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Discussion
Determining antibody
specificity
Figure 1 shows the patient's apparent antibody specificity to be
anti-D and possibly anti-C, although anti-G has not been excluded.
- As differentiating anti-D, -C, and -G is not clinically important in pretransfusion testing and as the laboratory did not have the
rGr (D-C-G+) cell necessary to do so, anti-G was not considered further.
- Other clinically significant antibodies have been excluded based on at least one negative with a cell from a homozygous donor.
- Antibodies to low frequency antigens such as
Kpa and Lua do not need to be excluded in the absence of unexplained results.
Passive anti-D
is to be expected due to the transfusion of IV RhIG (WinRhoTM
, Cangene, Winnipeg, Canada), as is
the positive autocontrol.
Confirming
anti-C
- Since
the patient’s serum reacts with only one D- C+ cell, to
achieve the minimum level of probability (p=0.05) for anti-C,
the patient’s plasma was tested with two additional D- C+ red
cells, confirming anti-C with acceptable probability (Figure
2).
- Since
the last RBC transfusion had occurred three months ago, red
cell phenotyping was
possible by standard methods and showed the patient to be
D+C-E+c+e+ (probable genotype R2r).
- The
anti-C could be a passive antibody derived from the IV RhIG or a
new alloantibody stimulated by prior transfusion.
TraQ self study question: What
is a "p value" and how can it be used as a tool when
identifying antibodies?
-
DAT
- Because the AHG
autocontrol was positive, a direct antiglobulin test (DAT) was
performed with polyspecific antiglobulin serum (anti-IgG and
anti-C3d), followed by DATs with monospecific anti-IgG and
anti-C3b/-C3d.
- The results showed
only IgG sensitizing the patient’s red cells.
Eluate
An eluate was prepared
from the patient’s red cells using acid elution. Anti-D was
identified (as expected) with a possible anti-E (Figure
3).
- The eluate was tested
with two additional D- E+ red cells, confirming anti-E with
acceptable probability (Figure
4).
- Because transfusion
occurred 3 months ago and the patient could be validly typed as
E+, the anti-E appears to be a passive antibody derived from the
IV RhIG and sensitizing the patient’s E+ red cells.
- Were transfusion more
recent, the detection of anti-E in the patient’s eluate would
need to be investigated to exclude the possibility of a
hemolytic transfusion reaction due to anti-E.
- NOTE
(added for TraQ):With
recent transfusion, the anti-E could be an alloantibody
stimulated by donor red cells (boosted by a secondary immune
response) and now attaching to E+ donor cells that are now
part of the patient's red cell population. If recently
transfused, the patient could not be antigen phenotyped by
normal means. Initially you would not know the patient;s E
antigen status and it would possible that the anti-E in the
eluate could be an alloantibody.
More Discussion...
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