Vincent Delatour

A multicentric evaluation of IDMS-traceable creatinine enzymatic assays.

Chronic kidney disease definition is based on glomerular filtration rate (GFR) estimations which are derived from creatinine-based equations. The accuracy of GFR estimation is thus largely dependent of those of serum creatinine assays. International recommendations highlight the need for traceable creatinine assays. The French Society of Clinical Biochemistry conducted a study for measuring accuracy of creatinine enzymatic methods. This evaluation involved 25 clinical laboratories. Creatinine was measured in serum pools ranging from 35.9±0.9 μmol/L to 174.5±3.1 μmol/L (IDMS determination) using 12 creatinine enzymatic methods. For all creatinine values greater than 74.4±1.4 μmol/L, the bias and imprecision did not exceed 5% and 5.9%, respectively. For the lowest value (35.9±0.9 μmol/L), the bias ranged from -1.8 to 9.9% (with one exception). At this level, the imprecision ranged from 1.9 to 7.8%. The true performances of the assays (couples of bias and relative standard deviation), were evaluated using Monte-Carlo simulations. Most of the assays fall within the maximum Total Error of 12% at all concentrations. This study demonstrates substantial improvements in the calibration, traceability and precision of the enzymatic methods, reaching the NKDEP recommendations. Moreover, most of these assays allowed accurate creatinine measurements for creatinine levels lower than 40 μmol/L.

Enzymatic but not compensated Jaffe methods reach the desirable specifications of NKDEP at normal levels of creatinine. Results of the French multicentric evaluation

The French Society of Clinical Biochemistry conducted this study to compare the accuracy and performances of the best creatinine enzymatic assays and the compensated Jaffe methods from the same manufacturers. Creatinine was measured in 3 serum pools with creatinine levels of 35.9±0.9 μmol/L, 74.4±1.4 μmol/L, and 97.9±1.7 μmol/L (IDMS determination). The performances of the assays (total error that includes the contribution of bias and imprecision) were evaluated using Monte-Carlo simulations and compared against desirable NKDEP criteria. The enzymatic assays always fell within the desirable total Error of 7.6%. By contrast, this requirement was never obtained for the compensated Jaffe methods at the critical level of 74.4±1.4 μmol/L. Only the compensated Jaffe creatinine on Olympus analyzer reached this specification at 35.9±0.9 and 97.9±1.7 μmol/L levels. This study demonstrates that, despite substantial improvement regarding traceability to the IDMS reference method and precision, compensated Jaffe creatinine methods, by contrast to enzymatic ones, do not reach the desirable specifications of NKDEP at normal levels of creatinine.

Multicenter Evaluation of Cystatin C Measurement after Assay Standardization

BACKGROUND Since 2010, a certified reference material ERM-DA471/IFCC has been available for cystatin C (CysC). This study aimed to assess the sources of uncertainty in results for clinical samples measured using standardized assays. METHODS This evaluation was performed in 2015 and involved 7 clinical laboratories located in France and Belgium. CysC was measured in a panel of 4 serum pools using 8 automated assays and a candidate isotope dilution mass spectrometry reference measurement procedure. Sources of uncertainty (imprecision and bias) were evaluated to calculate the relative expanded combined uncertainty for each CysC assay. Uncertainty was judged against the performance specifications derived from the biological variation model. RESULTS Only Siemens reagents on the Siemens systems and, to a lesser extent, DiaSys reagents on the Cobas system, provided results that met the minimum performance criterion calculated according to the intraindividual and interindividual biological variations. Although the imprecision was acceptable for almost all assays, an increase in the bias with concentration was observed for Gentian reagents, and unacceptably high biases were observed for Abbott and Roche reagents on their own systems. CONCLUSIONS This comprehensive picture of the market situation since the release of ERM-DA471/IFCC shows that bias remains the major component of the combined uncertainty because of possible problems associated with the implementation of traceability. Although some manufacturers have clearly improved their calibration protocols relative to ERM-DA471, most of them failed to meet the criteria for acceptable CysC measurements.

IFCC Working Group Recommendations for Assessing Commutability Part 1: General Experimental Design

Commutability is a property of a reference material (RM) that relates to the closeness of agreement between results for an RM and results for clinical samples (CSs) when measured by ≥2 measurement procedures (MPs). Commutability of RMs used in a calibration traceability scheme is an essential property for them to be fit for purpose. Similarly, commutability of trueness controls or external quality assessment samples is essential when those materials are used to assess trueness of results for CSs. This report is part 1 of a 3-part series describing how to assess commutability of RMs. Part 1 defines commutability and addresses critical components of the experimental design for commutability assessment, including selection of individual CSs, use of pooled CSs, qualification of MPs for inclusion, establishing criteria for the determination that an RM is commutable, generalization of commutability conclusions to future measurements made with the MPs included in the assessment, and information regarding commutability to be included in the certificate for an RM. Parts 2 and 3 in the series present 2 different statistical approaches to commutability assessment that use fixed criteria related to the medical decisions that will be made using the laboratory test results.

Performance of GFR Estimating Equations in African Europeans: Basis for a Lower Race-Ethnicity Factor Than in African Americans

References 1. Fuller DS, Pisoni RL, Bieber BA, Gillespie BW, Robinson BM. The DOPPS Practice Monitor for US dialysis care: trends through December 2011. Am J Kidney Dis. 2013;61(2):342-348. 2. Twentieth Annual Report 2011. Creutzfeldt-Jakob disease surveillance in the UK. The National CJD Research & Surveillance Unit, Western General Hospital, Edinburgh, EH4 2XU. http://www.cjd.ed.ac.uk/documents/report20.pdf. Accessed January 3, 2013.

