Units and Unit Conversions
The serum concentration of prolactin can be given in mass concentration (µg/L or ng/mL), molar concentration (nmol/L or pmol/L) or in international units (typically mIU/L). The current IU is calibrated against the third International Standard for Prolactin, IS 84/500. Reference ampoules of IS 84/500 contain 2.5 µg of lyophilized human prolactin, and have been assigned an activity of .053 International Units of prolactin. Measurements that are calibrated against the current international standard can be converted into mass units using this ratio of grams to IUs; prolactin concentrations expressed in mIU/L can be converted to µg/L by dividing by 21.2. Previous standards use other ratios.
The first International Reference Preparation (or IRP) of human Prolactin for Immunoassay was established in 1978 (75/504 1st IRP for human Prolactin) at a time when purified human prolactin was in short supply. Previous standards relied on prolactin from animal sources. Purified human prolactin was scarce, heterogeneous, unstable, and difficult to characterize. A preparation labelled 81/541 was distributed by the WHO Expert Committee on Biological Standardization without official status and given the assigned value of 50 mIU/ampoule based on an earlier collaborative study. It was determined that this preparation behaved anomalously in certain immunoassays and was not suitable as an IS. However, in the absence of an alternative, it was used. Three different human pituitary extracts containing prolactin were subsequently obtained as candidates for an IS. These were distributed into ampoules coded 83/562, 83/573, and 84/500. On the basis of collaborative studies involving 20 different laboratories, it was concluded that there was little difference between these three preparations. 83/562 appeared to be the most stable. This preparation was largely free of dimers and polymers of prolactin. On the basis of these investigations 83/562 was established as the Second IS for human Prolactin. Once stocks of these ampoules were depleted, 84/500 was established as the Third IS for human Prolactin.
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