Ultrasensitive Silicon Nanowire Biosensor with Modulated Threshold Voltages and Ultra-Small Diameter for Early Kidney Failure Biomarker Cystatin C

Acute kidney injury (AKI) is a frequently occurring severe disease with high mortality. Cystatin C (Cys-C), as a biomarker of early kidney failure, can be used to detect and prevent acute renal injury. In this paper, a biosensor based on a silicon nanowire field-effect transistor (SiNW FET) was studied for the quantitative detection of Cys-C. Based on the spacer image transfer (SIT) processes and channel doping optimization for higher sensitivity, a wafer-scale, highly controllable SiNW FET was designed and fabricated with a 13.5 nm SiNW. In order to improve the specificity, Cys-C antibodies were modified on the oxide layer of the SiNW surface by oxygen plasma treatment and silanization. Furthermore, a polydimethylsiloxane (PDMS) microchannel was involved in improving the effectiveness and stability of detection. The experimental results show that the SiNW FET sensors realize the lower limit of detection (LOD) of 0.25 ag/mL and have a good linear correlation in the range of Cys-C concentration from 1 ag/mL to 10 pg/mL, exhibiting its great potential in the future real-time application.

Advances in the Detection, Mechanism and Therapy of Chronic Kidney Disease

Chronic Kidney Disease (CKD) is characterized by the gradual loss of renal mass and functions. It has become a global health problem, with hundreds of millions of people being affected. Both its incidence and prevalence are increasing over time. More than $20,000 are spent on each patient per year. The economic burden on the patients, as well as the society, is heavy and their life quality worsen over time. However, there are still limited effective therapeutic strategies for CKD. Patients mainly rely on dialysis and renal transplantation, which cannot prevent all the complications of CKD. Great efforts are needed in understanding the nature of CKD progression as well as developing effective therapeutic methods, including pharmacological agents. This paper reviews three aspects in the research of CKD that may show great interests to those who devote to bioanalysis, biomedicine and drug development, including important endogenous biomarkers quantification, mechanisms underlying CKD progression and current status of CKD therapy.

Adult Reference Ranges for Serum Cystatin C, Creatinine and Predicted Creatinine Clearance

Serum cystatin C measurement has been previously shown by ourselves and others to be a better indicator of changes in glomerular filtration rate (GFR) than serum creatinine. However, the available literature on reference values for cystatin C concentration remains surprisingly sparse; we thus set out to determine an adult reference range. Blood was taken from 309 healthy blood donors and creatinine and cystatin C concentrations were measured using commercially available automated methodologies. In addition, predicted creatinine clearances were calculated using the Cockcroft and Gault formula. The 95% reference intervals for creatinine, predicted creatinine clearance and cystatin C for all blood donors, regardless of gender, were 68–118 μmol/L, 58–120 ml/min/1·73 m2 and 0·51–0·98 mg/L, respectively. For women, the intervals were 68–98 μmol/L, 60–119 ml/min/1·73 m2 and 0·49–0·94 mg/L; for men, they were 78–123 μmol/L, 57–122 ml/min/1·73 m2 and 0·56–0·98 mg/L. The mean 95% reference interval for cystatin C in all donors under 50 years of age was 0·53–0·92 mg/L; for those over 50 years of age it was 0·58–1·02 mg/L. The small difference between male and female ranges meant that a single reference range for cystatin C could be established for all adults under 50 years of age without adjustment for body surface area. Serum cystatin C measurement offers a simpler and more sensitive screening test than serum creatinine for early changes in GFR.

Serum Cystatin C in Pregnancies With Normal and Restricted Fetal Growth

The objective of this study was to investigate circulating levels of cystatin C (an important endogenous marker of renal function) in mothers, fetuses, and neonates from intrauterine growth-restricted (IUGR; characterized by impaired nephrogenesis) and appropriate-for-gestational-age (AGA) pregnancies. Serum cystatin C levels were measured by enzyme immunoassay in 40 parturients and their 20 IUGR (<or=3rd customized centile, due to gestational pathology) and 20 AGA fetuses and neonates on postnatal day 1 (N1) and 4 (N4). Comparatively, creatinine and urea concentrations were determined in the same samples. Fetal cystatin C levels were higher in the AGA than the IUGR group (P = .001). In both groups, maternal cystatin C levels were lower than fetal (P < .001), N1 (P < .001), and N4 (P < .001) levels. Fetal levels were higher than N1 (P < .001) and N4 (P < .001), and N1 levels were higher than N4 (P = .007) ones. In both groups, no correlation existed between maternal and fetal levels, but positive correlations were found between cystatin C, creatinine, and urea levels in maternal and neonatal samples (in all cases, r >or= 0.376 and P <or= .045). Cystatin C levels did not correlate with gestational age and did not differ between males and females. Fetal cystatin C serum levels are lower in the IUGR group, significantly decrease after birth, and do not correlate with maternal levels in both groups. In addition, serum cystatin C levels positively correlate with respective creatinine and urea levels in the perinatal period.

Cystatin C: a kidney function biomarker

Renal function is essential for homeostasis. The kidneys play important pleiotropic roles including removal of metabolic waste products and maintenance of water-electrolyte balance and blood pressure. Early diagnosis of renal dysfunction and institution of appropriate therapy are vital to survival. Unfortunately, common indicators of renal function lack necessary sensitivity and specificity. Recent evidence has, however, suggested that cystatin C (cysC) may be useful as a marker for glomerular filtration. CysC is a protein belonging to a group of cysteine proteases inhibitors produced primarily by nucleated cells. Due to low molecular weight and positive pI, it is easily filtered. Moreover, its serum concentration is independent of gender, age, or muscle mass, i.e., typical confounders in assessing glomerular filtration rate (GFR). This chapter discusses the structure and biologic function of cysC, its role as an indicator of GFR, and the most frequently used methods for its determination.

