Mats Flodin

Generation of a New Cystatin C–Based Estimating Equation for Glomerular Filtration Rate by Use of 7 Assays Standardized to the International Calibrator

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

Evaluation of Gentian cystatin C reagent on Abbott Ci8200 and calculation of glomerular filtration rate expressed in mL/min/1.73 m(2) from the cystatin C values in mg/L

Objective. Estimation of the glomerular filtration rate (GFR) is essential when evaluating patients with kidney disease and treating patients with drugs eliminated from the circulation by the kidneys. Cystatin C has been shown in several studies to be superior to creatinine in the estimation of GFR. At our hospitals, there is an increasing demand for cystatin C and at present we perform approximately 1500 cystatin C analyses a month. We thus need the assay available 24 h/day and to have it on our routine chemistry instrument to minimize handling time per test and time to reported test results. Material and methods. We have evaluated a new cystatin C immunoassay from Gentian (Gentian, Moss, Norway) on Architect ci8200 (Abbott Laboratories, Abbott Park, Ill., USA). A prerequisite at our hospital is that cystatin C results are reported as a calculated GFR in mL/min/1.73 m2, so we also made a comparison with iohexol clearance. Results. The Gentian cystatin C assay showed good agreement with the corresponding assay from Dade Behring (Deerfield, Ill., USA) and good inter‐laboratory concordance. The assay has very low total imprecision, good linearity and strong correlation with iohexol clearance (R2 = 0.956). The equation for the correlation curve is: y = 79.901x−1.4389. Conclusions. There was low inter‐laboratory variation between the three laboratories involved in the cystatin C evaluation, and thus all three laboratories can use the same equation for calculating the estimated GFR.

The revised Lund-Malmo GFR estimating equation outperforms MDRD and CKD-EPI across GFR, age and BMI intervals in a large Swedish population

Abstract Background: The performance of creatinine-based glomerular filtration rate (GFR) estimating equations may vary in subgroups defined by GFR, age and body mass index (BMI). This study compares the performance of the Modification of Diet in Renal Disease (MDRD) study and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations with the revised Lund-Malmö equation (LM Revised), a new equation that can be expected to handle changes in GFR across the life span more accurately. Methods: The study included 3495 examinations in 2847 adult Swedish patients referred for measurement of GFR (mGFR) 2008–2010 by plasma clearance of iohexol (median 52 mL/min/1.73 m2). Bias, precision [interquartile range (IQR)] and accuracy [percentage of estimates ±10% (P10) and ±30% (P30) of mGFR] were compared. Results: The overall results of LM Revised/MDRD/CKD-EPI were: median bias 2%/8%/11%, IQR 12/14/14 mL/min/1.73 m2, P10 40%/35%/35% and P30 84%/75%/76%. LM Revised was the most stable equation in terms of bias, precision and accuracy across mGFR, age and BMI intervals irrespective of gender. MDRD and CKD-EPI overestimated mGFR in patients with decreased kidney function, young adults and elderly. All three equations overestimated mGFR and had low accuracy in patients with BMI <20 kg/m2, most pronounced among men. Conclusions: In settings similar to the investigated cohort LM Revised should be preferred to MDRD and CKD-EPI due to its higher accuracy and more stable performance across GFR, age and BMI intervals.

Early and late outcome prediction of death in the emergency room setting by point-of-care and laboratory assays of cardiac troponin I

BACKGROUND Point-of-care (POC) assays of cardiac troponins are common in the emergency department setting. The question raised was as follows: What is the clinical impact of the results of POC assays of cardiac troponins as compared with sensitive laboratory assays? METHODS Patients admitted consecutively to the emergency department (N = 1,069) and on whom cardiac troponins were requested as part of their clinical work-up were included. Cardiac troponin I (cTnI) was measured by the POC assays-i-Stat (Abbott Diagnostics, Abbott Park, IL) and Stratus CS (Siemens Healthcare Diagnostics, Deerfield, IL)-and by the laboratory assays-Access AccuTnI (Beckman Coulter, Fullerton, CA) and Architect cTnI (Abbott Diagnostics). Results were related to early (14 days) and late outcome (median 3.3 months, range 0.1-35) as to death. RESULTS The laboratory assays identified more patients (P < .001) with elevated levels than the two POC assays (39%-74% vs 20%-27%). Adopting the 99th percentiles upper reference limit, the Access AccuTnI identified 88% and Architect cTnI identified 81% of all patients who died of cardiovascular disease as compared with 50% and 54% for i-Stat and Stratus CS, respectively (P < .001). Negative predictive values for the laboratory assays were 97% as compared with 89% to 93% for the POC assays. Negative likelihood ratios were 0.25 (CI 0.15-0.041) and 0.59 to 0.68 (CI 0.47-0.79), respectively. CONCLUSIONS The current POC cTnI assays are less sensitive for outcome prediction of patients with myocardial injury. The clinical judgment of the patient with suspected myocardial ischemia should not solely rely on results from POC assays. If a clinical suspicion of myocardial injury remains despite negative cTnI results with the POC assays, such results should be complemented by results from sensitive laboratory assays.

