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Coombs Test (Indirect)

Autoimmune disorder Biomarker

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Collection Type: Blood

Body System

Related System: Autoimmune disorder

Overview

The Indirect Coombs test (indirect antiglobulin test, IAT) detects antibodies directed against red blood cell (RBC) surface antigens that are present in a patient’s serum but not bound to cells. It measures whether circulating alloantibodies or autoantibodies exist that could react with donor blood or fetal RBCs. It is used when hemolytic anemia, transfusion compatibility concerns, or fetal/newborn isoimmunization (risk of hemolytic disease of the fetus and newborn) are suspected. Symptoms prompting testing include unexplained anemia, jaundice, dark urine, fatigue, or a history of transfusion/pregnancy. Results vary with pregnancy status, prior transfusions, autoimmune disease, age, and sex (women with prior pregnancies have higher alloimmunization risk).

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Test Preparation

Duly Filled Coombs (Direct/Indirect) Form is mandatory

Why Do I Need This Test

Profile: included in Autoimmune disorder / transfusion compatibility and antenatal antibody screening panels - Symptoms: unexplained hemolysis, anemia, jaundice, dark urine, or prior transfusion reaction - Conditions: detects alloimmunization, risk for hemolytic disease of the fetus/newborn, and antibody-mediated hemolytic anemia - Reasons for abnormal: prior transfusion, pregnancy, autoimmune disease, certain drugs, or recent exposure to foreign RBC antigens - Biological meaning: a positive test shows circulating RBC-directed antibodies that can cause hemolysis or transfusion incompatibility - Family history: prior family history of hemolytic disease in newborns or autoimmune hemolytic anemia increases likelihood of testing

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Understanding Test Results

A negative Indirect Coombs (no agglutination; titer <1:8) means no detectable RBC-directed antibodies in serum and low risk for antibody-mediated transfusion reactions or fetal hemolysis.
A low-positive/weak reaction or low titer (around 1:8–1:16, lab-dependent) may be of uncertain significance and typically prompts repeat testing or antibody identification.
Titers ≥1:16–1:32 (threshold varies by laboratory) are often considered clinically significant; rising or high titers indicate a substantial level of allo- or autoantibody and higher risk of hemolytic disease of the fetus/newborn or transfusion reactions.
Strongly positive results require antibody identification, crossmatch precautions, monitoring (e.g., maternal antibody titers, fetal MCA Doppler), and may necessitate antigen-negative blood or treatment.

Normal Range

Negative (no agglutination) OR titer <1:8

FAQs

Q: When is the indirect Coombs test done?

A: The indirect antiglobulin (Coombs) test is performed to screen a patient’s serum for red‑cell antibodies before transfusion and in pregnancy to detect maternal alloantibodies. In prenatal care it’s done at the first visit, around 28 weeks, and after sensitising events (bleeding, miscarriage, amniocentesis or transfusion). It’s also used for compatibility testing prior to transfusion and when unexplained hemolysis is suspected.

Q: What does a positive indirect Coombs test mean in pregnancy?

A: A positive indirect Coombs test in pregnancy means the mother has antibodies against fetal red blood cell antigens (often Rh or other blood groups). These antibodies can cross the placenta and may cause hemolytic disease of the fetus/newborn. Positive results prompt antibody identification and serial titers, closer fetal monitoring (ultrasound/Doppler), and treatment planning to prevent or manage fetal anemia.

Q: What if an indirect Coombs test is negative?

A: If an indirect Coombs test is negative, no significant antibodies to red blood cell antigens are detected in the patient’s serum. This indicates low immediate risk of immune-mediated hemolysis from transfusion or of fetal hemolytic disease. Routine care continues, but repeat testing is advised after sensitizing events (pregnancy, transfusion) or if unexplained hemolysis occurs, since low-level or newly formed antibodies may not yet be detectable.

Q: What is the difference between Coombs test and indirect Coombs test?

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Q: What if a Coombs test is positive?

A: A positive Coombs (direct antiglobulin) test means antibodies or complement are bound to red blood cells, indicating immune‑mediated hemolysis. Causes include autoimmune hemolytic anemia, drug‑induced reactions, transfusion reactions, or hemolytic disease of the newborn. Next steps are antibody identification, blood counts and hemolysis tests (bilirubin, LDH, haptoglobin, reticulocyte count), and treating the cause (steroids, stop offending drug, transfusion if needed).

Q: What to do if baby is Coombs positive?

A: A positive direct antiglobulin (Coombs) test means antibodies are attached to the baby’s red cells. Monitor closely for hemolysis and jaundice: check bilirubin, hemoglobin and reticulocyte counts and repeat clinical exams. Treat high bilirubin with phototherapy; consider IVIG or exchange transfusion for severe hemolysis. Ensure adequate feeding, arrange neonatal/pediatric hematology follow‑up, and treat anemia if present.