Sofie K. Van Houcke

IFCC international conventional reference procedure for the measurement of free thyroxine in serum

Abstract The IFCC Working Group for Standardization of Thyroid Function Tests proposes a candidate international conventional reference procedure (RMP) for measurement of the amount-of-substance concentration of free thyroxine in plasma/serum at physiological pH 7.40 and temperature (37.0°C). The unit for reporting measurement results is, by convention, pmol/L. The RMP is based on equilibrium dialysis isotope dilution-liquid chromatography/tandem mass spectrometry (ED-ID-LC/tandem MS). The rationale for proposing a conventional RMP is that, because of the physical separation step, it is unknown whether the measurement truly reflects the concentration of free thyroxine (FT4) in serum. Therefore, the ED part of the RMP has to strictly adhere to the following conditions: use of a dialysis buffer with a biochemical composition resembling the ionic environment of serum/plasma as closely as possible; buffering of the sample to a pH of 7.40 (at 37.0°C) before dialysis, however, without additional dilution; dialysis in a device with a dialysand/dialysate compartment of identical volume and separated by a membrane of regenerated cellulose and adequate cut-off; thermostatic control of the temperature during dialysis at 37.0°C±0.50°C. The convention does not apply to the ID-LC/tandem MS part, provided it is eligible to be nominated for review by the Joint Committee for Traceability in Laboratory Medicine. Here, we describe the ED procedure, inclusive its validation and transferability, in greater detail. We recommend a protocol for successful calibration, measurement and monitoring of the accuracy/trueness and precision of the candidate conventional RMP. For details on our ID-LC/tandem MS procedures, we refer to the Supplement.

A Progress Report of the IFCC Committee for Standardization of Thyroid Function Tests

Background: The IFCC Committee for Standardization of Thyroid Function Tests aims at equivalence of laboratory test results for free thyroxine (FT4) and thyrotropin (TSH). Objectives: This report describes the phase III method comparison study with clinical samples representing a broad spectrum of thyroid disease. The objective was to expand the feasibility work and explore the impact of standardization/harmonization in the clinically relevant concentration range. Methods: Two sets of serum samples (74 for FT4, 94 for TSH) were obtained in a clinical setting. Eight manufacturers participated in the study (with 13 FT4 and 14 TSH assays). Targets for FT4 were set by the international conventional reference measurement procedure of the IFCC; those for TSH were based on the all-procedure trimmed mean. The manufacturers recalibrated their assays against these targets. Results: All FT4 assays were negatively biased in the mid- to high concentration range, with a maximum interassay discrepancy of approximately 30%. However, in the low range, the maximum deviation was approximately 90%. For TSH, interassay comparability was reasonable in the mid-concentration range, but worse in the pathophysiological ranges. Recalibration was able to eliminate the interassay differences, so that the remaining dispersion of the data was nearly entirely due to within-assay random error components. The impact of recalibration on the numerical results was particularly high for FT4. Conclusions: Standardization and harmonization of FT4 and TSH measurements is feasible from a technical point of view. Because of the impact on the numerical values, the implementation needs careful preparation with the stakeholders.

Standardization activities in the field of thyroid function tests: a status report.

Abstract Background: Laboratory testing is an essential tool for diagnosis and management of thyroid diseases. However, the current status of standardization hampers the interchangeability of results. To improve this situation, the Working Group for Standardization of Thyroid Function Tests was established. Methods: Method comparisons were organized for measurement of human thyroid stimulating hormone (TSH), and free and total thyroid hormone in serum from apparently healthy donors. The aim was to assess the status of standardization and the quality of the performance of current routine assays. A second objective was to investigate the effect of mathematical recalibration of the results using their relationship to the overall mean (TSH) or the reference measurement procedure values (other thyroid hormones). Results: The need for standardization was shown to be highest for free thyroid hormone and total triiodothyronine measurements, while the majority of TSH and total thyroxine assays agreed within 10% of the reference. Most assays showed good performance. However, some could benefit from improved precision, consistency of calibration, or within- and between-run stability. Recalibration eliminated assay-specific bias. Thus, the residual spread was due to within-method effects. Not withstanding, sample-related effects remained. Conclusions: These studies confirmed the feasibility of standardization based on method comparison with native sera, but highlighted the need to resolve issues, such as sample-related effects. In view of the fact that in this phase the project worked with samples from individuals with euthyroid status, the next method comparison shall place emphasis on challenging the performance of the assays with clinical samples and expanding the covered measurement range. Clin Chem Lab Med 2010;48:1577–83.

