Comparison of Fresh Frozen Serum to Proficiency Testing Material in College of American Pathologists Surveys: α-Fetoprotein, Carcinoembryonic Antigen, Human Chorionic Gonadotropin, and Prostate-Specific Antigen

Commutability and traceability in EQA programs

OBJECTIVES

The concept of commutability of samples has focused laboratories on the importance of traceability. However, the critical role of External Quality Assurance (EQA) in achieving the primary role of traceability (i.e. facilitating comparable patient results in different laboratories) has largely been lost. The aim of this paper is to review the role of EQA in achieving traceable/commutable results.

DESIGN AND METHODS

The role of commutability and traceability in EQA and Internal Quality Control (IQC) are discussed. Examples of commutable EQA samples are given to highlight the problem of assuming EQA material does not behave like patient samples.

RESULTS

We provide the conventional traceability chain (top down) and the role of EQA in a "bottom up" model using conventional EQA samples.

CONCLUSIONS

The quest for commutable samples has compromised the value of EQA without an understanding that some EQA materials are commutable for some measurands. EQA plays a key role in performance improvement, but laboratories need to understand the importance of using a range of values appropriate to the assay to identify areas of quality need. Traceability and EQA using conventional samples are not mutually exclusive concepts.

Interpretation of EQA results and EQA-based trouble shooting

Important objectives of External Quality Assessment (EQA) is to detect analytical errors and make corrective actions. The aim of this paper is to describe knowledge required to interpret EQA results and present a structured approach on how to handle deviating EQA results. The value of EQA and how the EQA result should be interpreted depends on five key points: the control material, the target value, the number of replicates, the acceptance limits and between lot variations in reagents used in measurement procedures. This will also affect the process of finding the sources of errors when they appear. The ideal EQA sample has two important properties: it behaves as a native patient sample in all methods (is commutable) and has a target value established with a reference method. If either of these two criteria is not entirely fulfilled, results not related to the performance of the laboratory may arise. To help and guide the laboratories in handling a deviating EQA result, the Norwegian Clinical Chemistry EQA Program (NKK) has developed a flowchart with additional comments that could be used by the laboratories e.g. in their quality system, to document action against deviations in EQA. This EQA-based trouble-shooting tool has been developed further in cooperation with the External quality Control for Assays and Tests (ECAT) Foundation. This flowchart will become available in a public domain, i.e. the website of the European organisation for External Quality Assurance Providers in Laboratory Medicine (EQALM).

Mass spectrometry measurements of prostate-specific antigen (PSA) peptides derived from immune-extracted PSA provide a potential strategy for harmonizing immunoassay differences

OBJECTIVES

Harmonization of prostate-specific antigen (PSA) immunoassays is important for good patient care. The specificity of the antibodies used to detect circulating PSA could cause differences in the PSA measurements.

METHODS

We used mass spectrometry (MS) to quantitate the concentration of five peptides cleaved from trypsin digestion of PSA and compared these measurements with six automated immunoassays. Linear regression and a mixed-effects model were used to analyze the results.

RESULTS

PSA measurements from the immunoassays and the five MS peptide assays were highly correlated (R(2) > 0.99), but the recovery of the World Health Organization standard and the regression slopes differed across assays. The same relative patterns of immunoassay differences were seen in comparing their results with each of the five MS peptide measurements from different parts of the circulating PSA molecules.

CONCLUSIONS

Mass spectrometry quantitation of peptides derived from trypsin digestion of immune-extracted PSA could be used to harmonize PSA immunoassays.

Statistical validation of reagent lot change in the clinical chemistry laboratory can confer insights on good clinical laboratory practice

Verification of new lot reagent's suitability is necessary to ensure that results for patients' samples are consistent before and after reagent lot changes. A typical procedure is to measure results of some patients' samples along with quality control (QC) materials. In this study, the results of patients' samples and QC materials in reagent lot changes were analysed. In addition, the opinion regarding QC target range adjustment along with reagent lot changes was proposed. Patients' sample and QC material results of 360 reagent lot change events involving 61 analytes and eight instrument platforms were analysed. The between-lot differences for the patients' samples (ΔP) and the QC materials (ΔQC) were tested by Mann-Whitney U tests. The size of the between-lot differences in the QC data was calculated as multiples of standard deviation (SD). The ΔP and ΔQC values only differed significantly in 7.8% of the reagent lot change events. This frequency was not affected by the assay principle or the QC material source. One SD was proposed for the cutoff for maintaining pre-existing target range after reagent lot change. While non-commutable QC material results were infrequent in the present study, our data confirmed that QC materials have limited usefulness when assessing new reagent lots. Also a 1 SD standard for establishing a new QC target range after reagent lot change event was proposed.

