Hormone replacement therapy (oestradiol-only preparations): can the laboratory recommend a concentration of plasma oestradiol to protect against osteoporosis?

Target concentrations of plasma oestradiol to protect against osteoporosis have now been published in at least one textbook' and are increasingly discussed by clinicians. A survey of the literature revealed that many reports do not give details of oestradiol assay methods, despite the fact that there is a wide range of biases. Plasma oestradiol measurements made after ingestion of equine oestrogen preparations are not valid, and for oral oestradiol and oestradiol valerate preparations a single untimed sample does not give a reliable estimate of the average exposure over 24 h. Oestradiol measurements in plasma from women using patch, gel or implant preparations could be useful provided that consideration is given to the time of sampling and to the assay method used, although target levels have not been established. Further studies are required in this area.