Calcium, Magnesium, Albumin, and Total Protein Measurement in Serum as Assessed with 20 Fresh-Frozen Single-Donation Sera

To the Editor: The scope of external quality assessment (EQA)1 in laboratory medicine has evolved considerably (1). With the increasing worldwide interest in the use of common reference intervals and/or medical-decision limits, modern EQA schemes need to be better at assessing the standardization status of commercial in vitro diagnostic tests. This need has led to new challenges in the design of EQA surveys. We report the outcomes of a Norwegian pilot study that investigated the use of commutable, fresh-frozen single donations to assess the current standardization status as part of an initiative toward producing common reference intervals (2). The study covered measurements of calcium, magnesium, albumin, and total protein in serum from 20 single-donation blood samples obtained from Solomon Park Research Laboratories. Serum was generated according to CLSI protocol C37-A, with 2 U human thrombin (Sigma-Aldrich) added per milliliter of plasma to facilitate clotting (3); filtration was not used. Aliquots of these samples were sent to laboratories that used the same test systems (instrument, reagent, and calibrator from the same source). Five peer groups (n ≥ 6 laboratories each; N = 47) were considered: Abbott Diagnostics ARCHITECT, Ortho Clinical Diagnostics VITROS, …

Long-term stability of laboratory tests and practical implications for quality management

Abstract Background: Long-term stability of analytical performance is required for adequate patient management. We investigated the use of patient data to document test stability, and the relevance of observed instabilities on a surrogate medical outcome. We used multiyear patient and internal quality control (IQC) data from two laboratories for tests to monitor chronic kidney and thyroid disease. Methods: We plotted moving means of the 50th percentiles of stratified patient data and of the daily IQC means. We evaluated observed instabilities based on goals inferred from the analytes’ biological variation and investigated their effect on classification of results against reference intervals. Results: Patient and IQC data generally matched well, except for analytes, for which other than analytical variation sources prevailed. Analytical instabilities were predominantly due to reagent/calibrator lot changes, however, for immunoassays also to within-lot instabilities, urging frequent recalibrations. The relevance of biased results on medical decisions ranged from negligible to very pronounced, indicating the need for assessment of analytical performance in relation to quality goals inferred from biological variation. Conclusions: Patient percentiles offer great potential to assess/monitor the medium- to long-term analytical stability of a test within certain constraints. Differences in analytical quality between assays can significantly affect medical outcome.

Good samples make good assays – the problem of sourcing clinical samples for a standardization project.

Abstract Clinical samples are the cornerstone in all aspects related to in vitro diagnostic testing. They are particularly valuable in the process of establishing/validating metrological traceability, because they eliminate commutability issues potentially associated with artificial calibrators. Therefore, they are essential for IFCC standardization projects. However, sourcing clinical specimens is particularly challenging. It mostly turns out that only dedicated supply sources can accommodate the varying specifications within reasonable timelines. Here we describe the torturous experience in this regard of the IFCC Working Group for Standardization of Thyroid Function tests (since transformed into a Committee). We always focused on obtaining high quality samples in sufficient volume to serve all project participants. We applied a step-up approach: in phase I, we used high volume (200 mL of plasma/serum) single donations from apparently healthy individuals, and switched in phase II and III to medium-sized volume clinical samples (15–30 mL) from well-defined patient categories. In the first two phases we observed for some assays a sample-related discrepant analytical performance for total/free triiodothyronine and thyroid stimulating hormone (TSH), whereas in phase III we faced a severe delay in obtaining the relevant panels for free thyroxine (FT4) and TSH (n=90 and n=100, respectively). Additional experiments only allowed us to exclude hypothesized causes of the observations. We believe that there would be merit in a collaborative effort by chairholders of similar projects to establish a sample procurement infrastructure based on a solid relationship with commercial supply sources with the support of a significant number of committed clinicians.