Suitability of quality control materials for prostate-specific antigen (PSA) measurement: inter-method variability of common tumor marker control materials

BACKGROUND

Quality control materials with minimal inter-assay differences and clinically relevant proportions of different molecular forms of the analyte are needed to optimize intra- and inter-laboratory accuracy and precision.

METHODS

We assessed if clinically relevant total prostate-specific antigen (tPSA) levels were present in seven commercially available Multi Constituent Tumor Marker Controls (MC-TMC). Further, we determined the concentration of free PSA (fPSA) and calculated the percentage of free PSA (%fPSA) in all materials. Finally, we determined variability of TMC materials across several commonly used PSA platforms.

RESULTS

All MC-TMC materials contained at least one concentration of tPSA in normal and pathologic range. Control materials varied in the amount of fPSA and %fPSA, with most controls consisting of fPSA only and only one MC-TMC containing medically relevant levels of around 35% fPSA. Only a minority of MC-TMC materials showed minimal variability across four PSA methods while the majority of PSA controls showed wide inter-method differences.

CONCLUSIONS

Use of many commercially available controls for PSA could lead to biased PSA measurements because they contain medically irrelevant proportions of fPSA and show significant variation among different PSA assay platforms.

Proficiency testing/external quality assessment: current challenges and future directions

BACKGROUND

Proficiency testing (PT), or external quality assessment (EQA), is intended to verify on a recurring basis that laboratory results conform to expectations for the quality required for patient care.

CONTENT

Key factors for interpreting PT/EQA results are knowledge of the commutability of the samples used and the process used for target value assignment. A commutable PT/EQA sample demonstrates the same numeric relationship between different measurement procedures as that expected for patients' samples. Noncommutable PT/EQA samples frequently have a matrix-related bias of unknown magnitude that limits interpretation of results. PT/EQA results for commutable samples can be used to assess accuracy against a reference measurement procedure or a designated comparison method. In addition, the agreement of the results between different measurement procedures for commutable samples reflects that which would be seen for patients' samples. PT/EQA results for noncommutable samples must be compared to a peer group mean/median of results from participants who use measurement procedures that are expected to have the same or very similar matrix-related bias. Peer group evaluation is used to asses whether a laboratory is using a measurement procedure in conformance to the manufacturer's specifications and/or in conformance to other laboratories using the same technology. A noncommutable PT/EQA sample does not give meaningful information about the relationship of results for patients' samples between different measurement procedures.

SUMMARY

PT/EQA provides substantial value to the practice of laboratory medicine by assessing the performance of individual laboratories and, when commutable samples are used, the status of standardization or harmonization among different measurement procedures.

Roadmap for harmonization of clinical laboratory measurement procedures

Results between different clinical laboratory measurement procedures (CLMP) should be equivalent, within clinically meaningful limits, to enable optimal use of clinical guidelines for disease diagnosis and patient management. When laboratory test results are neither standardized nor harmonized, a different numeric result may be obtained for the same clinical sample. Unfortunately, some guidelines are based on test results from a specific laboratory measurement procedure without consideration of the possibility or likelihood of differences between various procedures. When this happens, aggregation of data from different clinical research investigations and development of appropriate clinical practice guidelines will be flawed. A lack of recognition that results are neither standardized nor harmonized may lead to erroneous clinical, financial, regulatory, or technical decisions. Standardization of CLMPs has been accomplished for several measurands for which primary (pure substance) reference materials exist and/or reference measurement procedures (RMPs) have been developed. However, the harmonization of clinical laboratory procedures for measurands that do not have RMPs has been problematic owing to inadequate definition of the measurand, inadequate analytical specificity for the measurand, inadequate attention to the commutability of reference materials, and lack of a systematic approach for harmonization. To address these problems, an infrastructure must be developed to enable a systematic approach for identification and prioritization of measurands to be harmonized on the basis of clinical importance and technical feasibility, and for management of the technical implementation of a harmonization process for a specific measurand.

