CHT Crossmatch Procedures & Interpretation 2

Question 1

Flow cytometric crossmatch (FCXM) is considered more sensitive than standard CDC crossmatch because:

Accepted Answer

It detects antibody binding directly, independent of complement fixation

Answer

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.

Question 2

In a flow cytometric T-cell crossmatch, a positive result is typically defined by:

Accepted Answer

A fluorescence channel shift exceeding the laboratory-defined threshold vs. negative control

Answer

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.

Question 3

A positive B-cell crossmatch with a concurrent negative T-cell crossmatch most likely indicates:

Accepted Answer

Anti-HLA class II antibodies directed at DR, DQ, or DP antigens

Answer

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.

Question 4

A 'virtual crossmatch' in transplantation is performed by:

Accepted Answer

Comparing recipient antibody specificities (from solid-phase assay) against the donor's HLA type

Answer

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.

Question 5

EDTA is sometimes added to crossmatch reaction tubes in order to:

Accepted Answer

Chelate divalent cations to inhibit complement activation

Answer

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.

CHT - Certified Histocompatibility Technologist Practice Test

CHT - Certified Histocompatibility Technologist Practice Test

CHT Crossmatch Procedures & Interpretation 2