Crossmatch Procedures & Interpretation Flashcards
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Flow cytometric crossmatch (FCXM) is considered more sensitive than standard CDC crossmatch because:
Answer: It detects antibody binding directly, independent of complement fixation
FCXM uses a fluorescent-labeled secondary anti-human antibody to detect IgG binding to cell-surface HLA, allowing detection of low-level antibodies insufficient to fix complement.
In a flow cytometric T-cell crossmatch, a positive result is typically defined by:
Answer: A fluorescence channel shift exceeding the laboratory-defined threshold vs. negative control
A positive FCXM is defined by a fluorescence shift (in channels or MFI) beyond a laboratory-established threshold relative to the negative control, indicating specific antibody binding.
A positive B-cell crossmatch with a concurrent negative T-cell crossmatch most likely indicates:
Answer: Anti-HLA class II antibodies directed at DR, DQ, or DP antigens
B cells express both HLA class I and class II; a positive B-cell but negative T-cell crossmatch suggests anti-class II specificity, since T cells lack constitutive HLA class II expression.
A 'virtual crossmatch' in transplantation is performed by:
Answer: Comparing recipient antibody specificities (from solid-phase assay) against the donor's HLA type
Virtual crossmatch matches the recipient's HLA antibody profile (from Luminex single antigen bead testing) against the donor's HLA typing to predict physical crossmatch outcome without requiring donor cells.
EDTA is sometimes added to crossmatch reaction tubes in order to:
Answer: Chelate divalent cations to inhibit complement activation
EDTA chelates Ca²⁺ and Mg²⁺ ions that are required for complement cascade activation, thereby preventing complement-mediated cytotoxicity and allowing detection of complement-independent antibodies.
The antihuman globulin (AHG)-enhanced CDC crossmatch increases sensitivity by:
Answer: Crosslinking IgG bound to cell-surface HLA, increasing complement-fixing complexes
AHG (anti-human IgG) binds to IgG already attached to cell-surface HLA antigens, crosslinking them and creating larger immune complexes that more efficiently fix complement.
Which cell preparation is most appropriate as a target for a T-cell crossmatch?
Answer: Peripheral blood T lymphocytes isolated by rosetting or immunomagnetic separation
Peripheral blood T lymphocytes are used because they constitutively express HLA class I antigens and are readily isolated from blood by rosetting with neuraminidase-treated sheep RBCs or CD3 magnetic beads.