Commutability Limitations Influence Quality Control Results with Different Reagent Lots

BACKGROUND

Good laboratory practice includes verifying that each new lot of reagents is suitable for use before it is put into service. Noncommutability of quality control (QC) samples with clinical patient samples may preclude their use to verify consistency of results for patient samples between different reagent lots.

METHODS

Patient sample results and QC data were obtained from reagent lot change verification records for 18 QC materials, 661 reagent lot changes, 1483 reagent lot change–QC events, 82 analytes, and 7 instrument platforms. The significance of between-lot differences in the results for QC samples compared with those for patient samples was assessed by a modified 2-sample t test adjusted for heterogeneity of QC and patient sample measurement variances.

RESULTS

Overall, 40.9% of reagent lot change–QC events had a significant difference (P < 0.05) between results for QC samples compared with results for patient samples between 2 reagent lots. For QC results with differences <1.0 SD interval (83.1% of total), 37.7% were significantly different from the changes observed for patient samples. For QC results with differences ≥1.0 SD interval (16.9% of total), 57.0% were significantly different from those for patient samples.

CONCLUSIONS

Occurrence of noncommutable results for QC materials was frequent enough that the QC results could not be used to verify consistency of results for patient samples when changing lots of reagents.

Imprecision of tumour biomarker measurements on Roche Modular E170 platform fulfills desirable goals derived from biological variation

Background

Monitoring of test imprecision is one of the most important quality indicators in clinical laboratories. Imprecision goals should be derived from biological variation. The aim of this study was to evaluate the imprecision of eight tumour biomarker assays routinely measured on the Modular E170 system.

Methods

Method coefficient of variations (CVs) were obtained by an appropriate Internal Quality Control programme based on the measurement every working day of a fresh–frozen human serum pool with biomarkers concentrations around the clinical cut-offs. We evaluated data collected along the whole year 2008 ( n range: 21–461); monthly CVs and their cumulative means were calculated and compared with corresponding goals.

Results

Biomarkers concentration means and average yearly CVs (desirable goals in parentheses) were as follows: α-fetoprotein, 9.6 μg/L, 3.9% (6.0%); CA125, 41.2 U/L, 2.8% (12.4%); CA15.3, 32.7 U/L, 3.1% (3.1%); CA19.9, 35.1 U/L, 2.8% (8.0%); CEA, 7.7 μg/L, 4.3% (6.4%); prostate-specific antigen (PSA), 4.1 μg/L, 4.3% (9.1%); CYFRA 21.1, 2.4 μg/L, 5.7% (11.3%); and ferritin, 64.5 μg/L, 4.0% (7.1%).

Conclusions

Our study shows that in routine laboratory practice and over a clinically and analytically relevant time-span, the imprecision of the tumour biomarker measurements on the Roche Modular E170 fulfills desirable goals. For four assays (CA125, CA19.9, PSA and CYFRA 21.1) the optimum CV can even be achieved.

Toward metrological traceability in the determination of prostate-specific antigen (PSA): calibrating Beckman Coulter Hybritech Access PSA assays to WHO standards compared with the traditional Hybritech standards

BACKGROUND

The metrological traceability of prostate-specific antigen (PSA) assay calibration to WHO standards is desirable to potentially improve the comparability between PSA assays. A method comparison was performed between the traditionally standardized Beckman Coulter Hybritech Access PSA and free PSA (fPSA) assays and a new alternate calibration of assays aligned to the WHO standards 96/670 and 96/668, respectively.

METHODS

Sera from 641 men with and without prostate cancer, various control materials and mixtures of different proportions of the WHO standards were measured with both assay calibrations.

RESULTS

Excellent comparability between the corresponding assay calibrations was observed, with correlation coefficients of at least 0.996. The Passing-Bablok slopes were 0.747 for total PSA (tPSA), 0.776 for fPSA and 1.02 for the percentage ratio of fPSA to tPSA (%fPSA), while the corresponding percentages of the new WHO-aligned assay results related to the traditional assays were 76.2%, 77% and 102.2%. Receiver operating characteristics revealed no differences between the two PSA assay calibrations.