Codon preference optimization increases prokaryotic cystatin C expression

Gene expression is closely related to optimal vector-host system pairing in many prokaryotes. Redesign of the human cystatin C (cysC) gene using the preferred codons of the prokaryotic system may significantly increase cysC expression in Escherichia coli (E. coli). Specifically, cysC expression may be increased by removing unstable sequences and optimizing GC content. According to E. coli expression system codon preferences, the gene sequence was optimized while the amino acid sequence was maintained. The codon-optimized cysC (co-cysC) and wild-type cysC (wt-cysC) were expressed by cloning the genes into a pET-30a plasmid, thus transforming the recombinant plasmid into E. coli BL21. Before and after the optimization process, the prokaryotic expression vector and host bacteria were examined for protein expression and biological activation of CysC. The recombinant proteins in the lysate of the transformed bacteria were purified using Ni(2+)-NTA resin. Recombinant protein expression increased from 10% to 46% based on total protein expression after codon optimization. Recombinant CysC purity was above 95%. The significant increase in cysC expression in E. coli expression produced by codon optimization techniques may be applicable to commercial production systems.

Early renal failure detection by cystatin C in Type 2 diabetes mellitus: varying patterns of renal analyte expression

AIM

The early stages of renal failure are poorly diagnosed by current routine tests. We studied cystatin C and routine renal analyte patterns in Type 2 diabetes mellitus.

METHODS

Type 2 diabetes mellitus patients (n = 48) were tested for serum cystatin C, urine albumin, haemoglobin A1c, serum creatinine, serum urea, urine creatinine, glucose, triglycerides and low density lipoproteins (LDL). Glomerular filtration rate (GFR) estimates were made using Cockroft-Gault and Modification of Diet in Renal Disease formulae.

RESULTS

The cystatin C (95%CI) reference range was 0.78-0.86 mg/L. While serum cystatin C showed general correlation with routine renal tests, a plateau was observed in analytes measured against cystatin C. Cystatin C improved sensitivity led to detection of renal abnormality in 19% of patients not diagnosed by routine tests.

CONCLUSIONS

Cystatin C is a more sensitive marker of renal disease in Type 2 diabetes mellitus where estimated GFR is unreported at >60 mL/min and where antihypertensive medications render microalbuminuria detection unreliable. Its incorporation into a panel of renal function tests is highly recommended.

Cystatin C: current position and future prospects

Cystatin C is a low-molecular-weight protein which has been proposed as a marker of renal function that could replace creatinine. Indeed, the concentration of cystatin C is mainly determined by glomerular filtration and is particularly of interest in clinical settings where the relationship between creatinine production and muscle mass impairs the clinical performance of creatinine. Since the last decade, numerous studies have evaluated its potential use in measuring renal function in various populations. More recently, other potential developments for its clinical use have emerged. This review summarises current knowledge about the physiology of cystatin C and about its use as a renal marker, either alone or in equations developed to estimate the glomerular filtration rate. This paper also reviews recent data about the other applications of cystatin C, particularly in cardiology, oncology and clinical pharmacology.

Age, gender, and race effects on cystatin C levels in US adolescents

BACKGROUND AND OBJECTIVES

The objective of this study was to describe the normal range of serum cystatin C and identify factors associated with variability in serum cystatin C contrasting with factors that are known to influence creatinine levels in the general US adolescent population.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Serum cystatin C and creatinine were measured in 719 participants aged 12 to 19 yr in the Third National Health and Nutrition Examination Survey, a national cross-sectional survey conducted in 1988 through 1994. We calculated gender- and race/ethnicity-specific cystatin C and creatinine ranges and conducted multivariable linear regression analyses to assess factors that contribute to variability in cystatin C and creatinine levels.

RESULTS

Overall, the mean serum cystatin C level was 0.84 mg/L and was higher in male than female individuals and higher in non-Hispanic white versus non-Hispanic black and Mexican American individuals. The mean serum creatinine was 0.71 mg/dl and was higher in male than in female individuals but lower in non-Hispanic white and Mexican American compared with non-Hispanic black individuals. Unlike creatinine, which increases with age from 12 to 19 yr, cystatin C levels decrease, particularly in female individuals. After adjustment for age, gender, and race/ethnicity, uric acid and blood urea nitrogen were significantly associated with cystatin C levels.

CONCLUSIONS

Serum cystatin C is significantly related to gender, age, race/ethnicity, uric acid, and blood urea nitrogen in adolescents.

Biochemistry and Clinical Role of Human Cystatin C

Low molecular-mass plasma proteins play a key role in health and disease. Cystatin C is an endogenous cysteine proteinase inhibitor belonging to the type 2 cystatin superfamily. The mature, active form of human cystatin C is a single non-glycosylated polypeptide chain consisting of 120 amino acid residues, with a molecular mass of 13,343-13,359 Da, and containing four characteristic disulfide-paired cysteine residues. Human cystatin C is encoded by the CST3 gene, ubiquitously expressed at moderate levels. Cystatin C monomer is present in all human body fluids; it is preferentially abundant in cerebrospinal fluid, seminal plasma, and milk. Cystatin C L68Q variant is an amyloid fibril-forming protein with a high tendency to dimerize. It forms self-aggregates with massive amyloid deposits in the brain arteries of young adults, leading to lethal cerebral hemorrhage. The main catabolic site of cystatin C is the kidney: more than 99% of the protein is cleared from the circulation by glomerular ultrafiltration and tubular reabsorption. The diagnostic value of cystatin C as a marker of kidney dysfunction has been extensively investigated in multiple clinical studies on adults, children, and in the elderly. In almost all the clinical studies, cystatin C demonstrated a better diagnostic accuracy than serum creatinine in discriminating normal from impaired kidney function, but controversial results have been obtained by comparing this protein with other indices of kidney disease, especially serum creatinine-based equations. In this review, we present and discuss most of the available data from the literature, critically reviewing conclusions and suggestions for the use of cystatin C in clinical practice. Despite the multitude of clinical data in the literature, cystatin C has not been widely used, perhaps because of a combination of factors, such as a general diffidence among clinicians, the absence of definitive cut-off values, conflicting results in clinical studies, no clear evidence on when and how to request the test, the poor commutability of results, and no accurate examination of costs and of its routine use in a stat laboratory.

Performance evaluation of a particle-enhanced turbidimetric cystatin C assay on the Hitachi 917 analyzer

BACKGROUND

Human cystatin C is a low molecular weight protein that has been proposed as a new endogenous marker of glomerular filtration rate. We investigated the performance of the Genzyme cystatin C assay on the Hitachi 917 analyzer.

METHODS

Imprecision, linearity, recovery, and interference studies were performed on the Hitachi 917 analyzer. For method comparison, split sample aliquots were assayed using the described method and 2 other commercially available cystatin C assays.

