Editing Bilirubin (section)

===Direct===

Direct bilirubin = Conjugated bilirubin + delta bilirubin<ref name="Tietze 2012 pp. 86–122" />

==== Conjugated ====
In the liver, bilirubin is conjugated with [[glucuronic acid]] by the enzyme [[glucuronyltransferase]], first to [[bilirubin glucuronide]] and then to [[bilirubin diglucuronide]], making it soluble in water: the conjugated version is the main form of bilirubin present in the "direct" bilirubin fraction. Much of it goes into the bile and thus out into the small intestine. Though most [[bile acid]] is reabsorbed in the [[terminal ileum]] to participate in [[enterohepatic circulation]], conjugated bilirubin is not absorbed and instead passes into the [[Large intestine|colon]].<ref name="handbook">{{cite book |title=Oxford American Handbook of Gastroenterology and Hepatology |vauthors=Cheifetz AS |publisher=Oxford University Press, USA |year=2010 |isbn=978-0199830121 |location=Oxford |page=165}}</ref>

There, colonic bacteria deconjugate and metabolize the bilirubin into colorless [[urobilinogen]], which can be oxidized to form [[urobilin]] and [[stercobilin]]. Urobilin is excreted by the kidneys to give urine its yellow color and stercobilin is excreted in the feces giving stool its characteristic brown color. A trace (~1%) of the urobilinogen is reabsorbed into the [[enterohepatic circulation]] to be re-excreted in the bile.<ref>{{cite book |last=Kuntz |first=Erwin |title=Hepatology: Textbook and Atlas |publisher=Springer |year=2008 |isbn=978-3-540-76838-8 |location=Germany |page=38}}</ref>

Conjugated bilirubin's half-life is shorter than delta bilirubin.<ref name="Sullivan Gourley 2011 pp. 176–186.e3">{{cite book |title=Pediatric Gastrointestinal and Liver Disease |vauthors=Sullivan KM, Gourley GR |publisher=Elsevier |year=2011 |isbn=978-1-4377-0774-8 |pages=176–186.e3 |chapter=Jaundice |doi=10.1016/b978-1-4377-0774-8.10017-x}}</ref>

====Delta bilirubin====

Although the terms direct and indirect bilirubin are sometimes used interchangeably with conjugated and unconjugated bilirubin, the direct fraction actually includes both conjugated bilirubin and delta bilirubin.<ref>{{Citation |last=Sullivan |first=Kara M. |title=17 - Jaundice |date=2011-01-01 |work=Pediatric Gastrointestinal and Liver Disease (Fourth Edition) |pages=176–186.e3 |editor-last=Wyllie |editor-first=Robert |url=https://linkinghub.elsevier.com/retrieve/pii/B978143770774810017X |access-date=2025-02-14 |place=Saint Louis |publisher=W.B. Saunders |doi=10.1016/b978-1-4377-0774-8.10017-x |isbn=978-1-4377-0774-8 |last2=Gourley |first2=Glenn R. |editor2-last=Hyams |editor2-first=Jeffrey S.}}</ref>

Delta bilirubin is albumin-bound conjugated bilirubin.<ref name="Tietze 2012 pp. 86–122" /> In the other words, delta bilirubin is the kind of bilirubin covalently bound to [[albumin]], which appears in the serum when hepatic excretion of conjugated bilirubin is impaired in patients with [[hepatobiliary disease]].<ref>{{cite book |title=Nelson Textbook of Pediatrics |vauthors=Moyer KD, Balistreri WF |publisher=Saunders |year=2011 |isbn=978-1-4377-0755-7 |veditors=Kliegman RM, Stanton BF, St Geme JW, Schor NF, Behrman RE |page=1405 |chapter=Liver Disease Associated with Systemic Disorders |chapter-url=http://www.mdconsult.com/books/page.do?eid=4-u1.0-B978-1-4377-0755-7..00352-3&isbn=978-1-4377-0755-7&type=bookPage&from=content&uniqId=433360670-2}}</ref> Furthermore, direct bilirubin tends to overestimate conjugated bilirubin levels due to unconjugated bilirubin that has reacted with diazosulfanilic acid, leading to increased azobilirubin levels (and increased direct bilirubin).{{cn|date=July 2024}}

δ bilirubin = total bilirubin – (unconjugated bilirubin + conjugated bilirubin)<ref name="Tietze 2012 pp. 86–122" />

=====Half-life=====

The half-life of delta bilirubin is equivalent to that of [[human serum albumin|albumin]] since the former is bound to the latter, yields 2–3 weeks.<ref name="Kalakonda John 2019 p.">{{cite book |title=StatPearls |vauthors=Kalakonda A, John S |publisher=StatPearls Publishing |year=2019 |location=Treasure Island (FL) |chapter=Physiology, Bilirubin article-18281 |pmid=29261920 |quote=This fraction of conjugated bilirubin gets covalently bound to albumin, and is called delta bilirubin or delta fraction or biliprotein. As the delta bilirubin is bound to albumin, its clearance from serum takes about 12–14 days (equivalent to the half-life of albumin) in contrast to the usual 2 to 4 hours (half-life of bilirubin). |access-date=2019-12-22 |chapter-url=http://www.ncbi.nlm.nih.gov/books/NBK470290/}}</ref><ref name="Medscape Reference 2019" />

A free-of-bound bilirubin has a half-life of 2 to 4 hours.<ref name="Kalakonda John 2019 p." />

{{further|Bilirubin glucuronide}}