Overview
The anti globulin test (Coomb's test), introduced by Coombs, Mourant and Race in 1945, detects antibodies that bind red cell antigens but cannot directly agglutinate them — so-called incomplete antibodies. It remains one of the most important tests in transfusion medicine and immunohematology.
There are two forms: the Direct Anti Globulin Test (DAT), which reflects in vivo sensitization of the patient's own red cells, and the Indirect Anti Globulin Test (IAT), which detects free antibody in the patient's serum using reagent red cells.
Learning Objectives
After this lesson you will be able to…- Describe the principle of the Anti-Globulin test.
- Explain the procedure of the Direct and Indirect Anti Globulin Test.
- Describe the clinical uses of the Direct and Indirect Anti Globulin Test.
- Interpret DAT and IAT results correctly, including positive/negative controls.
- List methods to increase the sensitivity of the IAT.
Clinical Story
Why This MattersA newborn develops jaundice within 24 hours of birth to an Rh-negative mother. The blood bank technologist performs a Direct Coomb's test on the cord blood — it turns positive, confirming that maternal anti-D antibodies have coated the baby's red cells. This single test result triggers urgent phototherapy and monitoring to prevent kernicterus.
Core Concepts
Antihuman globulin (AHG) antibodies combine with red cells coated with human immunoglobulin or complement (whether sensitized in vivo or in vitro). The AHG acts as a bridge connecting antibody-coated cells, causing visible agglutination.
DAT reflects in vivo antibody sensitization — patient's own red cells are tested directly against AHG reagent. Uses include detecting warm autoimmune hemolytic anemia, hemolytic transfusion reactions, hemolytic disease of the newborn, and drug-induced antibodies. Presence of agglutination = positive DAT (immunoglobulin/complement bound to RBCs); absence = negative DAT.
IAT detects the presence of IgG antibody in serum (in vitro sensitization) — patient's serum is tested against reagent red cells (Coomb's control cells, typically pooled O-positive cells). Used for cross matching, erythrocyte antigen typing, and detecting anti-D antibodies in an Rh-negative woman married to an Rh-positive man. A positive test means the patient's serum contains the antibody in question (e.g. anti-D).
Sensitivity of the IAT can be improved by optimizing incubation temperature at 37°C, increasing the serum-to-red-cell ratio, and adding potentiating reagents like LISS (low ionic strength saline), albumin or enzymes.
Laboratory Principle
Incomplete (non-agglutinating) antibodies bind red cell antigens but are too small or structurally unable to bridge the distance between adjacent red cells to cause direct agglutination. AHG reagent, containing antibodies against human immunoglobulin/complement, physically bridges these antibody-coated cells together, converting an invisible sensitization into a visible agglutination reaction.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| AHG reagent | Polyspecific/monospecific | Bridges antibody/complement-coated RBCs | 2–8°C |
| Positive control cells | IgG-coated red cells | Validate AHG reagent reactivity when DAT is negative | 2–8°C |
| Coomb's control cells (O positive) | 5% saline suspension, pooled from 3 donors | Reagent RBCs for IAT | 2–8°C |
| Anti-D serum | Positive control | Positive control for IAT | 2–8°C |
| Normal saline | 0.9% | Washing red cells | Room temperature |
Step-by-Step Procedure
Make a 5% cell suspension by washing patient blood 3 times with normal saline; decant supernatant completely.
Add 2 drops of AHG reagent to 1 drop of patient's cell suspension; mix well.
Centrifuge at 1500 RPM for 1 minute; gently resuspend and examine for agglutination with naked eye and microscope.
If negative, add a drop of IgG-coated positive control cells, mix, centrifuge, and confirm agglutination — a negative control result invalidates the test.
Set up Test, Positive Control and Negative Control tubes with serum/Coomb's control cells, wash 3 times, add AHG, incubate 5–10 min at room temperature, centrifuge and read for agglutination.
