Adult reference ranges for serum cystatin C, creatinine and predicted creatinine clearance

Glomerular filtration rate measurement and prediction equations

Chronic kidney disease (CKD) is defined as the presence of kidney damage or a glomerular filtration rate (GFR) <60 mL/min/1.73 m(2) for three or more months. Measurement of serum creatinine is the most commonly used method to evaluated kidney function, but it must be included in formulas to estimate GFR, adjusting for age, gender and ethnicity, such as the Modification of Diet in Renal Disease (MDRD) study equation. The performance of this equation is acceptable for patients with CKD but appears to under-estimate GFR in populations with unknown kidney status. A new formula has been developed recently. The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation appears to perform better than the MDRD equation. Cystatin C has been widely evaluated as a marker for GFR and seems to be more sensitive than creatinine. The aim of this review is to discuss the recommendations for detecting CKD, emphasizing the characteristics and limitations of GFR estimating equations and pitfalls in the evaluation of urinary albumin excretion.

Evaluation of trial outcomes in acute kidney injury by creatinine modeling

BACKGROUND AND OBJECTIVES

Clinical trials of acute kidney injury (AKI) use changes in creatinine as outcome metrics. This study investigated how outcome metrics and baseline creatinine affect trial outcome.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A one-compartment pharmacokinetic model of creatinine change resulting from a decrease in GFR was applied to a population of 10,000 simulated virtual inpatients. Treatment was simulated as an amelioration of GFR decrease by a specified percentage, the treatment efficacy, in 50%. Three categorical and two continuous outcome metrics were calculated and compared. Outcomes were compared for measured and estimated baseline creatinine levels that were back-calculated assuming a GFR of 100 or 75 ml/min.

RESULTS

The continuous metrics, the average value of creatinine and the average value of creatinine relative to baseline decreased approximately linearly with increase in treatment efficacy. The categorical metrics displayed a sigmoidal decrease and erroneously suggested perfect treatment when GFR decrease was ameliorated by only 60 to 80%. Using an estimate of baseline creatinine increased the number of patients who were classified as having AKI.

CONCLUSIONS

When used to determine clinical trial outcome, continuous metrics correctly detected the extent of intervention. At low treatment efficacy, categorical metrics underestimated and at high treatment efficacy overestimated the effect of treatment. These effects were exaggerated when the population contained a high proportion of patients with more severe AKI. An estimated baseline creatinine level will overestimate AKI prevalence compared with a measured baseline value. Clinical trials of AKI should use a continuous outcome metric and a measured baseline and report baseline median and interquartile range.

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.

Comparisons of cystatin C with creatinine for evaluation of renal function in chronic kidney disease

BACKGROUND

Because of the limitations of creatinine (Cr) as a marker for the glomerular filtration rate (GFR), cystatin C (CysC) has been proposed as an alternative substance. The aim here was to clarify the characteristics of CysC compared with Cr.

METHODS

CysC and Cr were measured in 199 patients with chronic kidney disease. Regression analysis between CysC and Cr and comparisons of the effect of gender, inflammation, prescription of prednisolone, smoking and diabetes mellitus (DM) on these markers were performed. Sensitivity and specificity of CysC and Cr to discriminate estimated GFR of less than 50 ml/min/1.73 m(2) were computed and evaluated by the receiver-operating characteristic curve (ROC).

RESULTS

The correlation coefficient between natural logarithmic Cr [ln(Cr)] and ln(CysC) was higher than that between these variables per se (0.941 vs. 0.906). When Cr was lower than 1.10 mg/dl, CysC rose more sharply than Cr. CysC divided by Cr (CysC/Cr) was higher in females (1.35 +/- 0.33 vs. 1.16 +/- 0.30; p < 0.001), in patients with elevated CRP (1.33 +/- 0.40 vs. 1.21 +/- 0.29; p < 0.001) and in patients prescribed with prednisolone (1.42 +/- 0.33 vs. 1.20 +/- 0.30; p < 0.001). A stepwise multiple linear regression model indicated that ln(CysC) was positively correlated with ln(Cr), age, female gender, prednisolone prescription, elevated CRP and DM (R = 0.964, p < 0.001). The area under the ROC curve of Cr was 0.900 and that of CysC was 0.925.

CONCLUSION

CysC is a promising marker for GFR because it was not gender- or age-related. However, inflammation, prednisolone and DM caused CysC to deviate higher than expected from GFR. CysC can rise sensitively in early renal dysfunction.

