MLT GlobeBlood Banking: Antibody screen & identification – page 4
90 Blood Banking MCQs on Antibody screen & identification with answers and explanations.
Proteolytic enzymes enhance some antibody reactions mainly because they:
Removing sialic acid reduces zeta potential and exposes some antigens such as Rh and Kidd, so cells come closer together.
A patient has anti-E and anti-K. About 70% of donors are E-negative and 91% are K-negative. About how many random units must be tested to find 2 compatible units?
0.70 × 0.91 = 0.64 of units are compatible. 2 ÷ 0.64 = 3.1, so about 4 units must be screened.
An antibody reacts only at AHG and is enhanced by ficin. It reacts with rr and R2R2 cells, not with R1R1 cells, and more strongly with c/c cells than with R1r cells. The specificity is:
Reaction with all c-positive cells, no reaction with c-negative R1R1 cells, and dosage point to anti-c. Anti-e and anti-f would not react with R2R2 cells.
Using monospecific anti-IgG instead of polyspecific AHG in the IAT mainly helps to:
Polyspecific AHG detects C3 bound by cold antibodies. Anti-IgG avoids this, though it may miss rare complement-only Kidd antibodies.
For pretransfusion antibody screening of a 3-week-old infant, which sample may be used?
Antibodies in young infants are maternal, so either the infant's or the mother's plasma can be used for the initial screen.
Rouleaux cause false positives at immediate spin but usually not in the antiglobulin phase because:
Rouleaux depend on abnormal plasma proteins. Washing removes them before AHG, so the AHG reading is unaffected.
When antibody titers are compared by testing samples in parallel, which change is usually considered significant?
Titration has a variation of about one dilution, so only a fourfold (two-tube) or greater change is considered real.
A patient has anti-Lea that reacts at room temperature but not at 37 °C or AHG. Which red cells are acceptable?
Lewis antibodies inactive at 37 °C are not clinically significant; antigen-negative units are not required, only AHG-compatible ones.
A cold panagglutinin reacts strongly and equally with adult A1, B and O cells and the autocontrol, but only weakly with group O cord cells. The specificity is:
Adult cells are rich in I and cord cells in i. Anti-H would react weakly with A1 cells.
In a neutralization test, a control of plasma plus saline (instead of the neutralizing substance) is included to show that:
Adding any fluid dilutes the plasma. The saline control must still react; otherwise loss of reactivity may be due to dilution.
A strong antibody gives weak reactions in undiluted plasma but strong reactions at 1:8 and 1:16. This is called:
In prozone, antibody excess blocks lattice formation; dilution restores agglutination. Postzone is antigen excess.
A patient has a positive IgG DAT and a negative antibody screen. The eluate reacts with all panel cells. The most likely explanation is:
Autoantibody may be fully adsorbed onto the patient's cells, leaving none in plasma. A pan-reactive eluate points to autoantibody.
Why do column (gel) IAT tests not need a cell washing step?
During centrifugation, red cells enter the gel containing AHG, while plasma stays above, so unbound IgG cannot neutralize the AHG.
A woman with two previous pregnancies has a positive antibody screen at AHG and a negative autocontrol. The most likely cause is:
Reactivity with screening cells but not with her own cells points to an alloantibody, likely stimulated by pregnancy. A warm autoantibody would also make the autocontrol positive.
22% bovine albumin enhances agglutination in antibody tests mainly by:
Albumin lowers the zeta potential (the repulsive charge between red cells), so IgG-coated cells can come close enough to agglutinate.
An antibody reacts at immediate spin but not at 37 °C or in the AHG phase. It is most likely of which class?
Large IgM antibodies agglutinate saline-suspended cells directly at room temperature. IgG antibodies usually need 37 °C incubation and AHG.
Treating panel cells with ficin or papain destroys which antigens?
Proteolytic enzymes cleave Fya, Fyb, M, N and (variably) S. They enhance reactions of Rh, Kidd, Lewis, P1 and I antibodies, which helps separate mixtures in antibody identification.
ZZAP reagent, used to prepare cells for adsorption, contains:
ZZAP combines DTT with papain or ficin. It removes bound IgG and makes cells adsorb better, but it destroys Kell, Duffy, MNS and other sensitive antigens.
A patient on high-dose intravenous penicillin has a positive DAT (IgG). The eluate does not react with untreated panel cells. The next useful test is:
Penicillin binds to red cells, and antibody to the drug reacts only with drug-coated cells (drug adsorption mechanism). A nonreactive eluate with a positive DAT in a treated patient suggests this.
In a PEG antiglobulin test, the tubes are not centrifuged and read after the 37 °C incubation. Why?
PEG concentrates antibody by removing water, but spinning cells in PEG causes nonspecific clumping. Cells are washed and read only at the AHG phase.