Harmonization of immunoassays to the all-procedure trimmed mean – proof of concept by use of data from the insulin standardization project

*Corresponding author: Linda M. Thienpont, Harelbekestraat 72, 9000 Gent, Belgium, Phone: +32 9 2648104, Fax: +32 9 2648198, e-mail: linda.thienpont@ugent.be Sofie K. Van Houcke, Katleen Van Uytfanghe and Linda M. Thienpont: Laboratory for Analytical Chemistry, Faculty of Pharmaceutical Sciences, Ghent University, Gent, Belgium Stefan Van Aelst: Department of Applied Mathematics and Computer Science, Ghent University, Gent, Belgium

Dual-wavelength recording, a simple algorithm to eliminate interferences due to UV-absorbing substances in capillary electrophoresis.

Analytical interferences have been described due to the presence of various exogenous UV‐absorbing substances in serum. Iodine‐based X‐ray contrast agents and various antibiotics have been reported to interfere with interpretation of serum protein pherograms, resulting in false diagnosis of paraproteinemia. In the present study, we have explored the possibility of measuring UV absorbance at two distinct wavelengths (210 and 246 nm) to distinguish between true and false paraproteins on a Helena V8 clinical electrophoresis instrument. This study demonstrates that most substances potentially interfering with serum protein electrophoresis show UV‐absorption spectra that are distinct from those of serum proteins. Scanning at 246 nm allows detection of all described interfering agents. Comparing pherograms recorded at both wavelengths (210 and 246 nm) enables to distinguish paraproteins from UV‐absorbing substances. In case of a true paraprotein, the peak with an electrophoretic mobility in the gamma‐region decreases, whereas the X‐ray contrast media and antibiotics show an increased absorption when compared to the basic setting (210 nm). The finding of iodine‐containing contrast media interfering with serum protein electrophoresis is not uncommon. In a clinical series, interference induced by contrast media was reported in 54 cases (of 13 237 analyses), corresponding with a prevalence of 0.4%. In the same series, 1631 true paraproteins (12.3%) were detected. Implementation of the proposed algorithm may significantly improve the interpretation of routine electrophoresis results. However, attention should still be paid to possible interference due to presence of atypical proteins fractions (e.g., tumor markers, C3).

Determination of iohexol and iothalamate in serum and urine by capillary electrophoresis

Estimation of glomerular filtration rate (eGFR) is essential to assess kidney function. Iodine‐containing contrast agents detection by HPLC has been proposed as a safe alternative for inulin or radioactive compounds. However, HPLC is a time‐consuming and labor‐intensive method. The aim of this study was to develop an assay for iohexol and iothalamate using capillary electrophoresis. Iohexol and iothalamate were directly analyzed by CE in serum and urine, using photometric detection (246 nm). Serum peak height was proportional to iohexol and iothalamate concentrations. Detection limits for iohexol and iothalamate were 10 and 5 mg/L. Limits of quantification were 13.0 and 15.0 mg/L. Within‐run CVs were 4.9 and 6.5%; between‐run CVs 3.1–9.9% and 3.8–13.7%. A good correlation was observed between CE and HPLC: y = 1.1703x + 5.017 (iohexol) and y = 0.7807x + 11.01 (iothalamate; (y = concentration obtained by CE [mg/L], x = concentration obtained by HPLC [mg/L]). In addition, CE allowed to determine urinary iohexol concentration. Although the detection limit for CE was higher than for HPLC, CE can still be used for eGFR determination. Advantages of this high‐throughput method are the absence of sample pretreatment and a minimal sample volume requirement.