Effects of Glucocorticoid Immunosuppression on Serum Cystatin C Levels

The aim of the present study is to describe the influence of glucocorticoid immunosuppression on serum cystatin C concentration in renal transplant patients. To evaluate the influence of immunosuppressive regimens, especially glucocorticoids, on serum cystatin C level, 38 clinically stable patients on immunosuppression therapy with low-dose glucocorticoids were compared to 30 clinically stable patients receiving cyclosporin A alone, and 18 clinically stable patients receiving cyclosporin A together with azathioprine. Clinical stability was defined as the absence of acute rejection, febrile infection, and cyclosporin A toxicity, as well as stability of creatinine clearance as estimated by the formula of Cockroft and Gault. All groups were compared for estimated creatinine clearance (CrCl) values and had comparable gender, age and time since transplantation. The group receiving short-course, high-dose methylprednisolone was analyzed at four time points: a) before methylprednisolone commencement (median, 15 days); b) the day methylprednisolone was introduced (before medication); c) after 3 days of methylprednisolone therapy; and d) on a follow-up 9-10 days after the last dose. Intravenous administration of high-dose methylprednisolone led to significant differences in cystatin C levels at different time points (before administration, after three doses, and 8 days after discontinuation). Glucocorticoid medication in adult renal transplant patients is associated in a dose-de pendent manner with increased cystatin C, leading to systematic under estimation of GFR. Moreover, our data illustrate the need for specific reference intervals in patients on glucocorticoid therapy. In clinical routine settings, as well as in future clinical studies, it is important to take glucocorticoid medication into account when interpreting serum cystatin C concentrations in renal transplant patients presumably, as well as in other patient groups.

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.

Cystatin C as a filtration marker--haemodialysis patients expose its strengths and limitations

OBJECTIVE

The reliability of serum cystatin C (s-Cys) as a filtration marker depends on the intra- and inter-individual variation and influence of non-renal factors of its production rate (Cys(pr)), non-renal clearance (CL(nr)) and sieving coefficient (S). Haemodialysis patients with no residual renal function would be the best population in which to investigate these variables, which otherwise require reliable GFR measurements.

MATERIAL AND METHODS

Seventy-nine haemodialysis (HD) patients with negligible residual renal function (Group 1) were investigated and compared with 55 HD patients with varying degrees of residual renal function (Group 2) and 923 non-dialysis patients (Group 3). The equation eGFR = Cys(pr)/s-Cys-CL(nr) was used to analyse the turnover and variation of cystatin C.

RESULTS

A formula for estimating GFR, eGFR = 99/s-Cys-14.1, calculated from Group 3, was shown to fit the HD patients. The measured s-Cys in Group 1 was 6.9+/-0.9 and 6.4+/-1.1 mg/L in Group 2. The calculated 95% confidence interval of eGFR of +/-(30-40) % increased sharply below GFR 20 mL/min/1.73 m(2), which means that s-Cys cannot be used for calculating low GFR, including the residual GFR of dialysis patients.

CONCLUSIONS

Nicotine users in Group 1 had significantly higher s-Cys than non-users (7.5+/-0.9 mg/L compared to 6.7+/-0.8; p = 0.0008), which may be a factor to include in the eGFR formulae. However, s-Cys was independent of non-renal factors such as sex, age, LBM, body weight, malnutrition and CRP and also of changes in CRP.

Interruption of antiretroviral therapy is associated with increased plasma cystatin C

BACKGROUND

Cystatin C has been proposed as an alternative marker of renal function. We sought to determine whether participants randomized to episodic use of antiretroviral therapy guided by CD4 cell count (drug conservation) had altered cystatin C levels compared with those randomized to continuous antiretroviral therapy (viral suppression) in the Strategies for Management of Antiretroviral Therapy trial, and to identify factors associated with increased cystatin C.

METHODS

Cystatin C was measured in plasma collected at randomization, 1, 2, 4, 8 and 12 months after randomization in a random sample of 249 and 250 participants in the drug conservation and viral suppression groups, respectively. Logistic regression was used to model the odds of at least 0.15 mg/dl increase in cystatin C (1 SD) in the first month after randomization, adjusting for demographic and clinical characteristics.

RESULTS

At randomization, mean (SD) cystatin C level was 0.99 (0.26 mg/dl) and 1.01 (0.28 mg/dl) in the drug conservation and viral suppression arms, respectively (P = 0.29). In the first month after randomization, 21.8 and 10.6% had at least 0.15 mg/dl increase in cystatin C in the drug conservation and viral suppression arms, respectively (P = 0.0008). The difference in cystatin C between the treatment arms was maintained through 1 year after randomization. After adjustment, participants in the viral suppression arm had significantly reduced odds of at least 0.15 mg/dl increase in cystatin C in the first month (odds ratio 0.42; 95% confidence interval 0.23-0.74, P = 0.0023).

CONCLUSION

These results demonstrate that interruption of antiretroviral therapy is associated with an increase in cystatin C, which may reflect worsened renal function.

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.

Cystatin C and carotid intima-media thickness in asymptomatic adults: the Multi-Ethnic Study of Atherosclerosis (MESA)

BACKGROUND

Persons with early kidney disease have an increased risk of cardiovascular events and mortality, but the importance of accelerated atherosclerosis in promoting these outcomes is unclear. We therefore explored whether serum cystatin C level is associated with carotid intima-media thickness (IMT) in ambulatory adults without clinical heart disease.

STUDY DESIGN

Cross-sectional study.

SETTING & PARTICIPANTS

We evaluated 6,557 ethnically diverse persons free of clinical cardiovascular disease aged 45 to 84 years at the baseline visit of the Multi-Ethnic Study of Atherosclerosis.

PREDICTORS

Kidney function was estimated by using 2 methods: serum cystatin C level and estimated glomerular filtration rate, based on creatinine and cystatin C levels.

OUTCOMES & MEASUREMENTS

Study outcomes were internal and common carotid IMT, measured by using high-resolution B-mode ultrasound. Multivariate linear and logistic regressions were used to evaluate the independent association of kidney function with carotid IMT.

