Editing International unit (section)

== For biologic preparations ==
[[File:Oxytocin intravenous bag.jpg|thumb|30 IU of oxytocin for intravenous infusion]]
[[Biologics]] are medications and other products made from biological sources. Depending on the form (powder vs solution), production method, or just batch-to-batch differences, there is often some variation in their relative potencies, so that a simple measure of [[mass]] would not suffice. The ''international unit'' allows one to compare the relative potency of all these forms, so that different forms or preparations with the same biological effect will contain the same number of IUs. To do so, the [[WHO Expert Committee on Biological Standardization]] (WHO ECBS) provides a reference preparation of the agent, arbitrarily sets the number of IUs contained in that preparation, and specifies a [[biological assay]] and/or an [[immunoassay]] procedure to compare other preparations of the same agent to the reference preparation. Since the number of IUs contained in a new substance is arbitrarily set, there is no equivalence between IU measurements of different biological agents.

=== Process of definition ===
To define the IU for a substance, an international collaborative study is organized by the [[WHO Expert Committee on Biological Standardization]] using various assay systems in several laboratories.<ref name="CSCC 1992">{{cite journal | author = Canadian Society of Clinical Chemists | title = Canadian Society of Clinical Chemists position paper: standardization of selected polypeptide hormone measurements. | journal = Clin. Biochem. | volume = 25 | issue = 6 | pages = 415–24 |date=Dec 1992 | pmid = 1477965 | doi=10.1016/0009-9120(92)90030-V}}</ref>  The main goal of the study is to reach a consensus regarding methods of analysis and the approach to standardization so that results can be compared.<ref name="Jeffcoate 1988">{{cite journal | author = Jeffcoate SL | title = What are we measuring in gonadotropin assays? | journal = Acta Endocrinol Suppl (Copenh) | volume = 288 | pages = 28–30 | year = 1988 | pmid = 3048031 }}</ref><ref name="Wicher 1991">{{cite journal | author = Wicher JT | title = Calibration is the key to immunoassay but the ideal calibrator is unattainable. | journal = Scand J Clin Lab Invest Suppl. | volume = 205 | pages = 21–32 | year = 1991 | doi = 10.3109/00365519109104599 | pmid = 1947745 }}</ref><ref name="Ekins 1991">{{cite journal | author = Ekins R | title = Immunoassay standardization | journal = Scand J Clin Lab Invest Suppl. | volume = 205 | pages = 33–46 | year = 1991 | doi = 10.3109/00365519109104600 | pmid = 1947747 }}</ref>

The study is performed using highly purified preparations of the [[Chemical substance|substance]], typically in [[lyophilize]]d form, called ''international reference preparations'', or ''IRPs''.<ref name="CSCC 1992" />  Each preparation is divided into precisely weighed samples, with each sample stored in its own ampoule labeled with a code corresponding to the source IRP.<ref name="CSCC 1992" />  Assays are performed using these samples and are calibrated against the previously available IU standard.  These results can be quite variable; the final IU value for samples of a given IRP are determined by consensus.<ref name="Jeffcoate 1988" />  The IRP that provides the best results and shows the best long term stability is selected to define the next IU.<ref name="CSCC 1992" />  This IRP is then referred to as the ''international standard''.<ref name="CSCC 1992" />

When a standard sample comes close to depleting, a new standard will have to be found. A new batch of the substance in question is produced and calibrated against the old one, so that the unit still represents the same amount of biological activity. Many substances have undergone several such standard sample changes. The ''third international standard'' of [[prolactin]] has nearly run out and in 2016 replacement was proposed.<ref>{{cite web |last1=Ferguson |first1=Jackie |title=WHO International Collaborative Study of the Proposed 4th International Standard for Prolactin, Human |date=2016 |url=https://www.who.int/publications/m/item/WHO-BS-2016.2292}}</ref>

=== Examples ===
* 1 IU of [[rhEGF]] is defined as the potency of 0.001 μg of a rhEGF in the "91/530" standard vial. One manufacturer reports that its rhEGF is 1.4 times as potent as the 91/530 standard.<ref name=R&D>{{cite web |title=Unit Conversion Table |url=https://www.rndsystems.com/resources/technical-information/unit-conversion-table |website=R&D Systems}}</ref>
* 12.5 IU of [[Oxytocin (medication)|oxytocin]] is defined as the potency of 21 μg of pure peptide in the "76/575" standard vial.<ref>{{cite web |title=WHO International Standard OXYTOCIN 4th International Standard NIBSC code: 76/575 Instructions for use (Version 4.0, Dated 30/04/2013) |url=https://www.nibsc.org/documents/ifu/76-575.pdf |website=nibsc.org}}</ref>

Before the purification of [[penicillin]] was perfected, the amounts were also described in "International Units" (and some non-international arbitrary "units" before them) to account for batch-to-batch variation. See {{section link|Penicillin|Penicillin units}}.