Indirect Coombs Test (Antibody Screen)
The indirect antiglobulin test (IAT) screens a patient's serum for unexpected antibodies against red blood cell antigens. It is mandatory before every blood transfusion to prevent hemolytic reactions, and during pregnancy — especially in Rh-negative women — to detect alloimmunization that could cause hemolytic disease of the newborn. A positive screen triggers identification of the specific antibody and selection of compatible donor blood.
Expected Result
Normal
Negative (no unexpected antibodies detected)
Routine screen; if positive, antibody identification panels are run.
This is a qualitative test — results are reported as positive or negative rather than as a numeric range. Interpretation may vary by laboratory method; always review with your doctor.
What a Negative Result Means
A negative antibody screen means no clinically significant unexpected red-cell antibodies are present — standard crossmatched donor blood can be given safely. In pregnancy, negative at booking and at 28 weeks is reassuring; Rh-negative mothers still receive anti-D prophylaxis to prevent future alloimmunization.
What a Positive Result Means
A positive antibody screen means the serum contains antibodies that could react with donor red cells. Identification panels determine the specific antibody (anti-D, anti-Kell, anti-Duffy, anti-Kidd, and others). Blood for transfusion must then lack the corresponding antigen. In pregnancy, a positive screen with anti-D or anti-Kell requires close fetal monitoring for hemolytic disease of the newborn.
Frequently Asked Questions
Why is the indirect Coombs test done in pregnancy?
Routine antibody screening in early pregnancy and at 28 weeks detects alloimmunization — usually from a previous pregnancy, transfusion, or Rh-incompatibility. Early detection allows anti-D prophylaxis, close fetal monitoring by middle cerebral artery Doppler, and planning for intrauterine transfusion if fetal anemia develops.
What antibodies matter most for transfusion?
Anti-D (most clinically significant), anti-Kell, anti-Duffy (Fya), anti-Kidd (Jka), and anti-c are high-priority. ABO antibodies are naturally occurring and handled through ABO typing. Antibodies like anti-Lewis or anti-M are common but rarely cause problems. Transfusion-service protocols identify and manage each type.
If my indirect Coombs is positive, can I still receive blood?
Yes — but with care. The specific antibody is identified and antigen-negative donor units are selected. This can take extra time and, in the case of rare antibodies, may require sourcing blood from a national registry. Crossmatched blood from an antigen-negative donor is as safe as standard transfusion for you.
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This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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