RESULTS

In unadjusted linear analysis, each SD (0.23 mg/L) greater cystatin C level was associated with 0.091-mm greater internal carotid IMT (P < 0.001), but this association was diminished by 70% after adjustment for age, sex, and race/ethnicity (0.027 mm; P < 0.001) and was no longer significant after adjustment for cardiovascular risk factors (0.005 mm; P = 0.5). Similarly, the strong unadjusted associations of cystatin C level with common carotid IMT disappeared after adjustment. Chronic kidney disease, defined by using either creatinine level or cystatin C-based estimated glomerular filtration rate less than 60 mL/min/1.73 m(2), had no independent association with internal and common carotid IMT.

LIMITATIONS

There were few participants with severe kidney disease.

CONCLUSIONS

Cystatin C level had no independent association with carotid IMT in a population free of clinical heart disease. This observation suggests that accelerated atherosclerosis is unlikely to be the primary mechanism explaining the independent association of cystatin C level with cardiovascular risk.

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.

Is serum cystatin-C a reliable marker for metabolic syndrome?

PURPOSE

Chronic kidney disease and metabolic syndrome are recognized as major cardiovascular risk factors. It has been shown that cystatin C has a stronger association with mortality risk than creatinine-based estimations of glomerular filtration rate. We measured cystatin values in dyslipidemic patients and looked for correlations between renal function, cystatin, and metabolic syndrome.

METHODS

There were 925 dyslipidemic patients prospectively included in this cross-sectional study and evaluated over 10 months. Each visit included clinical and biological assessment.

RESULTS

Most patients exhibited cardiovascular risk factors other than dyslipidemia: hypertension in 34%, diabetes in 11%, and smoking in 18%. Mean triglycerides were 149 +/- 136 mg/dL, mean high-density lipoprotein cholesterol 54 +/- 14 mg/dL, and low-density lipoprotein 167 +/- 48 mg/dL. Metabolic syndrome was present in 238 (26%) patients. Plasma creatinine did not differ between control group and metabolic syndrome patients (80 +/- 26 vs 82 +/- 20 micromol/L, respectively, P = .2), but creatinine clearance evaluated by abbreviated Modification of Diet in Renal Disease Study formula was lower in the metabolic syndrome group than in the non-metabolic-syndrome group (83.3 +/- 18.8 mL/min/1.73 m(2) vs 86.8+/-16.9 mL/min/1.73 m(2), respectively, P < .007). Cystatin value was significantly higher in metabolic syndrome patients than in others (0.86 +/- 0.23 vs 0.79 +/- 0.20 mg/L, respectively, P < .0001), independently of serum creatinine level and creatinine clearance. Furthermore, there was a progressive increase in cystatin, as a function of the number of metabolic syndrome components.

CONCLUSIONS

Our study shows that cystatin is associated with metabolic syndrome in dyslipidemic patients. Cystatin may be an interesting marker of metabolic syndrome and of increased cardiovascular and renal risk.

Serum cystatin C in the United States: the Third National Health and Nutrition Examination Survey (NHANES III)

BACKGROUND

Serum cystatin C increasingly is used as a marker of glomerular filtration rate and cardiovascular risk. However, information for serum cystatin C levels in the general population, specifically across a wide age range and different ethnicities, is lacking.

OBJECTIVES

To determine nationally representative serum cystatin C levels, estimate the prevalence of increased cystatin C levels in the general population, and identify factors associated with increased cystatin C levels.

STUDY DESIGN

Cross-sectional survey.

SETTING AND PARTICIPANTS

A nationally representative subsample of 7,596 participants aged 12 years or older in the Third National Health and Nutrition Examination Survey conducted in 1988-1994.

PREDICTORS

Age, sex, race/ethnicity, risk factors for chronic kidney disease.

OUTCOMES

Continuous serum cystatin C levels and serum cystatin C level greater than 1.12 mg/L.

MEASUREMENTS

Cystatin C was measured in 2006 from stored sera by using an automated particle-enhanced nephelometric assay.

RESULTS

Overall median serum cystatin C level was 0.85 mg/L. Median cystatin C levels increased steeply with age and were greater in males and non-Hispanic white persons, even in a healthy subgroup of 20- to 39-year-olds. Prevalences of increased serum cystatin C levels (>1.12 mg/L) were 1%, 41%, and greater than 50% in all persons aged younger than 20 years, 60 years or older, and 80 years or older. In persons aged 60 years and older, older age, non-Hispanic white ethnicity, hypertension, current smoking, lower levels of education and high-density lipoprotein cholesterol, and increased body mass index, C-reactive protein, and triglyceride values were associated significantly with increased serum cystatin C levels.

LIMITATIONS

No measured glomerular filtration rate, single measurement of cystatin C, cross-sectional study design.

CONCLUSIONS

Serum cystatin C level is related to sex and ethnicity, even in young healthy individuals. The prevalence of increased cystatin C levels increases dramatically with age, reaching greater than 50% after the age of 80 years in both sexes and all ethnic groups.

Cystatin C and aging success

BACKGROUND

To our knowledge, the effect of kidney function on successful aging has not been examined.