RESULTS

The assay was linear from 0.24 to 6.36 mg/l. Within-run coefficient of variation (CV) was 4.2 and 0.8% at cystatin C concentrations of 0.50 and 2.00 mg/l, respectively. Between-run CV was 4.3 and 2.7% at the same concentrations. The average analytical recovery was 99%. Bilirubin (< or =30 mg/dl), triglycerides (< or =1000 mg/dl), intralipid (L index < or =1000), and rheumatoid factor (< or =1000 IU/ml) did not interfere with the assay. A >10% change in cystatin C level was observed when hemoglobin concentration was >800 mg/dl. The assay compared well with the Dade Behring immunonephelometric assay and the Dako immunoturbidimetric assay.

CONCLUSION

The Genzyme cystatin C immunoassay is an acceptable method for the determination of cystatin C on the Hitachi 917 analyzer.

Utility of Serum Cystatin C as a Clinical Measure of Renal Function in Dogs

A human kit for cystatin C determination was evaluated for use with canine sera. A reference range was also established. The association between cystatin C and glomerular filtration rate (GFR) was evaluated in 60 dogs with various diseases, by using exogenous creatinine plasma clearance (ECPC) as a measure of GFR. The correlation between cystatin C and ECPC (correlation coefficient [r] = −0.630; P&lt;0.001) was stronger than the correlation between serum creatinine and ECPC (r = −0.572; P&lt;0.001). Nonrenal diseases (e.g., neoplasia, infection) did not influence serum cystatin C concentration. Test sensitivity was significantly better (P&lt;0.001) for cystatin C (76%) than for creatinine (65%). Specificities for the two tests were 87% and 91%, respectively.

Adult cystatin C reference intervals determined by nephelometric immunoassay

OBJECTIVE

The aim of this study was to determine the adult reference values for cystatin C (CysC) and to evaluate their consistence with those reported in the literature.

DESIGN AND METHODS

CysC was analyzed in a consecutive series of subjects (100 males and 100 females) by a nephelometric immunoassay. Medline was searched for CysC reference values.

RESULTS

CysC reference intervals showed 4-11% of variation at the upper limit. The mean upper limit was </=1.0 mg/L of CysC.

CONCLUSION

Nephelometric CysC reference intervals are consistent among different populations.

Localized surface plasmon coupled fluorescence fiber-optic biosensor with gold nanoparticles

A novel fiber-optic biosensor based on a localized surface plasmon coupled fluorescence (LSPCF) system is proposed and developed. This biosensor consists of a biomolecular complex in a sandwich format of <antibody/antigen/Cy5-antibody-gold nanoparticle (GNP)>. It is immobilized on the surface of an optical fiber where a <Cy5-antibody-GNP> complex forms the fluorescence probe and is produced by mixing Cy5-labeled antibody and protein A conjugated gold nanoparticles (Au-PA). The LSPCF is excited by localized surface plasmon on the GNP surface where the evanescent field is applied near the core surface of the optical fiber. At the same time, the fluorescence signal is detected by a photomultiplier tube located beside the unclad optical fiber with high collection efficiency. Experimentally, this novel LSPCF biosensor is able to detect mouse immunoglobulin G (IgG) at a minimum concentration of 1 pg/mL (7 fM) during the biomolecular interaction of the IgG with anti-mouse IgG. The analysis is expanded by a discussion of the amplification of the LSPCF intensity by GNP coupling, and overall, this LSPCF biosensor is confirmed experimentally as a biosensor with very high sensitivity.

The upregulation of cystatin C in oral submucous fibrosis

Cystatin C is a 13kDa non-glycosylated basic protein belonging to cystatin family. It is consistently and dramatically upregulated in a variety of fibrotic diseases. The aim of this study was to compare cystatin C expression in normal human buccal mucosa and oral submucous fibrosis (OSF) specimens and further explore the potential mechanism that may lead to induction of cystatin C expression. Twenty-five OSF specimens and six of normal buccal mucosa were examined by immunohistochemistry. The activity of cystatin C from fibroblasts cultured from OSF and normal buccal mucosa were evaluated by using reverse-transcriptase polymerase chain reaction and enzyme-linked immunosorbent assay. Furthermore, the effect of arecoline, the major areca nut alkaloid, was explored. Cystatin C expression was significantly higher in OSF specimens (p<0.05) and expressed mainly by fibroblasts, endothelial cells, and inflammatory cells. OSF demonstrated significantly higher cystatin C expression than normal buccal mucosa fibroblasts both in mRNA and protein levels (p<0.05). In addition, arecoline was also found to elevate cystatin C mRNA and protein expression in a dose-dependent manner (p<0.05). Taken together, the data demonstrate that cystatin C expression is significantly upregulated in OSF from areca quid chewers and arecoline may be responsible for the enhanced cystatin C expression in vivo.

Cystatin C reference values and aging

OBJECTIVES

The aim of this study was to determine the reference values for serum cystatin C (CysC) with a particular focus on the effect of aging.

DESIGN AND METHODS

The study was performed on a consecutive series of subjects (258 men and 396 women). Laboratory parameters and a detailed personal and family medical history were collected.

RESULTS

CysC showed a significant correlation with age in both sexes, which was confirmed with multivariate linear regression after adjustment for SCr (serum creatinine). Age-related reference intervals were established for cystatin C (<45 years, <0.95 mg/L and >45 years, <1.20 mg/L).

CONCLUSIONS

The use of CysC reference values adjusted for age should be carefully taken into consideration.

Assessment of renal injury in vivo

The ICH S7A (Safety Pharmacology for Human Pharmaceuticals) guidelines specify that potential adverse pharmacologic effects of a test substance on renal function should be evaluated in supplemental studies when there is a cause for concern (ICH, 2001). For the most part, this can easily be accomplished by examination of the appropriate analytes in urine and blood collected as part of the routine preclinical safety studies. This review will serve as an overview of the selection, interpretation and limitations of standard clinical pathology methods (serum chemistry and urinalysis) for assessment of renal function in such studies, as well as provide some information on emerging biomarkers of renal function.

Serum cystatin C level as a potentially good marker for impaired kidney function

OBJECTIVES

To evaluate the diagnostic significance of serum cystatin C levels in clinical practice.

