DISEASES OF THE NEWBORN
Section I. INVESTIGATION OF A POSITIVE DIRECT ANTIGLOBULIN TEST
AND IMMUNE HEMOLYTIC ANEMIA
a. Some of the causes of positive direct antiglobulin tests have been discussed
in Lesson 1, Section II. The results of a direct antiglobulin test should reflect what is
happening "in vivo," not what has occurred "in vitro," through use of inappropriate
techniques (for example, cold autoantibodies or complement sensitizing the RBCs at
room temperature or 4C).
b. It is important to note that "in vivo" RBC sensitization does not necessarily
mean an individual has autoimmune hemolytic anemia or even that an autoantibody has
caused the positive reaction. "In vivo" RBC sensitization can be associated with any of
the following situations.
(1) Alloantibodies present in the recipient's plasma may sensitize transfused
RBCs. Alloantibodies present in the donor's plasma can also sensitize recipient's
RBCs. Alloantibodies crossing the placenta may sensitize fetal RBCs.
(2) Autoantibodies can react with intrinsic RBC antigens, leading to
sensitization of the RBCs with immunogIobulins and/or complement components.
Hemolytic anemia may or may not be present.
(3) Antibodies against drugs may be formed, for example, penicillin
antibodies. These antibodies may sensitize drug-coated RBCs.
(4) Red blood cells may take up proteins through nonimmunologic
processes. This has been shown to occur following exposure to drugs of the
cephalosporin group where the RBCs membrane becomes chemically altered so that
many proteins are nonspecifically absorbed to the membrane.
Immune complex formation. See Lesson 1, Section II and later in this
c. In the serologic investigation of "in vivo" RBC sensitization, it is important to
exclude first the "in vitro" causes of false-positive reactions mentioned earlier. The most
common pitfall is to interpret the positive results obtained on cells from a refrigerated
clot as indicating in " vivo" sensitization. Before such a positive result is accepted, the
test must be repeated on a fresh blood sample, preferably taken into EDTA, to prevent
"in vitro" complement uptake. See Lesson 1, Section II on causes of false-positive