METHODS

We evaluated the relationship between cystatin C and aging success during a 6-year follow-up in the Cardiovascular Health Study, a community-based cohort of older adults (aged >or= 65 years). Successful aging was defined as remaining free of cardiovascular disease, cancer, and chronic obstructive pulmonary disease and having intact physical and cognitive functioning. In adjusted analysis, an accelerated failure time model was used to evaluate the percentage reduction in successful years by level of cystatin C. A separate Cox proportional hazards model evaluated whether cystatin C was related to incident physical and cognitive disability.

RESULTS

A total of 2140 participants had cystatin C measured and were free of the previously mentioned conditions at baseline. Their mean age was 74 years. The mean cystatin C level, creatinine level, and estimated glomerular filtration rate were 1.06 mg/L, 0.93 mg/dL, and 78 mL/min/1.73 m(2), respectively (to convert cystatin C to nanomoles per liter, multiply by 75; and to convert creatinine to micromoles per liter, multiply by 88.4). A total of 873 participants reached a first event in follow-up, 138 because of cognitive disability, 238 because of physical disability, 34 because of chronic obstructive pulmonary disease, 146 because of cancer, and 317 because of cardiovascular disease. The adjusted percentage reduction in successful life years in the highest vs the lowest quartile of cystatin C was 27% (95% confidence interval, 11%-39%). The highest vs lowest quartile of cystatin C also was independently associated with incident cognitive or physical disability (hazard ratio, 1.39; 95% confidence interval, 1.00-1.98).

CONCLUSION

A higher cystatin C level, even within a range of relatively normal kidney function, was associated with unsuccessful aging.

A new serum cystatin C-based equation for assessing glomerular filtration rate in liver transplantation

BACKGROUND

Cystatin C (CysC) has been identified to be an alternative marker of glomerular filtration rate (GFR), but no CysC-based equation has been developed for patients receiving liver transplantation.

METHODS

Serum CysC and (99m)Tc-DTPA clearance (true GFR) were measured simultaneously on post-transplant days 1, 4 and 7. A new equation was constructed based on an observation group of 30 patients and its predictive ability was compared with three other CysC-based equations (Hoek, Filler and Larsson) based on a validation group of 30 patients.

RESULTS

The new equation for calculating GFR was defined as 19.12+96.21x(1/CysC) and the derived GFR was estimated at 97.4+/-30.2 mL/min/1.73 m(2) and was close to the true GFR (96.8+/-32.8 mL/min/1.73 m(2)). Estimates of GFR by Hoek, Filler and Larsson formulas (61.4+/-25.4, 73.8+/-31.9 and 61.3+/-29.6mL/min/1.73 m(2), respectively) differed significantly from the true GFR. Correlation between the true GFR and all formulas showed no significant difference. Bias was neglectable for the new equation (mean difference: 0.6 mL/min/1.73 m(2)) but remarkable for the other three equations (mean difference: -22.9 to -35.4 mL/min/1.73 m(2)). Accuracy within 10%, 30% and 50% of the true GFR for the new equation (30.0%, 76.7% and 93.3%) was significantly higher than those of the other three equations (p<0.001 for all).

CONCLUSIONS

A new serum CysC-based equation was established in this study and it was shown to be accurate in estimating GFR after liver transplantation, compared to the formulas of Hoek, Filler and Larsson.

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.

Renal function--estimation of glomerular filtration rate

Assessment and follow-up of renal dysfunction is important in the early detection and management of chronic kidney disease. The glomerular filtration rate (GFR) is the most accurate measurement of kidney disease and is reduced before the onset of clinical symptoms. Drawbacks to the measurement of GFR include the high cost and incompatibility with routine laboratory monitoring. Serum creatinine determination is a mainstay in the routine laboratory profile of renal function. The measurement of serum cystatin C has been proposed as a more sensitive marker for GFR. According to National Kidney Foundation-K/DOQ1 clinical guidelines for chronic kidney disease, serum markers should not be used alone to assess GFR. Based on prediction equations, clinical laboratories should report an estimate of GFR, in addition to reporting the serum value. In this article, information is presented on how best to estimate GFR using prediction equations for adults and for children. Using serum creatinine concentration with the Modification of Diet in Renal Disease (MDRD) study equation offers a suitable estimation of GFR in adults. The cystatin C prediction equation with the use of a prepubertal factor seems superior to creatinine-based prediction equations in children of <14 years.

Urinary Cystatin C as a Marker of Tubular Dysfunction

Cystatin C (CysC) is a nonglycosylated 13 KD protein that belongs to the type II cystatin gene family. It is a strong inhibitor of cysteine proteinases, freely filtered by the kidney glomerulus and reabsorbed by the tubulus, where it is almost totally catabolized. Remainder of the nonmetabolized CysC is eliminated in urine and may represent a useful marker of tubular dysfunction. The aim of the study was to confirm the clinical importance of the quantitative determination of CysC by an automated immunonephelometric method (DADE Behring). Two groups of patients were examined: one with glomerular (GD, n=36) and one with tubular dysfunction (TD, n=31), and compared with the control group (CG, n=31) of healthy males and females from laboratory personnel (n=11) and patients on routine systematic examination (n=20), from 25 to 58 years old. The patient groups were categorised according to the urine analysis of total proteins, creatinine and adequate proteins electrophoretic panel. CysC concentration in CG was in the range of 0.02-0.15 mg/L; 0.01-0.48 mg/L and 0.25-18 mg/L in GD and TD groups respectively. Values of means ± SD for patient groups (GD=0.11 ± 0.14; TD=3.92 ± 3.75 mg/L) showed statistical significance (p<0.001) in the TD group in relation to GD and CG groups. It confirms that quantitative determination of CysC in one urine portion, with a fast laboratory method, might be a useful marker of tubular dysfunction, especially in emergency situations, taking into account that there is no interference of circadian variation on its concentration.