DESIGN AND METHODS

Serum (99m)Tc-DTPA clearance was compared with serum cystatin C, creatinine, beta(2)-microglobulin levels and creatinine clearance in a group of patients aged 42.61 +/- 7.55 years with glomerular filtration rates of 10-60 mL/min/1.73 m(2) (n = 52) and healthy controls aged 43.90 +/- 12.06 years (n = 52).

RESULTS

No effect of sex on serum cystatin C levels was observed, but average levels increased with age. No significant difference was evident between the mean cystatin C levels of three blood samples taken at 1 month intervals from healthy subjects. Reference clearance was correlated with creatinine clearance (r = 0.957), cystatin C (r = 0.828), beta(2)-microglobulin (r = 0.767) and creatinine (r = 0.682). 60 mL/min/1.73 m(2) was chosen as the borderline for receiver-operating characteristics analysis. The values for the cut-off point, sensitivity, specificity and the area under curve were determined for cystatin C as 1.36 mg/L, 98%, 99% and 0.99 +/- 00.1, respectively; for creatinine, the values were 103 micromol/L, 80%, 100% and 0.97 +/- 0.01, respectively, and for beta(2)-microglobulin, the values were 2.51 mg/L, 86%, 92% and 0.94 +/- 0.02, respectively.

CONCLUSION

Serum cystatin C level can be used as a marker for renal damage.

Cystatin C as a New Covariate to Predict Renal Elimination of Drugs

BACKGROUND AND OBJECTIVE

The individual dosing of drugs that are mainly eliminated unchanged in the urine is made possible by assessing renal function. Most of the methods used are based on serum creatinine (SCr) levels. Cystatin C (CysC) has been proposed as an alternative endogenous marker of the glomerular filtration rate (GFR). Carboplatin is one of the drugs for which elimination is most dependent on the GFR. A prospective clinical trial including 45 patients was conducted to assess the value of serum CysC as a predictor of carboplatin clearance (CL).

METHODS

The patients were receiving carboplatin as part of established protocols. Carboplatin was administered as a daily 60-minute infusion at doses ranging from 290 to 1700mg. A population pharmacokinetic analysis was performed using the nonlinear mixed effect modelling NONMEM program according to a two-compartment pharmacokinetic model.

RESULTS

Data from 30 patients were used to test the relationships between carboplatin CL and morphological, biological and demographic covariates previously proposed for prediction of the GFR. The interindividual variability of carboplatin CL decreased from 31% (no covariate) to 14% by taking into account five covariates (SCr, CysC, bodyweight [BW], age and sex). Prospective evaluation of these relationships using the data from the other 15 patients confirmed that the best equation to predict carboplatin CL was based on these five covariates, with a mean absolute percentage error of 13% as an assessment of precision. NONMEM analysis of the whole dataset (n = 45 patients) was performed. The best covariate equation corresponding to the overall analysis was: CL (mL/min) = 110 x (SCr/75)-0.512 x (CysC/1.0)-0.327 x (BW/65)0.474 x (age/56)-0.387 x 0.854sex, with SCr in micromol/L, CysC in mg/L, BW in kilograms, age in years and sex = 0 if male and 1 if female. To put the value of CysC as an endogenous marker of the GFR into perspective, covariate equations without SCr were also evaluated; a better prediction was obtained by considering CysC together with age and BW (interindividual variability of 16.6% vs 23.3% for CysC alone).

CONCLUSION

CysC is a marker of drug elimination that is at least as good as SCr for predicting carboplatin CL. The model based on five covariates was superior to those based on only four covariates (with BW, age and sex combined with either SCr or CysC), indicating that CysC and SCr are not completely redundant to each other. Further pharmacokinetic evaluation is needed to determine whether SCr or CysC is the better marker of renal elimination of other drugs.

Cystatin C-A novel marker of glomerular filtration rate: A review

Glomerular filtration rate is routinely assessed by measuring the serum markers such as urea nitrogen and serum creatinine. Although these markers are widely used to assess renal function but they do not perform optimally in certain clinical settings. There is thus a practical need for an easily automated alternative to plasma creatine, which would be more specific, sensitive and reliable from the analytical and clinical view point. Compared with the above endogenous markers, and time consuming laborious tests, Cystatin C facilitates the recognition of abnormal renal function in children, as its reference range is constant beyond the 1(st) year of life. This review mainly focuses on the diagnostic performance of Cystatin C against other renal markers in the pediatric population and in specific subpopulations of patients.

Clinical value of serum cystatin C by ELISA for estimation of glomerular filtration rate

The search for whether endogenous markers of changes in glomerular filtration rate (GFR) by serum cystatin C assay and serum cystatin C compare with creatinine clearance by the Cockeroft-Gault formula and the evaluation of its clinical significance as a marker of GFR is important in clinical practice at present. Serum cystatin C was determined by sandwich enzyme immunoassay using a kit. Control blood samples were collected from 70 healthy subjects and 168 patients with various kidney diseases. Creatinine clearance (Cockeroft-Gault formula) as a measure of GFR, in 168 patients with various kidney diseases, depends on the creatinine clearance; GFR parameters were used to divide patients into two groups. The GFR was >80 mL/min in 38 patients (group A) and <80 mL/min in 130 patients (group B). The two groups were analyzed by correlation coefficient and diagnostic sensitivity and specificity were assessed by the receiver-operating characteristic (ROC) plots (area under the curve). Of the 70 healthy control individuals, the serum level of cystatin C was measured as normal value range and a reference interval of 1.05+/-0.18 micro g/mL (mean+/-1.96 SD, 95% confidence limits for the upper references limit is 1.4 microg/mL). In group A, serum cystatin C had no correlation to the creatinine clearance (r=0.171, P>0.05) and in group B, serum cystatin C was closely correlated to the creatinine clearance (r=-0.771, P<0.001). Diagnostic sensitivity and specificity were assessed by the ROC plots for serum cystatin C (area under the curve=0.8461, SE=0.057) and creatinine clearance (area under the curve=0.7642, SE=0.068). These data suggest that combined measurement of serum cystatin C is useful to estimate GFR, especially to detect the reduction of GFR. Further studies are required to evaluate the whether serum cystatin C as a more sensitive marker of early renal injury might be extremely useful, particularly in nonproteinuric or unapparent renal disease.