Flow Diagram
Quality Control
Any negative DAT result must be validated by adding IgG-coated positive control cells — if no agglutination occurs even then, the test is invalid and must be repeated with fresh AHG reagent.
Participate in immunohematology proficiency testing schemes to ensure consistent DAT/IAT interpretation across technologists and shifts.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Positive DAT | Autoantibody or alloantibody bound to patient's own RBCs (warm AIHA, HDNB, transfusion reaction, drug-induced) | Identify antibody specificity; correlate with clinical picture |
| Negative DAT with positive controls reacting appropriately | No detectable in vivo sensitization | Consider alternative causes of hemolysis |
| Positive IAT | Free antibody present in serum (e.g. anti-D) | Cross-match carefully; monitor at-risk pregnancies |
Common Errors & How to Avoid Them
Cause: Residual free serum/globulin neutralizes AHG reagent before it can bridge sensitized cells.
Prevention: Wash cells at least 3 times with normal saline and decant supernatant completely before adding AHG.
Cause: Failing to add positive control cells after a negative result.
Prevention: Always confirm negative DAT/IAT results with the positive control step before reporting.
Cause: Excessive centrifuge speed/time can create pseudo-agglutination that resembles a true positive.
Prevention: Strictly follow standardized centrifuge speed (1500 RPM) and time (1 minute); gently resuspend before reading.
Laboratory Tips from the Bench
Always examine agglutination both macroscopically and microscopically — weak reactions can be missed by naked eye alone.
Adding LISS or albumin enhancement media significantly boosts IAT sensitivity for weak or low-titer antibodies.
"D for Direct, own cells Directly tested" — DAT tests the patient's own RBCs; IAT tests the patient's serum indirectly against reagent cells.
Important Notes
If the positive control shows no agglutination, or the negative control shows agglutination, the entire test run must be considered invalid and repeated.
Beyond antibody screening, IAT is essential for pre-transfusion cross-matching and erythrocyte antigen typing, not just anti-D detection.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer.
Clinical Case Study
Apply Your KnowledgeBorn to an Rh-negative mother who was not given anti-D prophylaxis in a prior pregnancy. Baby develops jaundice within the first 24 hours of life.
The positive DAT on cord blood confirms that maternal anti-D IgG antibodies crossed the placenta and coated the baby's Rh-positive red cells, consistent with hemolytic disease of the newborn.
- →DAT on cord blood is a key test in suspected HDNB.
- →Anti-D prophylaxis in earlier pregnancies could have prevented sensitization.
- →Maternal indirect Coomb's titre monitoring identifies at-risk pregnancies antenatally.
Frequently Asked Questions
Incomplete antibodies (usually IgG) are too small to span the distance between two red cells caused by their negative surface charge (zeta potential), so they sensitize but do not visibly agglutinate cells without an AHG bridge.
Yes — a positive DAT indicates antibody or complement is bound to the red cells, but does not always mean active hemolysis is occurring; clinical correlation with hemoglobin trends and bilirubin is essential.
Clinically significant antibodies like anti-D are typically IgG and react optimally at body temperature (37°C), maximizing sensitivity for detecting these antibodies.
Quick Revision
10-Minute ReviewKey Takeaways
- The Anti Globulin Test detects incomplete antibodies that sensitize but do not directly agglutinate red cells.
- DAT reflects in vivo sensitization; IAT detects free serum antibody in vitro.
- DAT is essential for diagnosing AIHA, transfusion reactions and HDNB.
- IAT underpins cross-matching, antigen typing and anti-D antibody detection in pregnancy.
- Positive and negative controls are mandatory to validate every test run.
- Enhancement media (LISS, albumin, enzymes) improve test sensitivity for weak antibodies.
Competency Checklist
Track Your MasteryReferences
- NIOS Hematology and Blood Bank Technique — Lesson 22: Anti Globulin Test.
- Coombs RRA, Mourant AE, Race RR. A new test for the detection of weak and incomplete Rh agglutinins. 1945.