Use of serum cystatin C to detect early decline of glomerular filtration rate in type 2 diabetes

OBJECT

The estimation of serum cystatin C and its practical use for the estimation of the glomerular filtration rate (GFR) in diabetic patients has been previously demonstrated, however, those studies did not use the chronic kidney disease GFR staging. Therefore, we performed this study in type 2 diabetic patients with the aim to examine the usefulness of serum cystatin C to detect early decline of GFR using the staging of chronic kidney disease defined by the National Kidney Foundation.

METHODS

A total of 102 Taiwanese type 2 diabetic patients were recruited from the Chung-Shan Medical University Hospital. Morning fasting blood and urine samples were obtained for basal metabolic parameters, serum creatinine, serum cystatin C, and albumin-creatinine ratio. GFR was determined by Cockcroft-Gault equation creatinine clearance (CG-CCr).

RESULTS

Of the 102 type 2 diabetic patients, 67, 25, and 10 had normo-, micro-, and macroalbuminuria, respectively. Serum cystatin C was superior to serum creatinine in detecting early decline of GFR. The diagnostic accuracy of serum cystatin C was better than serum creatinine for stage 1 and 2 chronic kidney disease (CG-CCr cut-off value of 90 ml/min and 60 ml/min). Furthermore, serum cystatin C was also correlated with urine albumin excretion, which was not true with serum creatinine.

CONCLUSIONS

These results suggest that serum cystatin C may be an alternative serum marker for the early identification of subjects with a slight reduction of renal function, and also it may be a marker for early glomerular dysfunction in type 2 diabetes.

Urinary cystatin C as a specific marker of tubular dysfunction

BACKGROUND

Cystatin C (CST3), a strong inhibitor of cysteine proteinases, is freely filtered by the kidney glomerulus and is reabsorbed by the tubules, where it is almost totally catabolized, with the remainder then eliminated in urine. In tubular diseases, it seems sensible to postulate that CST3 degradation would be reduced and consequently an increase in its urinary elimination would be observed.

METHODS

We report here the development of an automatic quantitative assay to measure CST3 concentrations in urine using a Behring N-Latex Cystatin C kit on a BNII laser nephelometer. We tested its clinical relevance on several kidney disease patients.

RESULTS

This assay is sensitive (limit of detection 0.008 mg/L) and precise (within- and between-day CVs < 4%). Reference values for freshly collected urine samples range from 0.03 to 0.18 mg/L. Mean urine CST3 concentrations obtained from 52 patients with kidney tubular disease (4.31 +/- 3.85 mg/L) were significantly higher than those for 60 controls (0.096 +/- 0.044 mg/L; p < 0.0001) and 47 glomerular disease patients (0.106 +/- 0.133 mg/L; p < 0.0001).

CONCLUSION

Increased urinary CST3 concentrations allow the accurate detection of tubular dysfunction among pure and mixed nephropathies. Because of its ability to be processed on automated clinical chemistry analyzers, this assay could easily be used as an adjunct to the standard panel used to screen kidney pathologies, even in emergency situations.

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.

Argatroban therapy in heparin-induced thrombocytopenia with hepatic dysfunction

STUDY OBJECTIVES

We evaluated the dosing requirements in argatroban-treated patients with heparin-induced thrombocytopenia (HIT) and hepatic dysfunction, and compared efficacy and safety outcomes with historical control patients.

DESIGN

Retrospective analysis.

SETTING

Inpatient setting.

PATIENTS

Patients with hepatic dysfunction, defined as total bilirubin > 25.5 micromol/L (1.5 mg/dL), aspartate aminotransferase >100 IU/L, and/or alanine aminotransferase >100 IU/L, were identified from previous multicenter, historical-controlled studies of argatroban therapy in HIT.

INTERVENTIONS

Argatroban, adjusted to maintain activated partial thromboplastin times (aPTTs) 1.5 to 3 times baseline in the experimental group, vs no direct thrombin inhibition in the historical control patients.

MEASUREMENTS AND RESULTS

The analysis population included 82 argatroban patients and 34 historical control patients with hepatic impairment, of whom approximately 50% in each group had renal dysfunction (defined as a serum creatinine level > 1.3 mg/dL). The argatroban dosage was 1.6 +/- 1.1 microg/kg/min (mean +/- SD) over a mean 5-day course of therapy. Significantly lower doses were used in patients with elevated vs normal total bilirubin levels (0.8 +/- 0.6 microg/kg/min vs 1.7 +/- 0.8 microg/kg/min, p = 0.0063) and in patients with hepatic/renal dysfunction vs hepatic dysfunction alone (1.2 +/- 1.1 microg/kg/min vs 2.0 +/- 1.1 microg/kg/min, p < 0.001). The aPTT 24 h after argatroban initiation was 69 +/- 22 s, with 80% of patients having a therapeutic level of anticoagulation. Thirty-four argatroban-treated patients (41.5%) and 17 control patients (50.0%) experienced the 37-day composite end point of death, amputation, or new thrombosis (p = 0.32). Argatroban significantly reduced new thrombosis (8.5% vs 26.5%, p = 0.012). Major bleeding was similar between treatment groups (4.9% vs 2.9%, p = 0.684).