Commentary: clinical diagnostic use of cystatin C

Clinicians recognize and compensate for limitations in estimating the glomerular filtration rate (GFR) using serum creatinine (sCr) measurements by the use of timed collections and mathematical manipulations of sCr. These limitations stem from that fact that sCr is affected by nonrenal influences, including muscle mass and disease state. In addition, sCr may not be sensitive enough to detect minimal declines in GFR in those patient populations in which it is important to recognize early decline. This brief review describes the limitations of sCr, and examines the contribution that sCysC may be able to make in the early recognition of declining renal function. The physiology of CysC is presented, as are the results of clinical investigations that suggest sCysC is in many instances superior to sCr in the recognition of early decline in renal function. Certain exceptions to this are noted.

Chronic Renal Insufficiency and Cardiovascular Events in the Elderly: Findings From the Cardiovascular Health Study

In the Cardiovascular Health Study, the authors sought to evaluate the impact of chronic renal insufficiency (CRI) on cardiovascular risk status and outcomes in a representative sample of community-dwelling elderly adults. Defined as a serum creatinine level > or =1.3 mg/dL in women and > or =1.5 mg/dL in men, CRI was present in 647 (11%) of 5808 participants. At baseline, the prevalence of clinical or subclinical cardiovascular disease was 64% in participants with CRI and 43% in those without CRI (odds ratio, 2.34; 95% confidence interval, 1.96-2.80). The incidence of cardiovascular disease events during follow-up was 3% per year in participants with creatinine levels <1.10 mg/dL and increased steadily to reach 7% per year in those with creatinine > or =1.70 mg/dL. Among the possible mediators for the association between CRI and cardiovascular morbidity are inflammatory (C-reactive protein, fibrinogen, and interleukin-6) and hemostatic (factor VII, factor VIII, plasmin-antiplasmin product, and D-dimer) biomarkers, all of which were significantly elevated in Cardiovascular Health Study participants with CRI. Future studies should evaluate the contribution of novel and traditional cardiovascular risk factors to the cardiovascular risk of elderly persons with CRI. The identification of CRI in the elderly and the use of cardiovascular prevention therapies represent a major opportunity to reduce their burden of cardiovascular morbidity.

A sol particle homogeneous immunoassay for measuring serum cystatin C

OBJECTIVES

We have developed a sol particle immunoassay (SPIA) for measuring serum cystatin C, an endogenous marker of glomerular filtration rate (GFR).

DESIGN AND METHODS

We used colloidal gold particles coated with anti-cystatin C antibodies.

RESULTS

The assay was linear in the range 0.2 to 8 mg/L and showed good correlation between theoretical and obtained values. The within and between-day coefficients of variation (CV) varied from 1.1 to 1.6% and 0.4 to 1.0%, respectively. Analytical recovery was 95.7 to 103.7%. No interference could be detected from bilirubin (up to 200 mg/L), hemoglobin (up to 3 g/L), chyle (up to 5,000 FTU), rheumatoid factor (up to 1,000 IU/mL) or anticoagulants. Serum samples (n = 101), from which turbidity had been removed, were measured either with our assay or with Dako Cystatin C PET kits, using a Model 7070 Hitachi automatic clinical analyzer. Comparing these two methods, the calculated linear regression equation and the correlation coefficient were y = 0.986 x -0.153 and r = 0.995, respectively.

CONCLUSIONS

Our new SPIA assay is a fully automated, homogeneous immunoassay that can readily be used in conjunction with various commercial analyzers that are currently available. The assay is sensitive, precise and suitable for clinical use and appears to offer advantages over other GFR markers such as creatinine.

Interest of cystatin C in screening diabetic patients for early impairment of renal function

We compared cystatin C, creatinine, and the Cockroft formula for assessment of early renal failure, defined as a (51)Cr-EDTA clearance < 80 mL/min, in 89 diabetic patients with various degrees of renal impairment (glomerular filtration rate [GFR], 11.4 to 196.5 mL/min). The relationships between cystatin C, creatinine, and (51)Cr-EDTA clearance were linearized by plotting the reciprocals of the values, and correlation coefficients were determined. Sensitivity and specificity for the diagnosis of early renal failure were calculated from receiver operating characteristic (ROC) curves. Over the whole population, cystatin C was as well correlated with GFR (r =.74) as was creatinine (r =.67) or the Cockroft formula (r =.88). Moreover, its diagnostic accuracy was comparable to that of the 2 other parameters. Its sensitivity (86.8%) was better than that of creatinine (77.4%) for screening GFR < 80 mL/min, although the Cockroft formula was more sensitive (96.2%). The study of albuminuric diabetics (n = 63) led to similar conclusions, except for a poor sensitivity of cystatin C. In the 36 patients whose plasma creatinine was < 1 mg/dL, 10 (27.7%) had GFR < 80 mL/min. The correlation of creatinine with GFR, its diagnostic accuracy, and sensitivity were significantly lower than those of cystatin C. In this population of patients with normal creatinine levels, the correlation coefficient of cystatin C, its sensitivity, and its diagnostic accuracy were comparable to those of the Cockroft formula. A moderate reduction in GFR may be present in diabetic patients with low creatinine levels. Although Cockroft formula remains the most reliable and the less expensive tool for the evaluation of renal function, cystatin C is a more reliable criterion for screening and assessment than creatinine and represents a useful alternative to the Cockcroft-Gault formula.

Serum cystatin C as a marker of the renal function in patients with spinal cord injury

OBJECTIVE

To investigate the relationship between serum cystatin C, serum creatinine, and (51)Cr-EDTA-clearance in patients with spinal cord injury.

SETTING

The Spinal Cord Unit, Viborg-Kjellerup County Hospital.

METHODS

Twenty-four men and seven women aged 20.3 to 68.0 years with motor complete spinal cord injury (ASIA A or B) were included. Serum cystatin C was measured by an automated particle-enhanced nephelometric immunoassay (Dade Behring), serum creatinine by an enzymatic method (Vitros 950), and (51)Cr-EDTA-clearance by a multiple plasma sample method.

RESULTS

A linear relationship was found between (51)Cr-EDTA-clearance and the reciprocal values of cystatin C and creatinine. The correlation coefficient between (51)Cr-EDTA-clearance and 1/cystatin C was 0.72 compared to the correlation coefficient between (51)Cr-EDTA-clearance and 1/creatinine being 0.26. Comparison of the area under the curves in the non-parametric receiver operating characteristics (ROC) plots for serum cystatin C (area under the curve (AUC)=0.912; SE=0.065), and serum creatinine (AUC=0.507; SE=0.115) revealed significant differences (P-values=0.0005).

