13.4 Direct Antiglobulin Testing
Key Takeaways
- DAT detects in-vivo coating of the patient’s red cells with IgG and/or C3; you add AHG to washed patient cells and do not add plasma.
- Start with polyspecific AHG, then split a positive into anti-IgG versus anti-C3; IgG-positive plus HTR, HDFN, or AIHA suspicion is an elution.
- False positives include Wharton jelly, overcentrifugation, dirty glassware, and cold agglutinin in gel; false negatives include underwashing, expired AHG, interrupted washing, and IgG below detection.
- IgG-coated check cells must agglutinate after every negative AHG test or the result is invalid.
- Interpret the DAT with clinical history — recently transfused, newborn, known warm auto, drugs, and IVIG are not the same story.
13.4 Direct Antiglobulin Testing
Quick Answer: The DAT detects in-vivo coating of the patient’s own red cells with IgG and/or C3. You wash the cells and add AHG — no plasma is added. Start with polyspecific AHG (anti-IgG + anti-C3), then split a positive with monospecific anti-IgG and anti-C3. Elute when the DAT is IgG-positive and transfusion, HDFN, or AIHA is suspected. Check cells (IgG-coated red cells) must agglutinate after every negative AHG test. False positives: Wharton jelly, overcentrifugation, dirty glassware, cold agglutinin in gel. False negatives: underwashing, expired AHG, interrupted washing, IgG below detection. Read the DAT with the history.
The June 9, 2026 outline lists DAT as IV.A.4, after grouping and compatibility and before antibody identification. Chapter 10 already defined DAT versus IAT. This section is the procedure, the artifacts, the eluate, and the check cells.
What the DAT is asking
IAT (screen, panel, AHG XM, weak D) asks whether antibody in plasma can coat cells in the tube. DAT asks whether the patient’s circulating red cells already wear IgG or C3. You do not add the patient’s plasma. Adding plasma converts the test into an autocontrol.
EDTA is the preferred specimen because it chelates calcium and stops in-vitro complement from landing after the draw. A clotted tube stored in the refrigerator is a classic false-positive C3 DAT — complement binds in the cold in vitro. Do not diagnose cold AIHA from a fridge-clot tube.
Indications, not a standing order on every CBC:
- Transfusion-reaction workup and unexplained anemia after recent transfusion (delayed HTR).
- HDFN / jaundiced newborn, especially of a group O or alloimmunized mother.
- Autoimmune hemolysis (warm or cold) and some drug reactions.
- Positive autocontrol on a screen or panel.
- Unexplained hemolysis when the clinician asks the blood bank the right question.
A DAT strength does not equal disease severity. A 4+ DAT with a stable hematocrit is not automatically worse than a 1+ DAT, a falling hemoglobin, and a climbing bilirubin.
Polyspecific first, then IgG versus C3
Polyspecific AHG contains anti-IgG plus anti-C3b/C3d. It is the DAT screen. If it is negative and check cells work, the DAT is negative by that method.
If polyspecific is positive, repeat with monospecific anti-IgG and anti-C3:
| Pattern | Typical story | Next thought |
|---|---|---|
| IgG only | Warm AIHA, delayed HTR, HDFN, many drugs (penicillin-type), IVIG, passive anti-D | Elution if you need the specificity |
| C3 only | Cold agglutinin disease (IgM binds in the periphery, fixes C3, then comes off at 37 °C), some drugs | Do not expect an IgG eluate |
| IgG + C3 | Warm AIHA commonly; some delayed HTR | Elute the IgG; complement is extra information |
| Both negative after poly positive | Technique, faded complement, or a reagent problem | Repeat; do not invent a new antibody class |
Elution pulls IgG off the cells so you can identify it. Do it when the DAT is IgG-positive and the question is what is the coating antibody — recent transfusion (is there an alloantibody on donor cells?), HDFN (is it ABO, anti-D, or something else?), or new AIHA. Acid elution is the routine IgG method; Lui freeze-thaw or heat elution is the classic ABO HDFN trick. The last-wash control must be negative; if the last wash still reacts, you did not isolate cell-bound antibody.