CONCLUSIONS

Hepatic dysfunction affects argatroban dosing, with reduced doses required particularly in patients with serum total bilirubin levels > 25.5 micromol/L (1.5 mg/dL) or combined hepatic/renal dysfunction. Individual mean aPTT-adjusted doses typically remain > or = 0.5 microg/kg/min, supporting the recommendation of 0.5 microg/kg/min as a conservative initial dose for most patients with hepatic impairment. Argatroban, with proper initial dosing and monitoring, can provide safe and effective antithrombotic therapy in patients with HIT and hepatic impairment.

Glomerular filtration rate estimated by cystatin C among different clinical presentations

Glomerular filtration rate (GFR) estimates from serum creatinine has not been generalizable across all populations. Cystatin C has been proposed as an alternative marker for estimating GFR. The objective of this study was to compare cystatin C with serum creatinine for estimating GFR among different clinical presentations. Cystatin C and serum creatinine levels were obtained from adult patients (n=460) during an evaluation that included a GFR measurement by iothalamate clearance. Medical records were abstracted for clinical presentation (healthy, native chronic kidney disease or transplant recipient) at the time of GFR measurement. GFR was modeled using the following variables: cystatin C (or serum creatinine), age, gender and clinical presentation. The relationship between cystatin C and GFR differed across clinical presentations. At the same cystatin C level, GFR was 19% higher in transplant recipients than in patients with native kidney disease (P<0.001). The association between cystatin C and GFR was stronger among native kidney disease patients than in healthy persons (P<0.001 for statistical interaction). Thus, a cystatin C equation was derived using only patients with native kidney disease (n=204). The correlation with GFR (r(2)=0.853) was slightly higher than a serum creatinine equation using the same sample (r(2)=0.827), the Modification of Diet in Renal Disease equation (r(2)=0.825) or the Cockcroft-Gault equation (r(2)=0.796). Averaged estimates between cystatin C and serum creatinine equations further improved correlation (r(2)=0.891). Cystatin C should not be interpreted as purely a marker of GFR. Other factors, possibly inflammation or immunosuppression therapy, affect cystatin C levels. While recognizing this limitation, cystatin C may improve GFR estimates in chronic kidney disease patients.

Improved estimation of GFR by serum cystatin C in patients undergoing cardiac catheterization

Clinical assessment of glomerular filtration rate (GFR) mainly relies on single determinations of serum creatinine (crea) which is commonly insensitive to mild renal dysfunction. Serum cystatin C (cysC) has been proposed as an alternative endogenous marker of GFR showing higher correlation to standard clearance methods such as inulin or iohexol clearance. We compared serum crea and cysC levels in n=127 patients undergoing cardiac catheterization. The clearance of the iodinated contrast dye iopromide served as reference method for GFR. Serum cysC was determined by a particle-enhanced immunonephelometric method. CysC showed higher non-parametric correlation (r=0.805) to the iopromide clearance compared to crea (r=0.652) and to the estimated GFR according to the Cockcroft-Gault formula (r=0.690), which underestimated true GFR systematically. Receiver operating curves revealed a greater area-under-the-curve (AUC) for cysC (0.957 vs. 0.801, p<0.05). At a cut-off level of >1.3 mg/l cysC exhibited an 88% sensitivity and a 96% specificity for detecting renal dysfunction which was defined as an iopromide clearance less than 80 ml/min/1.73 m2; best values for crea were 63% for sensitivity and 80% for specificity at a cut-off of >1.2 mg/dl. In conclusion, cysC detected reduced GFR more reliably and at an earlier stage in patients undergoing cardiac catheterization allowing a better identification of patients with renal dysfunction and those at risk for contrast damage.

Serum cystatin C concentration as a marker of acute renal dysfunction in critically ill patients

Introduction

In critically ill patients sudden changes in glomerular filtration rate (GFR) are not instantly followed by parallel changes in serum creatinine. The aim of the present study was to analyze the utility of serum cystatin C as a marker of renal function in these patients.

Methods

Serum creatinine, serum cystatin C and 24-hour creatinine clearance (C_Cr) were determined in 50 critically ill patients (age 21–86 years; mean Acute Physiology and Chronic Health Evaluation II score 20 ± 9). They did not have chronic renal failure but were at risk for developing renal dysfunction. Serum cystatin C was measured using particle enhanced immunonephelometry. Twenty-four-hour body surface adjusted C_Cr was used as a control because it is the 'gold standard' for determining GFR.

Results

Serum creatinine, serum cystatin C and C_Cr (mean ± standard deviation [range]) were 1.00 ± 0.85 mg/dl (0.40–5.61 mg/dl), 1.19 ± 0.79 mg/l (0.49–4.70 mg/l), and 92.74 ± 52.74 ml/min per 1.73 m² (8.17–233.21 ml/min per 1.73 m²), respectively. Our data showed that serum cystatin C correlated better with GFR than did creatinine (1/cystatin C versus C_Cr: r = 0.832, P < 0.001; 1/creatinine versus C_Cr: r = 0.426, P = 0.002). Cystatin C was diagnostically superior to creatinine (area under the curve [AUC] for cystatin C 0.927, 95% confidence interval 86.1–99.4; AUC for creatinine 0.694, 95% confidence interval 54.1–84.6). Half of the patients had acute renal dysfunction. Only five (20%) of these 25 patients had elevated serum creatinine, whereas 76% had elevated serum cystatin C levels (P = 0.032).

