Editing Coombs test (section)

== Direct Coombs test ==
The direct Coombs test, also referred to as the direct antiglobulin test (DAT), is used to detect if antibodies or [[complement system]] factors have bound to [[red blood cell|RBCs]] surface antigens.<ref>{{Cite book|title=Williams hematology|vauthors=Reid ME, Lomas-Francis C|publisher=McGraw-Hill|isbn=9780071833004|edition=9th|location=New York|chapter=Chapter 136: Erythrocytes Antigens and Antibodies|date=23 December 2015|oclc=913870019}}</ref> The DAT is not required for pre-transfusion testing<ref name="JPAC Guidelines">{{cite web |url=https://www.transfusionguidelines.org/red-book/chapter-13-patient-testing-red-cell-immunohaematology/13-3-pre-transfusion-testing |title=13.3: Pre-transfusion testing |author=<!--Not stated--> |date=4 October 2023 |publisher=Joint United Kingdom (UK) Blood Transfusion and Tissue Transplantation Services Professional Advisory Committee}}</ref> but may be carried out by some laboratories.{{clarify|date=October 2015}} Before transfusion, an ''indirect'' Coombs test is often done.

=== Uses ===
[[File:Algorithm in positive direct antiglobulin test (DAT, or direct Coombs test).png|500px|thumb|Algorithm for the main diagnoses in a positive DAT<ref>Image by Mikael Häggström, MD. Source for conditions: {{cite web|url=https://www.bbguy.org/2017/02/27/028/|title=028: Who DAT? with Sue Johnson|author=Joe Chaffin|date=2017-02-27}} Citing: C Feldman & J O'Connor.</ref>]]
The direct Coombs test is used clinically when immune-mediated [[hemolytic anemia]] (antibody-mediated destruction of RBCs) is suspected. A positive Coombs test indicates that an immune mechanism is attacking the patient's [[red blood cell|RBCs]]. This mechanism could be [[autoimmunity]], [[alloimmunity]] or a drug-induced immune-mediated mechanism.<ref name="Zantek et al. 2012"/>

====Examples of alloimmune hemolysis====
* [[Hemolytic disease of the newborn]] (also known as HDN or erythroblastosis fetalis)
** [[Rh D hemolytic disease of the newborn]] (also known as Rh disease)
** [[ABO hemolytic disease of the newborn]] (the direct Coombs test may only be weakly positive)
** [[Anti-Kell hemolytic disease of the newborn]]
** [[Rh c hemolytic disease of the newborn]]
** [[Hemolytic disease of the newborn (anti-RhE)|Rh E hemolytic disease of the newborn]]
** Other blood group incompatibility (RhC, Rhe, Kidd, Duffy, Lewis, MN, P and others)
* Alloimmune hemolytic [[blood transfusion#Complications|transfusion reactions]]

====Examples of autoimmune hemolysis/immunohemolytic hemolysis<!--more accurate term than "autoimmune hemolysis," as the former includes extrinsic causative factors like drug exposure see Kumar, et al. Robbins and Cotran Pathologic Basis of Disease, 8th ed., 2010, p. 653-->====
* [[Warm antibody autoimmune hemolytic anemia]]
** Idiopathic
** [[Systemic lupus erythematosus]]
** [[Evans' syndrome]] (antiplatelet antibodies and hemolytic antibodies)
* [[Cold agglutinin disease|Cold antibody immunohemolytic anemia]]
** [[Idiopathic cold hemagglutinin syndrome]]
** [[Waldenström's macroglobulinemia]]
** [[Infectious mononucleosis]]
** [[Paroxysmal cold hemoglobinuria]] (rare)

====Drug-induced immune-mediated hemolysis====
* [[Methyldopa]] (IgG mediated type II hypersensitivity)
* [[Penicillin]] (high dose)
* [[Quinidine]] (IgM mediated activation of classical complement pathway and Membrane attack complex, MAC)

(A [[mnemonic|memory device]] to remember that the ''DAT'' tests the [[red blood cell|RBCs]] and is used to test infants for ''haemolytic disease of the newborn'' is:
'''''R'''h '''D'''isease''; '''''R''''' = RBCs, '''''D''''' = DAT.)

===Laboratory===
The patient's [[red blood cell|RBCs]] are washed (removing the patient's own [[blood plasma|serum]]) and then centrifuged with [[Coombs test#Coombs reagent|antihuman globulin]] (also known as Coombs reagent).  If [[immunoglobulin]] or complement factors have been fixed on to the RBC surface [[in-vitro]], the antihuman globulin will [[Agglutination (biology)|agglutinate]] the RBCs and the direct Coombs test will be positive. (A visual representation of a positive direct Coombs test is shown in the upper half of the schematic).{{cn|date=November 2021}}