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

“…FT4 target values were assigned at Ghent University with the ED ID-LC/tandem MS cRMP [15]. The method's expanded uncertainty of measurement (k = 2, 95% confidence level) according to the measurement protocol used (3 replicates, in independent runs) is estimated at 7.6%.…”

“…The measurands needed to be defined and reference measurement procedures or a valid alternative developed. For free thyroxine (FT4), this led to an IFCC-approved international conventional reference measurement procedure (cRMP), based on equilibrium dialysis isotope dilution-liquid chromatography-tandem mass spectrometry (ED ID-LC/tandem MS) [15]. In the case of TSH, for which it is technically unlikely to have an RMP available in the mid- to short term, the C-STFT proposed a statistical alternative for ‘harmonization’ rather than standardization [16,17].…”

Standardization of FT4 and harmonization of TSH measurements — a request for input from endocrinologists and other physicians [Opinion]

“…We have worked over the years towards the goal of standardization of FT4 and TSH testing. Because, unlike FT4, there is no reference measurement procedure for TSH, we have proposed an alternative to standardization for this test called harmonization [1][2][3]. Different phases of method comparison studies were performed to investigate and confirm the feasibility of standardization of FT4 and harmonization of TSH tests, but also with attention to fit-for-the-purpose analytical quality [4][5].…”

The Use of TSH in Determining Thyroid Disease: How Does It Impact the Practice of Medicine in Pregnancy?

“…Heterophile antibodies represent a group of relatively weak multispecific, polyreactive antibodies with specificity for poorly defined antigens that react with immunoassays derived from two or more species [14, 15]. Most frequently, such heterophile antibody interferences result from IgM rheumatoid factor or HAMA.…”