Conclusion

Cystatin C is an accurate marker of subtle changes in GFR, and it may be superior to creatinine when assessing this parameter in clinical practice in critically ill patients.

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.

Serum Cistatin C in patients with delayed graft function

Despite recent studies showing that serum Cystatin C(CysC) is a better marker for glomerular filtration rate (GFR) than the ubiquitously used creatinine, the clinical utility of this remains to be evaluated. This marker is very sensitive for alograft function after renal transplantation. Concentration of CysC was compared with that of creatinine. Decreased renal function was followed in 64 transplanted patients. Plasma CysC significantly correlated (r=0.625, p<0.001) with creatinine in healthy controls. In these patients the mean plasma creatinine and Cystatin C concentrations were: 81+/-13 mmol/L,0.90 +/-0.22 mg/L, respectively. Plasma Cystatin C and creatinine significantly correlated throughout the post-transplantation period (r=0.686, p<0.001), but we confirmed differences between kinetics of these parameters. In the first four days after transplantation the CysC concentration was normalized faster than creatinine concentration. Development of acute rejection episode ( between 5 and 7 days) showed high sensitivity and specificity of the changes of CysC compared with those of creatinine.

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.

Factors influencing serum cystatin C levels other than renal function and the impact on renal function measurement

BACKGROUND

It is well known that serum creatinine may be used as a marker of renal function only if taking into account factors that influence creatinine production, such as age, gender, and weight. Serum cystatin C has been proposed as a potentially superior marker than serum creatinine, because serum cystatin C level is believed to be produced at a constant rate and not to be affected by such factors. However, there are limited data on factors that may influence serum cystatin C levels, and there are limited data comparing cystatin C-based estimates of renal function with creatinine-based estimates that adjust for such factors, especially in individuals with normal, or mildly reduced, renal function.

METHODS

This was a cross-sectional study of 8058 inhabitants of the city of Groningen, The Netherlands, 28 to 75 years of age. Serum cystatin C and serum creatinine levels were measured, and creatinine clearance was determined from the average of two separate 24-hour urine collections. We performed multivariate analyses to identify factors independently associated with serum cystatin C levels after adjusting for creatinine clearance. Then, partial Spearman correlations were obtained after adjusting for factors that may influence serum cystatin C and creatinine levels. We also compared the goodness-of-fit (R(2)) of different multivariate linear regression models including serum cystatin C level and serum creatinine level for the outcome of creatinine clearance.

RESULTS

Older age, male gender, greater weight, greater height, current cigarette smoking, and higher serum C-reactive protein (CRP) levels were independently associated with higher serum cystatin C levels after adjusting for creatinine clearance. After adjusting for age, weight, and gender, the partial Spearman correlations between creatinine and, respectively, serum cystatin C level and serum creatinine level were -0.29 (P < 0.001) and -0.42 (P < 0.001), respectively. The R(2) values for serum cystatin C level and serum creatinine level adjusted for age, weight, and gender were 0.38 and 0.42, respectively. The addition of cigarette smoking and serum CRP levels did not improve the R(2) value for the multivariate serum cystatin C-based model.

CONCLUSION

Serum cystatin C appears to be influenced by factors other than renal function alone. In addition, we found no evidence that multivariate serum cystatin C-based estimates of renal function are superior to multivariate serum creatinine-based estimates.

Biochemical indicators of cardiac and renal function in a healthy elderly population

OBJECTIVES

To examine the distributions of NT-proBNP and cystatin C and their relation to age, gender, and other physiological factors in an apparently healthy elderly population.

METHOD

NT-proBNP and cystatin C were analyzed in 407 and 408 healthy individuals, median age: 65 (range 40-76).

RESULTS

Increasing age, female gender and CRP were independently associated to higher NT-proBNP levels. Age, body mass index, and CRP level were independently associated to the cystatin C level. In women and men, < or =65 years, the 97.5th percentile value for NT-proBNP was 268 ng/l and 184 ng/l, in those older, 391 ng/l and 269 ng/l. For those < or =65 years the 97.5th percentile value for cystatin C was 1.12 mg/l, and for those older 1.21 mg/l.

CONCLUSION

In a healthy elderly population, NT-proBNP is influenced by age and gender, whereas cystatin C is influenced by age but not by gender. Both markers seem to be associated to the CRP level.

