Editing Bioavailability (section)

{{Short description|Pharmacological measurement}}

In [[pharmacology]], '''bioavailability''' is a subcategory of [[Absorption (pharmacology)|absorption]] and is the fraction (%) of an administered [[medication|drug]] that reaches the [[systemic circulation]].<ref name="Hebert 2013 pp. 17–39">{{cite book | last=Hebert | first=Mary F. | title=Clinical Pharmacology During Pregnancy | chapter=Impact of Pregnancy on Maternal Pharmacokinetics of Medications | publisher=Elsevier | year=2013 | isbn=978-0-12-386007-1 | doi=10.1016/b978-0-12-386007-1.00003-9 | pages=17–39}}</ref>

By definition, when a medication is administered [[intravenous]]ly, its bioavailability is 100%.<ref>{{cite book|last=Griffin|first=J. P.|title=The Textbook of Pharmaceutical Medicine|date=7 December 2009|edition=6th|location=Jersey|publisher=BMJ Books|isbn=978-1-4051-8035-1}}{{page needed|date=February 2013}}</ref><ref name="Flynn 2007 pp. 1–3">{{cite book | last=Flynn | first=Edward | title=xPharm: The Comprehensive Pharmacology Reference | chapter=Pharmacokinetic Parameters | publisher=Elsevier | year=2007 | isbn=978-0-08-055232-3 | doi=10.1016/b978-008055232-3.60034-0 | pages=1–3}}</ref> However, when a medication is administered via [[route of administration|routes]] other than intravenous, its bioavailability is lower due to intestinal epithelium absorption and [[first-pass metabolism]]. Thereby, mathematically, bioavailability equals the ratio of comparing the [[Area-under-curve (pharmacokinetics)|area under the plasma drug concentration curve versus time]] (AUC) for the extravascular formulation to the AUC for the intravascular formulation.<ref name="Davis 2018 pp. 79–137">{{cite book | last=Davis | first=Jennifer L. | title=Equine Internal Medicine | chapter=Pharmacologic Principles | publisher=Elsevier | year=2018 | isbn=978-0-323-44329-6 | doi=10.1016/b978-0-323-44329-6.00002-4 | pages=79–137}}</ref> AUC is used because AUC is proportional to the dose that has entered the systemic circulation.<ref name="Johanson 2010 pp. 153–177">{{cite book | last=Johanson | first=G. | title=Comprehensive Toxicology | chapter=Modeling of Disposition | publisher=Elsevier | year=2010 | isbn=978-0-08-046884-6 | doi=10.1016/b978-0-08-046884-6.00108-1 | pages=153–177}}</ref>

Bioavailability of a drug is an [[mean|average value]]; to take [[Statistical population|population variability]] into account, [[standard deviation|deviation range]] is shown as [[±]].<ref name="Davis 2018 pp. 79–137"/> To ensure that the drug taker who has poor absorption is dosed appropriately, the bottom value of the deviation range is employed to represent real bioavailability and to calculate the drug dose needed for the drug taker to achieve systemic concentrations similar to the intravenous formulation.<ref name="Davis 2018 pp. 79–137"/> To dose without knowing the drug taker's absorption rate, the bottom value of the deviation range is used in order to ensure the intended efficacy, unless the drug is associated with a narrow [[therapeutic window]].<ref name="Davis 2018 pp. 79–137"/>

For [[dietary supplements]], herbs and other nutrients in which the route of administration is nearly always oral, bioavailability generally designates simply the quantity or fraction of the ingested dose that is absorbed.<ref>{{cite journal |first1=Robert P. |last1=Heaney |title=Factors Influencing the Measurement of Bioavailability, Taking Calcium as a Model |journal=The Journal of Nutrition |pmid=11285351 |year=2001 |volume=131 |issue=4 |pages=1344S–8S|doi=10.1093/jn/131.4.1344S |doi-access=free }}</ref><ref name="SANDSTEAD AU 2007 pp. 925–947">{{cite book | last1=SANDSTEAD | first1=HAROLD H. | last2=AU | first2=WILLIAM | title=Handbook on the Toxicology of Metals | chapter=Zinc**Dr. Carl-Gustaf Elinder was the author of this chapter in the 2nd edition of the Handbook on Toxicology of Metals; his text provided guidance. | publisher=Elsevier | year=2007 | isbn=978-0-12-369413-3 | doi=10.1016/b978-012369413-3/50102-6 | pages=925–947 | quote=Bioavailability is the major factor affecting dietary requirements (Sandstrom, 1997). Flesh foods facilitate bioavailability, although indigestible Zn-binding ligands decrease bioavailability (Mills, 1985).  }}</ref><ref name="Solomons 2003 pp. 6272–6277">{{cite book | last=Solomons | first=N.W. | title=Encyclopedia of Food Sciences and Nutrition | chapter=ZINC &#124; Physiology | publisher=Elsevier | year=2003 | isbn=978-0-12-227055-0 | doi=10.1016/b0-12-227055-x/01309-2 | pages=6272–6277 | quote=Bioavailability strictly refers to both the uptake and metabolic utilization of a nutrient.}}</ref>