Overview
The direct antiglobulin test (DAT) detects immunoglobulin (typically IgG) and/or complement components already bound to a patient's red cells in vivo, using washed patient cells tested directly against antihuman globulin reagent.
Classification
| Reagent | Detects | Role |
|---|---|---|
| Polyspecific AHG | IgG and/or complement | Initial DAT screen |
| Monospecific anti-IgG | IgG only | Localizes coating type after positive screen |
| Monospecific anti-C3d | Complement (C3d) only | Localizes coating type after positive screen |
Morphologic Features
Not a cell-morphology topic. Findings are agglutination reactions (including possible mixed-field patterns) read after AHG addition to washed patient red cells.
Laboratory Characteristics
- Thorough washing of patient cells before AHG addition prevents false-negative results
- Run polyspecific DAT first; follow with monospecific reagents if positive, per SOP
- Document mixed-field reactivity (can suggest coated transfused cells or an early process)
- Run appropriate positive/negative controls per method
Reference Intervals
No numeric reference interval, result is reported as positive/negative (and reagent-specific if monospecific testing performed). Method sensitivity and reporting conventions are institution-specific.
Clinical and Laboratory Significance
Core test for evaluating suspected autoimmune hemolytic anemia, hemolytic disease of the fetus and newborn, drug-induced hemolysis, and transfusion reaction workups (especially delayed hemolytic reactions).
Differential Considerations
Autoantibody-mediated coating vs alloantibody-coated transfused cells vs drug-associated mechanism vs HDFN (maternal antibody on neonatal cells), distinguished with clinical history, monospecific reagent pattern, and, when performed, elution/identification studies.
Comparison Tables
DAT result framing
| Finding | Consider | Next step mindset |
|---|---|---|
| Polyspecific+, IgG+, C3d− | IgG-mediated coating (e.g., warm autoantibody, alloantibody) | Clinical correlation; possible elution studies |
| Polyspecific+, IgG−, C3d+ | Complement-only coating | Consider cold-antibody or complement-mediated process |
| Mixed-field agglutination | Coated transfused (donor) cells among uncoated cells | Correlate with transfusion timing |
| Positive DAT, no hemolysis markers | Coating without significant hemolysis | Clinical judgment; do not withhold transfusion by DAT alone |
Classification Frameworks
Not applicable, laboratory test procedure, not a WHO disease classification.
Laboratory Notes
- DAT tests the patient's own cells, do not confuse with the antibody screen (which tests plasma)
- Positive DAT informs, but does not by itself dictate, transfusion decisions
- Specificity of the coating antibody requires further workup beyond the DAT itself
References
Authoritative textbooks, guidelines, and reviews supporting this reference entry. Verify reference intervals, critical limits, and reflex criteria against institutional protocols and current guideline editions.
Textbooks
- Cohn CS, Delaney M, Johnson ST, Katz LM, eds. Technical Manual. 21st ed. AABB; 2023.
- Harmening DM. Modern Blood Banking & Transfusion Practices. 7th ed. F.A. Davis; 2019.
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2021.
CLSI and Professional Guidelines
- AABB. Standards for Blood Banks and Transfusion Services (verify current edition and institutional adoption).
- Clinical and Laboratory Standards Institute. Related immunohematology and specimen identity standards (verify current documents and institutional adoption).
Frequently Asked Questions
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