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The International Standard for Pituitary FSH (IS; in ampoules coded 83/575) was assayed in terms of the Second International Reference Preparation of Human Pituitary FSH and LH for Bioassay (IRP 78/549) by 27 laboratories in 13 countries using bioassays, receptor assays and immunoassays. Estimates of the FSH content of the IS by in-vivo bioassay were homogeneous both within and between laboratories and gave a combined geometric mean (with 95% fiducial limits) of 79·9 (74·6–85·4) i.u./ampoule. Estimates by different in-vitro bioassays and receptor assays were also homogeneous between assays and laboratories, and gave a combined geometric mean (with 95% fiducial limits) of 31·2 (28·8–33·9) i.u./ampoule. However, estimates by the 19 different immunoassay systems were heterogeneous and varied between 5 and 31 i.u./ampoule.

The material in ampoules coded 83/575 was established by the World Health Organization as the International Standard for Pituitary FSH. It was assigned a unitage of 80 i.u./ampoule on the basis of its calibration by in-vivo bioassay, because this assay best identifies and defines the hormone. However, the introduction of the new IS will necessitate the recalibration of immunoassay kits.

FSH 84/530, prepared in the same way as the IS from the same FSH preparation, did not differ significantly from the IS in any of the assay systems studied and appeared to be equally suitable as a standard.

Four highly purified preparations of human FSH (FSH A–D), differing in their isoform compositions and in their in-vivo: in-vitro bioactivity ratios, were also studied. The ranking order of the specific activities of FSH A–D by in-vitro bioassays paralleled their order by receptor assays and the order of their content of FSH isoforms with isoelectric points > 4·5. (Potency estimates of FSH B and C in terms of the IS were greater by receptor assay than by in-vitro bioassay.) The overall ranking order of the specific activities of FSH A–D by immunoassays was different. Contrary to expectation, estimates in terms of the IS of specific activities by immunoassay differed more between preparations than those by in-vitro bioassay or receptor assay.

Differences in specificity between immunoassay systems were demonstrated not only in the calibration of the IS in terms of the crude FSH of IRP 78/549 but also in the comparisons of the highly purified FSH in the IS and FSH A–D. The differences in the immunoreactivities and bioactivities of FSH preparations differing in their isoform compositions greatly complicate the standardization of assays for FSH.

Journal of Endocrinology (1989) 123, 275–293

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