CONCLUSION

In patients with spinal cord injury serum cystatin C is a better marker of the renal function compared to serum creatinine.

Serum cystatin C is superior to serum creatinine as a marker of kidney function: a meta-analysis

BACKGROUND

Serum cystatin C (Cys C) has been proposed as a simple, accurate, and rapid endogenous marker of glomerular filtration rate (GFR) in research and clinical practice. However, there are conflicting reports regarding the superiority of Cys C over serum creatinine (Cr), with a few studies suggesting no significant difference.

METHODS

We performed a meta-analysis of available data from various studies to compare the accuracy of Cys C and Cr in relation to a reference standard of GFR. A bibliographic search showed 46 articles until December 31, 2001. We also retrieved data from eight other studies presented and published in abstract form.

RESULTS

The overall correlation coefficient for the reciprocal of serum Cys C (r = 0.816; 95% confidence interval [CI], 0.804 to 0.826) was superior to that of the reciprocal of serum Cr (r = 0.742; 95% CI, 0.726 to 0.758; P < 0.001). Similarly, receiver operating characteristic (ROC)-plot area under the curve (AUC) values for 1/Cys C had greater identity with the reference test for GFR (mean ROC-plot AUC for Cys C, 0.926; 95% CI, 0.892 to 0.960) than ROC-plot AUC values for 1/Cr (mean ROC-plot AUC for serum Cr, 0.837; 95% CI, 0.796 to 0.878; P < 0.001). Immunonephelometric methods of Cys C assay produced significantly greater correlations than other assay methods (r = 0.846 versus r = 0.784; P < 0.001).

CONCLUSION

In this meta-analysis using currently available data, serum Cys C is clearly superior to serum Cr as a marker of GFR measured by correlation or mean ROC-plot AUC.

Cystatin C: An Improved Estimator of Glomerular Filtration Rate?

Background: Glomerular filtration rate (GFR) is routinely assessed by measuring the concentrations of endogenous serum markers such as blood urea nitrogen and serum creatinine (SCr). Although widely used, these endogenous markers are not ideal and do not perform optimally in certain clinical settings. The purpose of this review is to critically review the potential utility of cystatin C (CysC), especially in patient populations in which CysC may have an advantage over routinely used endogenous markers of GFR.

Approach: In a narrative approach, we extensively review publications, primarily from the last 5 years, that address the development of methods to measure CysC, reference intervals, and the diagnostic accuracy of CysC to assess GFR. Between June 2000 and September 2001 Medline was searched using “cystatin c” as a textword, and articles that examined &gt;75 individuals (except for renal transplant studies) and/or used accepted “gold standards” for assessing GFR were selected for inclusion. A total of 17 studies are reviewed that provide reference interval data for several populations. A total of 24 studies make conclusions about the utility of CysC vs SCr and/or creatinine clearance, with 20 providing data on the sensitivity and specificity of CysC for detecting impaired GFR. These publications are organized into subgroups that deal with specific patient populations or clinical situations.

Content: This review focuses on two areas: (a) the evolution of immunoassays used to determine the concentration of CysC in serum, their analytic sensitivity, and reference intervals; and (b) the diagnostic performance of CysC against other renal markers in the general population and in specific subpopulations of patients.

Summary: Studies of reference intervals for CysC overwhelmingly demonstrated that CysC values in blood are independent of age and sex. Of the 24 studies that examined clinical utility, 15 concluded that CysC is superior to SCr, whereas 9 concluded that CysC is equivalent but provides no advantage. Summary ROC plot analysis of 20 studies that provide sensitivity and specificity data strongly suggests that CysC will be superior to SCr for detecting impaired GFR. Taken together, it is clear that CysC performs at least as well as SCr in the population at large and that it is likely to be superior to SCr in specific patient populations.

Determination of serum cystatin C: biological variation and reference values

Human cystatin C is a low molecular weight protein which has been proposed as a better marker of glomerular filtration rate than creatinine. To be able to interpret results obtained in different patient populations it is necessary to define cystatin C reference values. We measured serum concentration of cystatin C in 1223 subjects using a particle-enhanced nephelometric assay. Subjects were aged 4 to 79 years and were selected among apparently healthy individuals who came to the Centre for Preventive Medicine in Vandoeuvre-Lès-Nancy, France. We observed a Gaussian distribution of cystatin C concentration in serum. We did not find any effect of age or gender in children, hormonal status in women (puberty, menopause, oral contraceptives or hormone replacement therapy) or alcohol intake. Cystatin C concentration was slightly lower in female than in male adults below the age of 60 years. Cystatin C levels significantly increased above the age of 60 in both males and females, probably due to physiological aging of renal function. No other significant differences were observed between males and females. Using multiple regression analysis, moderate correlations were observed between body mass index and cystatin C, and between smoking and cystatin C, but these were not biologically significant. According to the literature, only methylprednisolone and cyclosporin A increased and decreased cystatin C levels, respectively. The reference values for cystatin C obtained in a carefully selected population were 0.75+/-0.089 mg/l for children aged 4-19 years, 0.74+/-0.100 mg/l for males and 0.65+/-0.085 mg/l for females (aged 20-59 years), and 0.83+/-0.103 mg/l for older individuals (> or =60 years).

Elevation of serum cystatin C concentrations in patients with chronic liver disease

OBJECTIVE

We examined serum cystatin C concentrations in patients to explore the possible clinical application of cystatin C as a marker of disease severity in cases of chronic liver diseases.

METHODS

Serum cystatin C concentrations were determined by an enzyme-linked immunosorbent assay kit in 103 patients with various chronic liver diseases and compared with concentrations in healthy control volunteers.

RESULTS

The mean cystatin C concentration was 0.68 +/- 0.03 mg/l in chronic hepatitis patients, 1.13 +/- 0.09 mg/l in liver cirrhosis patients and 1.16 +/- 0.10 mg/l in hepatocellular carcinoma patients, all significantly higher than concentrations in the control volunteers (P < 0.0001). Significant correlations were observed between cystatin C concentrations and total bilirubin levels, albumin levels, platelet levels, type IV collagen levels and hyaluronic acid levels. Serum cystatin C concentrations correlated well with histological stages despite the lack of correlation with histological grades.

