The C-STFT proposed a harmonization approach for measurement of TSH. This glycoprotein circulates in the bloodstream typically as mixture of components comprising disease-specific glycoforms. Current TSH immunoassays all are traceable to the same WHO IRP TSH 80/558 (10), and express their measurements in mIU/L (defined by that IRP). The latter is from human cadaver pituitary and may comprise glycosylation forms that differ from those typically present in serum TSH. Therefore, the above issues of non commutability of the IRP apply, and hence, IRP-traceable TSH measurements do not give equivalent or interchangeable results.
Definition of the measurand TSH
Important considerations: protein/TSH measurement is mixture analysis. This implies that each component in the mixture must contribute significantly to the diagnostic application of the assay used. The C-STFT defined the measurand as "TSH, intact, total, glycosylation encountered in diagnostic applications which should be specified".
The "All Procedure Trimmed Mean, APTM" as surrogate RMP for TSH
The C-STFT decided to preserve the traceability of TSH assays to the WHO IRP and recommends the "All Procedure Trimmed Mean" (APTM, expressed in IU) as surrogate RMP. It is proposed to statistically derive the APTM from data of a dedicated method comparison study with native human samples (14). The APTM targeted panel becomes the key tool in the harmonization process with the advantage that it is composed of samples containing typical glycoforms of TSH in blood circulation, thus as commutable as possible.