Coombs' Antiglobulin, Indirect

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Coombs’ Antiglobulin, Indirect

Synonym/acronym: Indirect antiglobulin test (IAT), antibody screen.

Common use

To check recipient serum for antibodies prior to blood transfusion.


Serum (1 mL) collected in a red-top tube.

Normal findings

(Method: Agglutination) Negative (no agglutination).


The indirect antiglobulin test (IAT) detects and identifies unexpected circulating complement molecules or antibodies in the patient’s serum. The first use of this test was for the detection and identification of anti-D using an indirect method. The test is now commonly used to screen a patient’s serum for the presence of antibodies that may react against transfused red blood cells (RBCs). During testing, the patient’s serum is allowed to incubate with reagent RBCs. The reagent RBCs used are from group O donors and have most of the clinically significant antigens present (D, C, E, c, e, K, M, N, S, s, Fya, Fyb, Jka, and Jkb). Antibodies present in the patient’s serum coat antigenic sites on the RBC membrane. The reagent cells are washed with saline to remove any unbound antibody. Antihuman globulin is added in the final step of the test. If the patient’s serum contains antibodies, the antihuman globulin will cause the antibody-coated RBCs to stick together or agglutinate. (See monograph titled “Blood Groups and Antibodies” and Effects of Natural Products on Laboratory Values online at DavisPlus for more information regarding transfusion reactions.)

This procedure is contraindicated for



  • Detect other antibodies in maternal blood that can be potentially harmful to the fetus
  • Determine antibody titers in Rh-negative women sensitized by an Rh-positive fetus
  • Screen for antibodies before blood transfusions
  • Test for the weak Rh-variant antigen Du. Development of anti-D antibodies occur when Rh-negative women become sensitized by an Rh-positive fetus. Antibody titers should be performed as soon as a subsequent pregnancy becomes known in order to appropriately anticipate management of hemolytic disease of the newborn (HDN). The IAT has also been used to test for the weak Rh-variant antigen Du. Modern technology and more potent reagents provide better sensitivity, and for this reason women who have the Du variant will likely be typed as Rh-positive. It is for this reason the AABB has determined weak D testing is no longer necessary to be used on obstetric patients. Women who have the weak D variant are tested using less-sensitive reagents, and those typed as Rh-negative will be candidates for immunization with Rh-immune globulin. Administration of Rh-immune globulin to these candidates is not harmful. Whether the test is used currently varies among facilities.

Potential diagnosis

Positive findings in:

  • Circulating antibodies or medications attach to the patient’s RBCs, and hemolysis occurs. Agglutination is graded from 1+ to 4+ in manual testing systems; with 4+ being the strongest degree of agglutination. Automated testing systems are capable of reporting 1+ to 4+ graded results, or providing images of the tested material so laboratory professionals can interpret the results, or providing computer assisted interpretation of the test results as positive or negative findings.

  • Hemolytic anemia (drug-induced or autoimmune)
  • Hemolytic disease of the newborn (related to ABO or Rh incompatibility)
  • Incompatible crossmatch
  • Infections (mycoplasma pneumonia, mononucleosis)
Samples in which the patient’s antibodies exhibit dosage effects (i.e., stronger reaction with homozygous than with heterozygous expression of an antigen) and reagent erythrocyte antigens contain single-dose expressions of the corresponding antigen (heterozygous) Samples in which reagent erythrocyte antigens are unable to detect low-prevalence antibodies Samples in which sensitization of erythrocytes has not occurred (true negative, complete absence of antibodies)

Critical findings


Interfering factors

  • Drugs that may cause a positive IAT include meropenem, methyldopa, penicillin, phenacetin, quinidine, and rifampin.
  • Recent administration of dextran, whole blood or fractions, or IV contrast media can result in a false-positive reaction.

Nursing Implications and Procedure


  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this test can assist in assessing for blood compatibility prior to transfusion.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s hematopoietic system as well as results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Note any recent procedures that can interfere with test results.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture. Prenatal mothers may be concerned about blood collection from their newborn. Explain that a cord sample of blood taken from the infant at the time of delivery does not result in infant blood loss.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • There are no food, fluid, or medication restrictions unless by medical direction.


  • Potential complications:
  • Acute hemolytic reactions, whether immune mediated or developed due to drug sensitivities, can be immediate and life threatening. Chronic hemolytic anemia is also a significant condition that requires timely identification of the problem in order to treat the condition.

  • Positive findings in the pregnant patient may require further investigation by amniocentesis. Any sampling method that involves penetration of natural tissue barriers carries the risk of infection.

  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Promptly transport the specimen to the laboratory for processing and analysis.


  • Inform the patient that a report of the results will be made available to the requesting health-care provider (HCP), who will discuss the results with the patient. It is important for the patient to be made aware of the presence of unusual antibodies. A person may have circulating antibodies, other than ABO/Rh group antibodies, which may respond to transfused blood. The antibodies attach to the person’s red blood cells, damaging the integrity of the cell wall, and hemolysis occurs. Therefore, it is important to screen for the presence of antibodies in the recipient’s serum prior to transfusion. Unexpected antibodies, other than ABO/Rh, can develop at any time. If present in maternal blood, they can be potentially harmful to the fetus, which makes antibody screening an important test in prenatal care.
  • Inform pregnant women that negative tests during the first 12 wk of gestation should be repeated at 28 wk to rule out the presence of an antibody.
  • Positive test results in pregnant women after 28 wk of gestation indicate the need for antibody identification testing.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include bilirubin, blood groups and antibodies, CBC hematocrit, CBC hemoglobin, Coombs’ direct antiglobulin (DAT), and haptoglobin.
  • Refer to Effects of Natural Products on Laboratory Values online at DavisPlus at the end of the book for further information regarding laboratory studies used in the investigation of transfusion reactions, findings and potential nursing interventions associated with types of transfusion reactions.
  • Refer to the Hematopoietic System table at the end of the book for related tests by body system.
Handbook of Laboratory and Diagnostic Tests, © 2013 Farlex and Partners
References in periodicals archive ?
The results from this investigation indicate that the performance of antenatal titrations by column agglutination technology (CAT) is at least as good as the performance by the indirect antiglobulin test (IAT) tube method.
(3,4) Similarly, automated testing methods are superior to standard tube low-ionic-strength solution (LISS) indirect antiglobulin test (IAT) (tube LISS-IAT) in detecting antibodies of known clinical significance, therefore increasing blood safety for transfused patients.
All 'D' negative samples by tube method were confirmed for weak D by an indirect antiglobulin test and subsequently by column agglutination technology.