CONCLUSION

Our results show that serum cystatin C increases with the progression of chronic liver disease and that it is a potential marker for liver fibrosis.

Serum cystatin C, a potent inhibitor of cysteine proteinases, is elevated in asthmatic patients

The effect of asthma pathogenesis on serum cystatin C, a potent inhibitor of cysteine proteinases and a newly proposed marker of the renal function, has not been yet determined. The objectives were to determine the 24-h pattern of cystatin C and creatinine concentrations in sera of asthmatic patients in order to test whether their concentrations might reflect circadian rhythms, the disease severity and the effect of therapy. Serum concentrations of cystatin C and creatinine were determined in steroid-independent and steroid-dependent asthmatics before and after 1 week of treatment with methylprednisolone and cyclosporin A, respectively. Samples were collected every 4 h during a 24-h period. Little or no significant effects of time on cystatin C and creatinine concentrations over a 24-h period were observed in healthy and asthmatic sera. However, significantly higher cystatin C concentrations were found in asthmatic patients compared to controls which suggests its role in the pathogenesis of asthma. Methylprednisolone increased and cyclosporin A decreased serum cystatin C concentrations after 1 week of therapy. Additionally these results support the need for the evaluation of cystatin C as a marker of glomerular filtration rate determination in asthma.

Serum cystatin C as a new marker for noninvasive estimation of glomerular filtration rate and as a marker for early renal impairment

Cystatin C is a nonglycosylated basic protein produced at a constant rate by all investigated nucleated cells. It is freely filtered by the renal glomeruli and primarily catabolized in the tubuli (not secreted or reabsorbed as an intact molecule). Because serum cystatin C concentration is independent of age, sex, and muscle mass, it has been postulated to be an improved marker of glomerular filtration rate (GFR) compared with serum creatinine level. We compared serum cystatin C level with other markers of GFR, such as serum creatinine level and creatinine clearance, and analyzed their variations based on iothalamate labeled with iodine 125 ((125)I-iothalamate) clearance ((125)I-ICl), used as the gold standard for GFR. The concentrations of the two different markers of GFR in patients with impaired renal function were classified according to (125)I-ICl. Twenty individuals with normal renal function ((125)I-ICl, 128 +/- 23 mL/min/1.73 m(2)) were used as the control group. Serum cystatin C level showed a greater sensitivity (93.4%) than serum creatinine level (86.8%). Also, serum cystatin C showed the greatest proportion of increased values in patients with impaired renal function (100%) compared with serum creatinine level (92.15%). Serum cystatin C levels started to increase to greater than normal values when GFR was 88 mL/min/1.73 m(2), whereas serum creatinine level began to increase when GFR was 75 mL/min/1.73 m(2). These data suggest that measurement of serum cystatin C may be useful to estimate GFR, especially to detect mild reductions in GFR, and therefore may be important in the detection of early renal insufficiency in a variety of renal diseases for which early treatment is critical.

Developments in the assessment of glomerular filtration rate

The assessment of the glomerular filtration rate (GFR) is the most commonly used test of renal function. The accepted reference procedure employs an exogenous clearance marker whilst the most popular test is that of serum or plasma creatinine. All of these tests have limitations, although the surrogate endogenous markers are the most practical. Cystatin C, a low molecular weight protein which can be measured by light scattering immunoassay, possesses many of the attributes required of the ideal GFR marker. Data on reference ranges indicate that circulating cystatin C levels reflect the variation in GFR throughout life and the marker demonstrates a better correlation with the reference procedure than serum creatinine.

Cystatin C--properties and use as diagnostic marker

This chapter focuses on the most well characterized inhibitors—cystatin C—and provide some information on its structure, biochemical properties, its role in normal and abnormal physiological processes, as well as on its use as a diagnostic marker. A major part of the cysteine proteases are evolutionary related to the structurally well–defined cysteine protease papain and are called papain–like cysteine proteases. The biological roles and the cystatin superfamily inhibitors of papain–like cystein proteases are also discussed. The aminoacid sequence and schematic structure of human cystatin C is also presented. The evolutionary relationships among all known inhibitory active human cystatins and kininogen cystatin domains are diagrammatically represented. The distribution of cystatins in body fluids and additional functions attributed to cystatin C are described. The serum or plasma cystatin C is used as a marker for glomerular filtration rate (GFR). The urine cystatin C is used as a marker for proximal tubular damage. The two types of brain hemorrhage associated with Cystatin C amyloid deposits are also demonstrated. The conditions connected with deposition of amyloid β–protein in cystatin C and cerebral hemorrhage is also provided.

Relationships among serum cystatin C, serum creatinine, lean tissue mass and glomerular filtration rate in healthy adults

In an effort to increase our knowledge of the optimal use of serum cystatin C and creatinine as glomerular filtration rate (GFR) markers, these variables, as well as lean tissue mass and GFR, were determined in a population of 42 healthy young adults (men and women with normal GFR). Dual-energy X-ray absorptiometry and measurement of the plasma clearance of iohexol were used to measure lean tissue mass and GFR, respectively. Serum creatinine was significantly correlated to lean tissue mass (r=0.65; p < 0.0001) but not to GFR (1/creatinine vs. GFR: r=0.11; p=0.106). In contrast, serum cystatin C correlated with GFR (1/cystatin C vs. GFR: r=0.32; p=0.0387), especially in men (1/cystatin C vs. GFR: r=0.64; p=0.0055), but not to lean tissue mass. These results might explain previous observations that serum cystatin C seems to be a better marker for GFR than serum creatinine, particularly for individuals with small to moderate decreases in GFR. However, the results also show that the serum concentrations of both creatinine and cystatin C are determined not only by GFR, but also by other factors. Since these additional factors differ for cystatin C and creatinine, it seems justified to use serum creatinine and cystatin C in conjunction to estimate GFR, at least until it is known in what situations serum creatinine or cystatin C is the preferable marker.

Changes in Plasma Cystatin C after Renal Transplantation and Acute Rejection in Adults

Background: Cystatin C has recently been proposed as an alternative marker of glomerular filtration rate. The diagnostic value of plasma cystatin C for the longitudinal assessment of kidney function after renal transplantation, however, has not been addressed.