CONCLUSIONS

The WHO calibration yields results approximately 25% lower for tPSA and fPSA values when compared with the conventional Hybritech calibration. Using the WHO-aligned PSA assay, a tPSA cut-off of 3 microg/L should be considered in clinical practice, while %fPSA cut-offs could be retained.

Variations observed for insulin concentrations in an interlaboratory quality control program may be due to interferences between reagents and the matrix of the control materials

The present study was carried out to observe the behaviour of insulin concentrations in an interlaboratory quality control program from BioRad Laboratories (Irving, CA) applied to Immulite 2000 (Diagnostics Product Corporation, Los Angeles, CA) for three control materials of Lyphocheck Immunoassay Plus Control. Insulin was measured for 261 consecutive working days in a year using a solid-phase immunometric chemiluminescent assay; likewise insulin was measured for 55 days during a period of 4 months in a pool of sera obtained from patients with insulin concentrations within the normal range of our laboratory. The results from each control material were classified in three groups according to the closeness among concentrations and time; mean concentrations were significantly different between consecutive groups for the three control materials (p<0.0001). However, no differences were observed in samples from pool sera. The variations observed in insulin concentrations of the control materials may be due to some interferences or matrix effect on the control material caused by the reagents to quantify insulin in the Immulite 2000.

Improving the comparability of immunoassays for prostate-specific antigen (PSA): Progress and problems

BACKGROUND

Commutability of immunoassay test results is an important objective for laboratory medicine.

METHODS

PSA is a clinically important analyte for which, as a consequence of a number of national and international initiatives over the last decade, considerable progress has been made towards improving method comparability. However, results from different assays are still not interchangeable, a situation that is only likely to improve once broad recommendations can be made about the most clinically relevant antibody combinations.

CONCLUSIONS

Universal implementation of such recommendations would almost certainly improve between-method agreement substantially, provided careful attention were paid to assay design and use of appropriately pure secondary standards ensured.

A Comparison of Human Chorionic Gonadotropin– Related Components in Fresh Frozen Serum With the Proficiency Testing Material Used by the College of American Pathologists

Context.—As part of a College of American Pathologists (CAP) proficiency testing survey, a comparison is made between human chorionic gonadotropin (hCG) results from an actual patient pool and a similarly targeted artificial sample. The goal is to gain insight into the possible source(s) of bias attributable to the proficiency testing material (PTM) with a view toward creating more appropriate survey materials.

Objective.—To compare hCG and related variants in a pool of fresh frozen sera (FFS) with that found in PTM.

Design.—The 2003 CAP K/KN-A Survey included a FFS specimen along with admixtures of PTM. The FFS (K-02) and 1 PTM admixture (K-01) had similar mean hCG values. Five hCG-related analytes were measured on these 2 samples by a reference laboratory.

Participants.—Approximately 1800 clinical laboratories and diagnostic test kit manufacturers participated in the K/ KN-A Survey.

Main Outcome Measures.—Method imprecision (coefficient of variation) and method bias (relative difference between peer group mean and all-method median) were computed for the 2 samples. Differences were evaluated with respect to hCG-related analytes levels.

Results.—All-method hCG results were 12.9 mIU/mL (12.9 IU/L) for the PTM material and 21.6 mIU/mL (21.6 IU/L) for the FFS material. Method biases for 14 manufacturers were greater for PTM than for FFS (−40% to +35% and −16% to +23%). Twelve of 14 methods had higher coefficients of variation on PTM. Total hCG and free β hCG measurements by the reference laboratory were 14.1 mIU/ mL (14.1 IU/L) for the PTM material and 18.5 mIU/mL (18.5 U/L) for the FFS material (FFS), and 2.4 (PTM) and 0.7 (FFS) mIU/mL (2.4 and 0.7 IU/L), respectively. On a molar basis, free β represented 17% and 4% of the total hCG, respectively. Levels of hyperglycosylated hCG, nicked hCG, and β core fragment were not measurable in either sample.

Conclusions.—It is unlikely that the hCG added to the PTM is the source of the increased bias and variability. The main difference is a 3-fold increase in free β found in the PTM, but methods previously found to strongly react with free β were not systematically elevated. The biases between manufacturers found for the FFS specimen are likely attributable to calibration differences.