Accuracy of GFR estimating equations combining standardized cystatin C and creatinine assays: a cross-sectional study in Sweden

Abstract Background: The recently established international cystatin C calibrator makes it possible to develop non-laboratory specific glomerular filtration rate (GFR) estimating (eGFR) equations. This study compares the performance of the arithmetic mean of the revised Lund-Malmö creatinine and CAPA cystatin C equations (MEANLM-REV+CAPA), the arithmetic mean of the Chronic Kidney Disease Epidemiology Collaboration equation (CKD-EPI) creatinine and cystatin C equations (MEANCKD-EPI), and the composite CKD-EPI equation (CKD-EPICREA+CYSC) with the corresponding single marker equations using internationally standardized calibrators for both cystatin C and creatinine. Methods: The study included 1200 examinations in 1112 adult Swedish patients referred for measurement of GFR (mGFR) 2008–2010 by plasma clearance of iohexol (median 51 mL/min/1.73 m2). Bias, precision (interquartile range, IQR) and accuracy (percentage of estimates ±30% of mGFR; P30) were compared. Results: Combined marker equations were unbiased and had higher precision and accuracy than single marker equations. Overall results of MEANLM-REV+CAPA/MEANCKD-EPI/CKD-EPICREA+CYSC were: median bias –2.2%/–0.5%/–1.6%, IQR 9.2/9.2/8.8 mL/min/1.73 m2, and P30 91.3%/91.0%/91.1%. The P30 figures were about 7–14 percentage points higher than the single marker equations. The combined equations also had a more stable performance across mGFR, age and BMI intervals, generally with P30 ≥90% and never <80%. Combined equations reached P30 of 95% when the difference between eGFRCREA and eGFRCYSC was <10% but decreased to 82% at a difference of ≥40%. Conclusions: Combining cystatin C and creatinine assays improves GFR estimations with P30 ≥90% in adults. Reporting estimates of both single and combined marker equations in clinical settings makes it possible to assess the validity of the combined equation based on the agreement between the single marker equations.

Calibration of the Siemens cystatin C immunoassay has changed over time.

To the Editor: The Siemens cystatin C immunoassay has been widely used in clinical research, particularly in the US. In recent years, however, the results obtained with the method appear to have changed. The glomerular filtration rate (GFR) is generally accepted as the best overall indicator of kidney function and is an important measure for assessing kidney disease. Several studies have shown cystatin C to be superior to creatinine for estimation of the GFR (1), which is usually expressed as the relative GFR [in units of mL · min−1 · (1.73 m2)−1]. This practice has led to the development of formulas to convert cystatin C measurements in milligrams per liter to a calculated GFR in these units, without the need for demographic coefficients (2, 3). The formulas were developed from studies that compared cystatin C concentrations with measured GFRs by using such exogenous markers as iohexol, diethylenetriamine pentaacetic acid or 51Cr-EDTA clearance (4). We were concerned that the calibration of the Siemens cystatin C method had changed during the last 5 years, because we noted that the cystatin C concentrations of participants in a longitudinal cohort improved substantially …

Estimated glomerular filtration rate (eGFRCystC) from serum cystatin C shows strong agreement with iohexol clearance in patients with low GFR

