Deciding to Perform an Elution for the Workup of a Positive Direct Antiglobulin Test in the Newborn Infant

 

If the mother's DAT is negative, the differential diagnosis of a positive DAT in a newborn infant is:

(1) false positive (due to contamination from Wharthon's jelly of the umbilical cord, etc.)

(2) anti-ABO (typically a type O mother with a type A or type B infant)

(3) anti-D

(4) another alloantibody

 

If both the mother and infant are of the same D status (with Du considered D-positive), then anti-D will not be formed by the mother to the infant. If the mother is D-negative and the infant D-positive, then anti-D can be formed by the mother but may not be.

 

In alloimmune hemolytic disease of the newborn, the antibody is formed in the mother and must pass across the placenta to enter the fetal circulation. This means:

(1) in the absence of a maternal-to-fetal hemorrhage, IgM antibody should not be found as only IgG can cross the placental barrier.

(2) if the mother's antibody screening test is negative, then an unexpected alloantibody (non-ABO antibody) is unlikely.

 

Procedure

 

Step 1: Perform the direct antiglobulin test on the cord sample.

 

Step 2: If the DAT on cord blood is positive,

(2a) Repeat the direct antiglobulin test on a heelstick specimen to confirm the positive DAT and to exclude a false positive DAT.

(2b) Perform a DAT on the maternal blood.

 

Step 3: If the heelstick DAT is positive and the maternal DAT is negative, then decide if an elution needs to be performed by matching the findings in the 3 tables below.

(3a) If findings are "Yes" in all 3 tables, then findings are consistent with an ABO-incompatibility and an elution does not need to be done.

(3b) If a "No" is returned in one or more of the tables, then an elution needs to be performed to identify the antibody.

 

Table 1

Mother's ABO Blood Type

	Type A	Type B	Type AB	Type O
Baby Type A	No	Yes (anti-A)	No	Yes (anti-A and anti-A,B)
Baby Type B	Yes (anti-B)	No	No	Yes (anti-B and anti-A,B)
Baby Type AB	Yes (anti-B)	Yes (anti-A)	No	Yes (anti-A, anti-B, and anti-A,B)
Baby Type O	No	No	No	No

 

 

Table 2	Mother Rh-positive	Mother Rh-negative
Baby Rh positive	Yes	No
Baby Rh negative	Yes	Yes
Baby Rh indeterminate due to positive DAT	Yes	No

 

 

Table 3	Negative	Positive
Mother's Antibody Screen	Yes	No

 

Special Situations

(1) mother treated with Rhogam prior to delivery

(1a) Since Rhogam is IgG anti-D, it can cross the placenta and coat fetal cells. Transfused immunoglobulin can circulate up to 3 months before disappearing (although this time is usually much faster).

(1b) If an elution is performed, then anti-D will be recovered.

(1c) Wait for 4 months and see if anti-D is still present in the maternal serum. If present, this indicates active antibody production and maternal alloimmunization to D-antigen.

(2) Du mother or infant:

(2a) The Du (weak expression of the D antigen) is treated as if D-positive.

(2b) Very rarely patients who are Du or D-positive can produce "anti-D" due to inheritance of a partial D antigen which allows formation of antibody to the portion of the missing antigen.

(3) significant maternal-to-fetal hemorrhage

(3a) This can result in maternal IgM antibody reaching the fetal circulation.

(3b) This would be unlikely to affect the above algorithm.

(4) maternal autoimmune disease:

(4a) This usually will be detected by history or a positive DAT on the maternal blood.

(4b) Maternal autoimmune disease can rarely be a cause of hemolytic disease of the newborn.