THE DIRECT ANTIGLOBULIN TEST(DAT) and Elution/Eluate Testing

When performed as part of an antibody identification

Jean Purcelli MT(ASCP)SBBMay, 2011

Version 2, May 2012

Detects in vivo coating of red cells with IgG or complement components or both.

Blood collected in EDTA is the best source of redcells to use for a DAT.

Red cells from clotted samples may give false positive results when using polyspecific AHG or monospecific anti-C3d AHG.

The DAT may be ordered by physicians to investigate: • Autoimmune hemolytic anemia• Drug induced hemolylsis• Hemolytic disease of the fetus or (HDFN)• Alloimmune reactions due to recently transfused rbcs.

• The DAT is also included in a basic antibody investigation:

• The DAT has similar significance as the auto control, but they cannot be substituted for one another.

The DAT may be positive in:

•An immediate or delayed transfusion reaction. Circulating donor rbcs become coated with antibodies in the patient’s plasma.

•Auto immune diseases : cold and warm autoimmune hemolytic anemia.

DAT positive

• Hemolytic disease of the fetus or newborn (HDFN)

• The cord cells and circulating fetal rbcs are coated with maternal antibodies.

• The fetus’ rbcs may be coated with antibodies from any clinically significant maternal antibody including the ABO system.

ABO antibodies are usually IgM and do not cross the placenta.

Group O individuals often have an IgG component of anti-A and –B, as well.

HDN due to ABO system antibodies are usually not severe and high levels of bilirubin are controlled with bilirubin lights.

DAT positive

• Collagen-vascular diseases: SLE, RA

• Drug induced hemolytic anemia (alpha methyldopa, phenacetin, penicillin, quinine, any cephalosporins, procainamide.

• Paroxysmal Cold Hemolysis (PCH)

DAT positive

•Treatment with IVIG: may contain ABO or anti-D antibodies or other specificities.

•Bone marrow or organ transplant: passenger lymphocytes of donor origin may produce ABO or other antibodies against the recipient cells.

DAT positive

•Treatment of transplant patients with antilymphocyte globulin (ALG) or antithymocyte globulin (ATG).

•Treatment with IV RHIG.

•Nonspecific adsorption of proteins.

In the laboratory, the DAT is included as part of an antibody investigation.

Washed rbcs are directly tested with AHG reagents.

Polyspecific and/or monospecific reagents are used.

Polyspecific AHG contains both anti-IgG and anti-complement components.

Monospecific anti-IgG contains only anti-IgG.

Monospecific anti-complement contains anti-C3d and usually anti-C3b.

Polyspecific AHG is used as a screening test.

If the test is negative the DAT is reported as negative.

If the test is positive, repeat the test with monospecific reagents.

A saline control tube is included in all DAT tests.

Microscopic examination should be included.

“mixed field” agglutination may be observed in the recently transfused patient.

After examination at the IS (immediate spin) phase:1.The results are recorded.

2.If the tube with anti-IgG is nonreactive, Checkcell is added.

3.If non-reactive, the polyspecific tube and the complement tube are placed at RT for 5-10 minutes, centrifuged and examined micro again, recorded and Check Cell added.

Checkcells are IgG coated red cells and will not react with tubes containing anti-C3d,-C3b.

An anti-complement “check cell” should be used.

ELUTION and ELUATE TESTING

• An elution should be performed as part of the antibody identification ONLY if the patient was recently transfused.

• Each reference lab must establish their own guidelines for defining “recently”. In the USA, 4 weeks to 3 months is usually used.

Elution/Eluate

An eluate is prepared and tested to detect a newly formed antibody that is not yet detectable in the serum.

Antibody production peaks at about 10-18 days after exposure to a foreign antigen.

Elution/Eluate• The eluate is tested by the IAT and usually the

same panel cells that are chosen to test the serum are also used for the eluate. Enhancement media is commonly used.

• A1 and B cells should also be tested if there is ABO incompatibility between a mother and baby.

• Also in adults if there is a history of recent transfusion of ABO incompatible platelets or plasma.

Elution/Eluate

• Elutions are not performed on rbcs coated only with complement.

• The last wash is always saved and tested for unusual dissociation of antibody from the red cells.

• The last wash test, is an antibody screen with enhancement media by IAT.

Elution/Eluate

• The last wash should be negative or additional washes of the red cells must be performed.

• A positive “last wash” would make the results of the eluate testing invalid.

• It is recommended that LISS or normal saline, kept at 4 C is used for washing the red cells.

The final report may contain comments about eluate, for example:1.Anti-Jka was identified in the eluate and may indicate the patient is having a delayed transfusion reaction.2.The eluate was non-reactive.3.The eluate was reactive with all rbcs without apparent specificity.

If the DAT is positive with anti-IgG, but the eluate is nonreactive it may indicate:

1.The DAT was induced by drugs

2.The DAT was caused by non-specific uptake of proteins.

• Pease refer to “Selected Procedures for Fast Referral”.

• In the lab we will use Elukit IITM reagents and procedure.

Preparing red cells for elution

The goal is to wash a large volume of red cells so no trace of contaminating unattached plasma

proteins remain.

Large 13 mm x 100 mm or 16 mm x 100 mmtest tubes are preferred.

• LISS or normal saline stored a 4oC is recommended for washing the red cells to prepare them for elution.

• It is recommended to change to clean test tubes 2x during the washing procedure.

• If 13 mm or 16 mm x 100 are used, five washes are adequate, then washed a final time in a 12 mm x 75 mm tube.

• An aliquot of the last wash is saved and tested before the elution is performed.

• If 12 x 75 mm are used, then 10 washes would be adequate with 3 tube changes.

“Last Wash”

• The last wash is tested using the three antibody screening cells.

• Add two drops of last wash to each tube.

• Add two drops of LISS or PEG.

• Place at 370 for 15’ to 30’, then wash 3-4 times and convert to the antiglobulin test.

• The last wash test should be non-reactive before proceeding with the elution.

• If the last wash is reactive, the red cells should be washed 4 more times and the “last wash” saved and tested again.

Elution

1. To 1 ml washed red cells, add 1 ml of Eluting Solution

1. Cover with parafilm and gently mix 4X.

1. Centrifuge for 90” and quickly harvest the eluate. The red cells may be discarded.

Elution

4. Centrifuge the eluate and transfer to a clean tube.

5. Add drops of Buffering Reagent until the color turns to blue. 6. Centrifuge again and transfer to a clean tube.

Eluate Testing

• Test the eluate with the same cells you tested with the serum.

• LISS or PEG may be used for enhancement.

• In general, if the DAT is 2-4+, use LISS for the last wash and the eluate. If the DAT is 1+ or micro +, use PEG for testing.