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The second International Standard for Human Pituitary LH (in ampoules coded 80/552; 2nd IS) and LH 81/535 (prepared in the same way as the 2nd IS from the same LH preparation) were compared with the International Reference Preparation of Human Pituitary LH for Immunoassay (IRP 68/40) by 19 laboratories in 11 countries, using in-vivo and in-vitro bioassays, a receptor assay and immunoassays.

Geometric mean estimates of the LH content of the 2nd IS (with 95% fiducial limits) in terms of IRP 68/40 were: 34·6 (29·1–41·0) IU/ampoule by in-vivo bioassays; 35·8 (27·0–47·4) IU/ampoule by in-vitro bioassays; 58·6 IU/ampoule by one receptor assay; and 36·8 (35·5–38·1) IU/ampoule by immunoassays. The close agreement between the relative activities of the 2nd IS and IRP 68/40 in the wide range of assay systems studied appears to reflect the fact that both standards contain highly purified LH with similar isoform compositions as judged by isoelectric focusing.

Estimates of the LH content of LH 81/535 in terms of IRP 68/40 and in terms of the 2nd IS tended to be lower than those for the 2nd IS across all methods, but the differences were not statistically significant.

The 2nd IS was found to be as suitable as IRP 68/40 as a standard for the in-vitro bioassay and immunoassay of LH in the two serum samples studied. However, the mean estimates of serum LH in terms of either of these standards were more than 150% larger by in-vitro bioassays than by immunoassays and more than 50% larger by one-site than by two-site immunoassays. This may be a reflection of the differences in the isoform composition of the highly purified LH of the 2nd IS and IRP 68/40 and that of the LH in the sera. The significant intra- and inter-laboratory variability observed in this study for LH estimates, especially by in-vitro bioassays but also by immunoassays, is very pertinent to the interpretation of published comparisons of LH bioactivity and immunoreactivity.

Estimates of the LH content of ampoules of the 2nd IS and LH 81/535 kept at elevated temperatures showed that both the 2nd IS and LH 81/535 appeared to be adequately stable under normal storage conditions; the in-vivo and in-vitro bioactivities and receptor binding activities of LH were more sensitive than its immunoreactivities to thermal degradation of the LH structure.

On the basis of these results, the World Health Organization Expert Committee on Biological Standardization established the preparation in ampoules coded 80/552 as the second International Standard for Human Pituitary Luteinizing Hormone, and assigned an activity of 35 International Units of human pituitary LH to the contents of each ampoule.

Journal of Endocrinology (1993) 138, 345–359

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