You do not elute every DAT. A C3-only DAT has no IgG to recover. A patient with long-standing warm AIHA whose eluate was already panagglutinin last week does not need a daily eluate unless something changed (new transfusion, new specificity). IVIG and RhIG can produce a positive DAT with anti-D or a panreactive eluate that is passive, not a new alloantibody.
Check cells are not optional
After every negative AHG test — DAT, IAT screen, panel, AHG XM, weak D — you add IgG-coated check cells (Coombs control cells). They must agglutinate.
Check cells prove three things at once: AHG was added, the AHG is still potent, and enough unbound IgG was washed away that AHG was not neutralized. If check cells fail, the negative result is invalid. Repeat the test. Do not report “DAT negative, check cells 0.”
Check cells that work do not prove you washed the original cells correctly in every philosophic sense, but a failed check-cell reaction is an automatic stop.
False positives and false negatives
False-positive DAT (agglutination that is not true in-vivo IgG/C3 coating of interest):
- Wharton jelly on poorly washed cord cells — sticky aggregates that look like agglutination.
- Overcentrifugation packing cells into a button that will not resuspend cleanly.
- Dirty glassware or contaminated saline (metal ions, residual detergent, bacterial film).
- Cold agglutinin in gel — gel DAT is sensitive; a cold can look like a positive DAT, especially if the card sat cold.
- Refrigerated clot sample (in-vitro C3).
- Overreading, fibrin, or septic/contaminated cell suspensions.
False-negative DAT:
- Underwashing in tube IAT/DAT — residual plasma IgG neutralizes AHG. Check cells often fail here; that is the gift.
- Interruption of washing — IgG already bound can dissociate if washed cells sit in saline.
- Expired or impotent AHG.
- Low-level IgG below the method’s detection (some ABO HDFN, early delayed HTR).
- Using anti-IgG only when the coating is C3 only (cold AIHA).
- Improper cell suspension or undercentrifugation so a weak button is shaken away.
Gel DAT reduces wash errors and increases cold noise. Tube DAT does the opposite. Know which artifact belongs to which platform.
Interpret with clinical history
The same 2+ IgG DAT is four different cases:
- Transfused 10 days ago, falling hemoglobin, newly positive screen: delayed HTR. Eluate and serum may show the new alloantibody (honor Kidd).
- Group O mother, group A neonate, spherocytes, modest DAT: ABO HDFN. DAT may be weak or negative; do not use a negative DAT to rule it out (Chapter 12).
- Untransfused adult, anemia, spherocytes, panagglutinin: warm AIHA. Eluate reacts with all cells.
- Just received IVIG or antenatal RhIG: passive coating. Do not open a full AIHA workup until the history is read.
Mixed-field DAT after recent transfusion means two cell populations — usually coated donor cells plus the patient’s own. That is the delayed-serologic postcard, not a gel manufacturing defect.
Exam traps
- Adding patient plasma to a DAT and calling it “more sensitive.” That is an autocontrol.
- Skipping check cells on a negative AHG test.
- Eluting a C3-only DAT and being surprised the eluate is empty.
- Diagnosing cold AIHA from a refrigerated clot tube.
- Ruling out ABO HDFN because the DAT is negative.
- Calling Wharton jelly a new antigen on the cord.
- Reporting a tube DAT after an interrupted wash or underwash because the button “looked negative.”
A tube DAT with polyspecific AHG is nonreactive. IgG-coated check cells added to the same tube are also nonreactive. What is the correct interpretation?
Which set correctly pairs a DAT artifact with the direction of the error?
A DAT is 2+ with anti-IgG and 0 with anti-C3 in a patient transfused 8 days ago who now has a falling hemoglobin. What is the most appropriate next serologic step?