistent result. In this study, CKD-EPI2012Scr-cys had a relatively better diagnosis consistency of GFR stage between the eGFR and rGFR in the whole cohort.

CONCLUSION

CKD-EPI2012Scr-cys appeared less biased and more accurate in overall participants. Neither of the new CKD-EPI equations achieved ideal accuracy in senior participants with moderately-severely injured GFR. A large-scale study with more subjects and cooperating centers to develop new formulae for the elderly is assumed to be necessary.