Assessment of the potency of different standards in the immunoradiometric assay of ACTH

There is no generally accepted human international reference preparation for ACTH. The different centres offering clinical ACTH measurement therefore select assay standards from the limited range of human-purified and synthetic preparations currently available. We have examined the relative potencies of three synthetic ACTH standards in comparison with the NIBSC human-purified 1-39 (code 74/555) in a two-site immunoradiometric assay (IRMA) for ACTH. All of the standards produced parallel curves in the IRMA but there was a wide variation in potency between the preparations giving highly significant differences in quality control and patients' ACTH results. Standards supplied by the National Hormone and Pituitary Programme, Bachem UK Ltd and Universal Biologicals Ltd gave lower ACTH results than results calculated using the NIBSC preparation. The potency differences between these standards emphasise the need for a reference preparation and, in the meantime, the necessity to define normal reference ranges for each laboratory.