b. False Positive Results.
(1) Cells having a positive direct antiglobulin test cannot be used with
reagent antiserums that require an antiglobulin phase because all such cells will be
agglutinated by the antiglobulin serum.
(2) Bacterial contamination of test cells or septicemia in a patient may result
in a positive antiglobulin test. If the RBCs are T-activated, they may react as some
antglobuIin sera contain anti-T.
(3) Extreme reticulocytosis has been reported to give a positive result
because of transferrin bound to reticulocytes reacting with antitransferrin in the
antiglobulin reagents. Most antiglobulin reagents today have little antitransferrin activity.
(4) Saline stored in glass bottles may contain coilcical silica leached from
the container; this has been reported to cause false positive reactions.
(5) Saline stored in metal containers, or used in equipment with metal parts,
may contain metallic ions that may bring about nonspecific protein-coating of the RBCs.
(6) Improperly prepared antiglobulin serum may contain traces of
species-specific antibodies (This should not be a problem if tests are performed with
licensed AHG reagents).
(7) When all the antiglobulin tests are weakly positive, the cause may be
(8) Overcentrifugation may give false-positive results (aggregation vs.
(9) Patients' or donors' serums can contain a naturally occurring cold
autoantibody (normal incomplete cold antibody) that can sensitize their own or other
cells with complement. Usually this only occurs at 4C, but it may occur up to room
temperature. If antiglobulin sera contain potent anticomplement, positive reactions may
occur with RBCs from refrigerated clots. These positive reactions have been found to
be largely a result of C4 sensitization, and can be avoided if the RBCs from
anticoagulanted blood is used. For cross matching, RBCs from ACD or CPD segments
can be used for direct antiglobulin tests; however, EDTA is preferable. These
anticoagulants will chelate Ca++ and Mg++, thus preventing any "in vitro" complement
uptake, without interfering with the complement already bound to the RBC "in vivo".
(10) Red blood cells may be autoagglutinated, before they are washed, and
this agglutination may persist through washing, leading to a false-positive reaction when
antiglobulin serum is added.