Serum cystatin C assay for the detection of early renal impairment in diabetic patients

The ability to assess renal function in diabetes patients rapidly and early is of major importance. This study was designed to determine whether cystatin C can replace serum creatinine as the screening marker for reduced glomerular filtration rate (GFR) in type 2 diabetes patients. The study was performed on 51 type 2 diabetic patients. GFR was estimated by the plasma clearance of (99m)Tc-DTPA. The correlation between (99m)Tc-DTPA clearance and levels of serum cystatin C, serum creatinine, and creatinine clearance was determined. Sensitivity and specificity for the diagnosis of renal impairment (defined as GFR<68 ml/min) were calculated by a receiver operating characteristic (ROC) curve for serum cystatin C, serum creatinine, and creatinine clearance. The correlation coefficients with (99m)Tc-DTPA clearance were -0.744 for serum cystatin C, -0.658 for serum creatinine, and +0.625 for creatinine clearance (P<0.001). With a cutoff value of 68 mL/min, the area under the ROC curve (AUC) was 0.891 for cystatin C, 0.77 for creatinine, and 0.753 for creatinine clearance. The AUC was statistically different between serum cystatin C and creatinine clearance (P<0.05). The ROC plot indicates that cystatin C is superior to serum creatinine and creatinine clearance for detecting impaired GFR. Serum cystatin C appropriately reflects GFR in diabetes, and is more efficacious than serum creatinine and creatinine clearance in detecting reduced GFR in type 2 diabetes patients.

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.

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.

Comparative performance of serum cystatin-c versus serum creatinine in diabetic subjects

OBJECTIVE

Serum cystatin-C, a protein with constant production rate, undergoes glomerular filtration. Cystatin-C is a candidate surrogate marker, allegedly superior to serum creatinine, for estimating glomerular filtration rate, due to its high correlation with absolute measurement of the latter. The aim of this study was to assess from intra- and inter-subject variability the performance of cystatin-C in a cohort of diabetic patients spanning a wide range of kidney function and to compare it to that of serum creatinine.

RESEARCH DESIGN AND METHODS

98 consecutive diabetic in-patients (45 type 1 and 53 type 2) were included. Mean age was 53 +/- 15 years (1SD). Creatinine clearance was 98 ml/min (median; range: 16-244). We used the discriminant ratio (DR) methodology to compare the performance of serum cystatin-C vs. that of creatinine to segregate subjects according to their glomerular filtration rate.

RESULTS

Serum creatinine values on day 1 and 2 were 1.10 +/- 0.76 and 1.07 +/- 0.89 mg/dl and concentrations of cystatin-C were 1.10 +/- 0.60 and 1.06 +/- 0.63 mg/L. A close linear relationship was observed between means of duplicates for creatinine and cystatin-C (Pearson product-moment correlation 0.92). DR was obtained from the ratio of the underlying between-subject to the within-subject standard deviations. DR values were 5.23 for creatinine and 8.82 for cystatin-C (P<0.0001), implying superior discriminating ability for cystatin-C. Once adjusted for attenuation, measured Pearson product-moment correlation rose from 0.92 to 0.97. The DR methodology allowed for deriving an unbiased linear regression equation between methods, with slope and intercept at 0.79 and 0.23, respectively.

CONCLUSIONS

Serum cystatin-C better discriminates among a population of type 1 and 2 diabetic patients with regard to their estimated glomerular filtration rate when compared with conventional serum creatinine measurement.

Evaluation of cystatin C and beta-2 microglobulin as markers of renal function in patients with type 2 diabetes mellitus

BACKGROUND

Despite recent studies showing that serum cystatin C (CC) is a better marker for GFR than the ubiquitously used serum creatinine, its clinical utility remains under evaluation.

METHODS

To evaluate their usefulness in patients with type 2 diabetes mellitus (DM), serum concentrations of CC, beta-2 microglobulin (B2M) and creatinine were measured in 105 (38 males, 67 females) Kuwaiti patients with type 2 DM. The results were compared with creatinine clearance (Ccr), which was measured (mCcr) and estimated (eCcr) with the Cockroft-Gault formula, and correlated with 24-h urine protein and early morning urine albumin/creatinine excretion ratio.

RESULTS

In patients with eCcr and mCcr results (n=51), eCcr and mCcr showed significant correlation with each other (r's=.86, P<.0001) with no significant difference between the two. In all patients (n=105), CC and B2M showed significant correlation with each other (r's=.82, P<.0001) and with serum creatinine concentration (r's=.77 and.84, respectively, P<.0001). Serum CC, B2M and creatinine showed significant (P<.001) inverse correlation with eCcr (r's=-.63, -.61 and -.76, respectively). Partial correlations after correcting for age and sex improved the correlation of serum creatinine with eCcr (r=-.81, P<.0001), but there was no significant change in the correlations of CC and B2M with eCcr (r=-.65, P<.0001 and r=-.62, P<.0001, respectively). Receiver operating characteristic (ROC) plots for serum CC, B2M and creatinine for detection of changes in the eCcr showed that the area under the ROC curve+/-S.E. is 0.897+/-0.119 for CC, 0.871+/-0.091 for B2M and 0.785+/-0.087 for serum creatinine. There was no statistically significant difference between the areas under the curve (AUC) for serum creatinine and CC (P=.07) and B2M (P=.12). CC had the highest sensitivity for detection of eCcr (<60 ml/min/1.73 m(2)) at routinely used cutoff values. CC was also the best discriminator when patients with normoalbuminuria were compared with patients with microalbuminuria.

CONCLUSION

Although there is no significant difference in the overall diagnostic accuracies of CC, B2M and creatinine for the detection of changes in the GFR, CC is the most sensitive marker at routinely used cutoff values and would be more clinically useful than B2M or serum creatinine in diabetic patients.