Methods: Renal function was evaluated in 30 adults receiving renal transplants (46 ± 9 years, mean ± SD) and in 56 healthy controls (38 ± 10 years) using cystatin C. Plasma cystatin C was determined daily starting the day of surgery and for 3 weeks after surgery by an immunonephelometric assay.

Results: Plasma concentration significantly decreased during the first week (−44% vs −29% for creatinine). Plasma cystatin C correlated with plasma creatinine (r = 0.741; P &lt;0.0001) and the reciprocal of the creatinine clearance estimated by the Cockcroft-Gault formula (r = 0.882; P &lt;0.001). In all three cases of acute renal impairment, the increase in plasma cystatin C values was more prominent than that of creatinine.

Conclusions: Plasma cystatin C is an alternative and accurate marker of allograft function in adult transplant patients. Increased sensitivity compared with creatinine for the detection of acute reduction in glomerular filtration rate allows in some cases a more rapid diagnosis of acute rejection or treatment nephrotoxicity.

Serum cystatin C as an endogenous marker of the renal function--a review

Since 1985, cystatin C has been suggested to be a marker of the renal function. Cystatin C is a proteinase inhibitor with a low molecular weight (M(r) = 13359). It is produced at a constant rate in all nucleated cells investigated to date, freely filtered in the renal glomeruli and reabsorbed and catabolised in the proximal tubules. The concentration of serum cystatin C is mainly determined by glomerular filtration, which makes cystatin C an endogenous marker of glomerular filtration rate (GFR). There are few data describing the influence of various factors on the production and elimination of cystatin C. Fully automated assays using particle-enhanced turbidimetry or particle-enhanced nephelometry are available and the assays are precise, rapid and usable in clinical routine practice. Reference intervals have been determined for cystatin C in adults and in children older than one year. It has been suggested that the same reference interval can be used in children older than one year and in adults without gender differences, on the assumption that the same method with the same standardisation is used. Several studies including adults and children with different renal diseases with various kidney function have suggested serum cystatin C to be a better marker of GFR than serum creatinine.

Evaluation of the Dade Behring N Latex Cystatin C assay on the Dade Behring Nephelometer II System

The Dade Behring N Latex Cystatin C assay, a particle-enhanced nephelometric immunoassay for measuring serum cystatin C, was evaluated on the Dade Behring Nephelometer II. The assay time was 6 min and the throughput was 75 samples per hour. The sample volume was 40 microL and the measuring range was 0.25-7.90 mg/L. Imprecision studies revealed within-run CVs < 1.8% and between-run CVs < 1.8% in the concentration range 0.87-4.63 mg/L. Recovery was 92.4-101.3%. Linearity studies showed excellent correlation between the theoretical and obtained values. No interferences were detected from haemoglobin < 1.0 mmol/ L, bilirubin <512 micromol/L and Intralipid <20 g/L. Stability of cystatin C in serum was 7 days at temperatures from 20 degrees C to 20 degrees C and 6 months at -80 degrees C. Measurements of cystatin C in heparin-plasma and EDTA-plasma did not differ significantly from cystatin C measured in serum. Fifty patient samples run on the Dade Behring Nephelometer II (y) were compared to the Dako Cystatin C assay (x). The Passing-Bablok regression analysis revealed y = 1.105x - 0.340. In conclusion, the Dade Behring N Latex Cystatin C assay was precise and correlated with the Dako Cystatin C assay.

Serum cystatin C as a determinant of fasting total homocysteine levels in renal transplant recipients with a normal serum creatinine

Serum creatinine, a surrogate for both renal function and homocysteine generation, is an important determinant of fasting plasma total homocysteine levels in stable renal transplant recipients. In this study, it is hypothesized that among stable renal transplant recipients with normal creatinine levels (i.e., < or = 1.5 mg/dl), serum cystatin C, a more sensitive indicator of GFR, would better predict fasting total homocysteine levels compared with serum creatinine. Fasting plasma total homocysteine, folate, vitamin B12, and pyridoxal 5'-phosphate levels, along with serum cystatin C, creatinine, and albumin levels, were determined in 28 consecutive renal transplant recipients (mean age 47 +/- 14 yr; 60.7% men) with stable allograft function, whose serum creatinine was < or = 1.5 mg/dl. General linear modeling with analysis of covariance revealed that serum cystatin C was independently predictive (partial R = 0.494; P = 0.023) of fasting total homocysteine levels after adjustment for age, gender, vitamin status, albumin, and creatinine levels. In contrast, creatinine levels were not predictive of fasting total homocysteine levels in this model (P = 0.110) or an identical model that excluded cystatin C (P = 0.131). Serum cystatin C levels may reflect subtle decreases in renal function that independently predict fasting total homocysteine levels among stable renal transplant recipients with a normal serum creatinine.

Serum cystatin C as a marker of the renal function

The protease inhibitor cystatin C is a non-glycosylated low molecular weight protein (Mr=13359) which is produced by all nucleated cells at a constant rate, freely filtered by the renal glomeruli, and catabolized in the tubuli. The aim of the study was to elucidate the applicability of serum cystatin C as a marker of glomerular filtration rate (GFR) in patients with various kidney diseases with a wide range of renal function and in dialysis patients. Seventy-six patients with various kidney diseases (aged 20 to 79 years) and 61 dialysis patients (aged 21 to 82 years) were included. Serum cystatin C was measured by automated particle-enhanced immunoturbidimetry, serum and urine creatinine by an enzymatic method, and GFR by 99mTc-DTPA-clearance using a single plasma sample method. Serum cystatin C in patients with various kidney diseases was 1.90+/-0.98 mg/L (mean+/-SD) and in dialysis patients 7.14+/-1.91 mg/L. In the non-dialysis patients a linear relationship was found between 99mTc-DTPA-clearance and 1/serum cystatin C (r=0.91, p-value<0.0001), 1/serum creatinine (r=0.89, p-value<0.0001), and creatinine-clearance (r=0.88, p-value<0.0001). Comparison of the non-parametric ROC plots for serum cystatin C (area under the curve (AUC)=0.9665; SE=0.0169), serum creatinine (AUC=0.9554; SE=0.0205), and creatinine-clearance (AUC=0.9731; SE=0.0160) revealed no significant differences (p-values: 0.50, 0.78, and 0.49). In conclusion, cystatin C may be a likewise good marker of the GFR as serum creatinine and creatinine-clearance, cystatin C having the advantage being independent of gender and muscle mass.