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Case Study O1 - Discussion Part 1 - Determining antibody specificity
 

TraQ Program of the BC PBCO

Sunday, 20 August 2017

Case Study O1 - Discussion Part 1 - 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

  • Sincethe 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

1. What is a "p value" and how can it be used as a tool when identifying antibodies?

Answer

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.

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Last modified on Thursday, 18 May 2017 07:42