Quantitative detection of amyloid-β peptides by mass spectrometry: state of the art and clinical applications

Abstract Alzheimer’s disease (AD) is the most common form of dementia in humans, and a major public health concern with 35 million of patients worldwide. Cerebrospinal fluid (CSF) biomarkers being early diagnostic indicators of AD, it is essential to use the most efficient analytical methods to detect and quantify them accurately. These biomarkers, and more specifically amyloid-β (Aβ) peptides, are measured in routine clinical practice using immunoassays. However, there are several limits to this immunodetection in terms of specificity and multiplexing of the multiple isoforms of the Aβ peptides. To overcome these issues, the quantification of these analytes by mass spectrometry (MS) represents an interesting alternative, and several assays have been described over the past years. This article reviews the different Aβ peptides quantitative MS-based approaches published so far, compares their pre-analytical phase, and the different quantitative strategies implemented that might be suitable for clinical applications.

Clinical Mass spectrometry proteomics (cMSP) for medical laboratory: What does the future hold?

BACKGROUND Mass spectrometry (MS) methods are being widely used these days in medical laboratories for quantifying many small molecular analytes as well as for microbiological purposes. METHODS Little use has been made so far, however, of MS for analyzing peptides and proteins in clinical laboratory (an approach known as clinical MS proteomics (cMSP)). The explanation for this situation may be that cMSP assays are more complex to implement than conventional assays, require large investments in terms of equipment and training, and have not yet been sufficiently validated for clinical applications. In addition, the protein analysis assays currently used in medical laboratories mostly meet both laboratory and clinical requirements in terms of analytical performances, ease of use, and turn-around-time. RESULTS With the spread of MS methods in laboratories, increasing interest seems to be focusing on the development of MS for quantifying new analytes. MALDI-TOF MS methods have already been replacing classical methods of bacterial classification in clinical laboratories, for example, and this can be said to be an important step in this direction. CONCLUSIONS In this paper, the literature available on the topic of clinical MS proteomics is reviewed and the pre-analytical, analytical, and post-analytical challenges which will have to be met in connection with this approach are discussed.

Antibody-free quantification of seven tau peptides in human CSF using targeted mass spectrometry

Tau protein concentration in cerebrospinal fluid (CSF) is currently used as a sensitive and specific biomarker for Alzheimers disease. Its detection currently relies on ELISA but the perspective of using mass spectrometry (MS) to detect its different proteoforms represents an interesting alternative. This is however an analytical challenge because of its low concentration in the CSF, a biological fluid collected in small volume by lumbar puncture, and with a high structural heterogeneity. To overcome these issues, instead of using immunocapture as previously done, we rather relied on an original two steps pre-fractionation technique of CSF: perchloric acid (PCA) followed by micro solid phase extraction (μSPE). We could then measure seven tau trypsic peptides by Multiple Reaction Monitoring (MRM) on a triple quadrupole mass spectrometer. Quantification was performed using isotopically labeled 15N- recombinant tau protein as internal standard and validated using CSF pools with low, medium, or high tau concentrations (HTCs). Repeatability, intermediate precision, linearity, limit of quantification (LOQ), and recovery were calculated for the different peptides. This new MRM assay, which allowed for the first time CSF tau protein quantification without immunocapture, has important potential application to follow tau metabolism in both diagnostic and therapeutic research.

IFCC Working Group Recommendations for Assessing Commutability Part 2: Using the Difference in Bias Between a Reference Material and Clinical Samples

A process is described to assess the commutability of a reference material (RM) intended for use as a calibrator, trueness control, or external quality assessment sample based on the difference in bias between an RM and clinical samples (CSs) measured using 2 different measurement procedures (MPs). This difference in bias is compared with a criterion based on a medically relevant difference between an RM and CS results to make a conclusion regarding commutability. When more than 2 MPs are included, the commutability is assessed pairwise for all combinations of 2 MPs. This approach allows the same criterion to be used for all combinations of MPs included in the assessment. The assessment is based on an error model that allows estimation of various random and systematic sources of error, including those from sample-specific effects of interfering substances. An advantage of this approach is that the difference in bias between an RM and the average bias of CSs at the concentration (i.e., amount of substance present or quantity value) of the RM is determined and its uncertainty estimated. An RM is considered fit for purpose for those MPs for which commutability is demonstrated.

Continuous improvement of medical test reliability using reference methods and matrix-corrected target values in proficiency testing schemes: Application to glucose assay

The reliability of biological tests is a major issue for patient care in terms of public health that involves high economic stakes. Reference methods, as well as regular external quality assessment schemes (EQAS), are needed to monitor the analytical performance of field methods. However, control material commutability is a major concern to assess method accuracy. To overcome material non-commutability, we investigated the possibility of using lyophilized serum samples together with a limited number of frozen serum samples to assign matrix-corrected target values, taking the example of glucose assays. Trueness of the current glucose assays was first measured against a primary reference method by using human frozen sera. Methods using hexokinase and glucose oxidase with spectroreflectometric detection proved very accurate, with bias ranging between -2.2% and +2.3%. Bias of methods using glucose oxidase with spectrophotometric detection was +4.5%. Matrix-related bias of the lyophilized materials was then determined and ranged from +2.5% to -14.4%. Matrix-corrected target values were assigned and used to assess trueness of 22 sub-peer groups. We demonstrated that matrix-corrected target values can be a valuable tool to assess field method accuracy in large scale surveys where commutable materials are not available in sufficient amount with acceptable costs.