Objective. Estimation of glomerular filtration rate (eGFR) is essential in the diagnosis and monitoring of patients with kidney disease and for correct dosage of drugs eliminated from the circulation by the kidneys. Cystatin C has been shown in several studies to be superior to creatinine in estimating eGFR. However, there are few studies on the performance of cystatin C estimated eGFR (eGFRCystC) in patients with advanced kidney disease and low GFR. Material and methods. We measured serum cystatin C, together with serum creatinine, during iohexol clearance in patients with iohexol clearance below 30 mL/min/1.73 m2. The cystatin C values were used to calculate eGFRCystC using the formula eGFR (mL/min/1.73m2) = 79.901*(cystatin C value in mg/L)−1.4389. Results. There was good correlation between eGFRCystC and iohexol clearance (r = 0.88) in patients with iohexol clearance <30 mL/min/1.73 m2 and none of the patients had a difference between eGFRCystC and iohexol clearance exceeding 50 %. The Modification of Diet in Renal Disease (MDRD) equation and corrected MDRD eGFR showed a positive bias and weaker correlations with iohexol eGFR (MDRD = 5.32+1.22*iohexol clearance; corrected MDRD = 4.76+1.10*iohexol clearance; r = 0.59). For MDRD eGFR, 42 of 94 (44.7 %) samples showed more than 50 % difference to iohexol clearance. Conclusions. eGFRCystC is an efficient, practical and cost‐effective alternative to iohexol clearance in patients with reduced GFR.

Variations in assay protocol for the Dako cystatin C method may change patient results by 50% without changing the results for controls

Abstract Background: Cystatin C is increasingly used as a glomerular filtration marker, but so far only a few companies produce most of the cystatin C reagents suited for turbidimetry or nephelometry use in clinical laboratories. Methods: We studied different protocols for measuring cystatin C on an Architect ci8200 system using cystatin C reagents from Dako (Glostrup, Denmark). The results were compared with those obtained with Dade Behring reagents (Deerfield, IL, USA) on a BN ProSpec system. Results: Differences in assay protocol on the same instrument with the Dako reagent yielded an up to 50% difference in glomerular filtration rate calculated from the cystatin C results when analyzing patient samples, but had no effect on the results for controls. There were also significant differences regarding linearity and kinetics between samples and controls/calibrators. Conclusions: The results indicate different reactivity of the Dako antibodies against calibrators and controls in comparison with patient samples, highlighting the importance of using controls and calibrators that do not differ from patient samples. Clin Chem Lab Med 2006;44:1481–5.

Increased Serum Concentrations of Carbohydrate-Deficient Transferrin (CDT) in Patients with Cystic Fibrosis

Carbohydrate-deficient transferrin (CDT) has been reported to be one of the best laboratory markers in serum (S) for detection of alcohol abuse. We have studied S-CDT values in cystic fibrosis (CF) patients and show that CF patients have increased S-CDT values without high alcohol consumption. CF patients have abnormalities in their protein glycosylation and sialylation, which may explain the increased S-CDT values.

Performance Characteristics of a Cystatin C Immunoassay with Avian Antibodies

Background: A new particle-enhanced turbidimetric immunoassay (PETIA) with avian antibodies for measuring serum/plasma cystatin C has been developed. The performance characteristics of the assay are described. Methods: Measurements were performed on a Roche Modular P and on an Abbott Architect ci8200 using Gentian cystatin C immunoassay. Results: Measuring range was 0.3–8.0 mg/L. Reference range was 0.57–1.09 mg/L. Total analysis time was 10 minutes. Linearity was absolute over the whole assay range. Recovery of samples and controls was within 98.6–109.4%. Total imparticle enhanced nephelometric cystatin C immunoassay (PENIA) by linear regression resulted in a slope within 0.97–1.02 and intercept within ±0.05 mg/L. Interference studies with drugs, anticoagulants, intralipid ( 11g/L), triglycerides ( 14 g/L) and bilirubin (420mg/L) antibodies, no interference with rheumatoid factor was observed. No carry-over was 6%) were both below 0.33 mg/L, which is less than the lowest standard. Sample stability was up to one month at 2–8°C. Stability of the reagents at 2–8°C was estimated to be 24 months. Stability of the reagents in use was minimum 9 weeks. Conclusions: Gentian cystatin C PETIA is shown to have excellent performance between methods. Interference results are improved due to avian antibodies and a broader span of the calibration curve. Avian antibodies are also known to have better immune response than mammalian antibodies towards mammalian antigens.