MLT GlobeBlood Banking: Antibody screen & identification
90 Blood Banking MCQs on Antibody screen & identification with answers and explanations.
A patient on long-term methyldopa develops anaemia. Which test confirms that his red cells are coated with antibody in vivo?
The DAT detects IgG or complement already bound to red cells, as in drug-induced immune haemolysis. The IAT detects free antibody in plasma.
The direct antiglobulin test (DAT) detects:
The DAT tests the patient's own red cells with anti-IgG or anti-C3d to find in vivo coating, as in HDFN, transfusion reactions and autoimmune hemolysis. The indirect test (IAT) finds free antibody in plasma.
Reagent red cells used for the routine antibody screen are group:
Group O cells lack A and B antigens, so the patient's anti-A or anti-B cannot react and unexpected antibodies can be detected. A1 and B cells are used for ABO reverse typing.
During panel interpretation, a specificity is usually 'ruled out' when:
If a cell with the antigen (preferably homozygous) does not react, the antibody is unlikely to have that specificity. Using double-dose cells avoids missing weak antibodies with dosage.
Why is an autocontrol tested along with an antibody identification panel?
A positive autocontrol shows antibody or complement on the patient's own cells, pointing to an autoantibody, drug effect or recent transfusion reaction. A negative autocontrol with panel reactivity suggests alloantibody.
Which test is an application of the indirect antiglobulin test rather than the DAT?
The IAT first sensitizes reagent cells with plasma antibody in vitro, then adds AHG; this is used in antibody screens, crossmatches and antigen typing. The other options test cells already coated in vivo.
Low-ionic-strength saline (LISS) is used in the IAT mainly because it:
Lowering the ionic strength reduces the shielding cloud of ions, so antibody binds faster and incubation can be shortened to 10–15 minutes. AHG is still needed to detect IgG.
In a column (gel) agglutination test, a strong positive (4+) reaction appears as:
Large agglutinates cannot enter the gel and stay at the top. Unagglutinated cells pass through and form a button at the bottom, which is a negative reaction.
After identifying anti-Fya, the patient's own red cells should be typed and are expected to be:
A person cannot normally make an alloantibody to an antigen on his own cells. Finding the patient Fy(a−) supports the identification.
After a panel suggests anti-E, some specificities are still not excluded. The next step is usually to:
Selected cells lacking E but carrying the unexcluded antigens show whether another antibody is hiding. Repeating the same panel gives no new information.
An antibody reacts only at immediate spin, is weak, and all reactive cells are P1-positive. This antibody is:
Anti-P1 is usually a naturally occurring cold IgM that rarely causes hemolysis. Antibodies reacting only below 37 °C are generally not clinically significant.
An antibody titration shows 2+ at 1:32, 1+ at 1:64 and no agglutination at 1:128. The titer is reported as:
The titer is the reciprocal of the highest dilution giving 1+ agglutination, here 1:64. The 1:128 tube is negative and is not counted.
The sensitization step of the indirect antiglobulin test is performed at:
IgG antibodies bind best at body temperature, so plasma and cells are incubated at 37 °C before washing and adding AHG. 56 °C is used for heat elution.
A patient transfused 3 weeks ago needs more red cells. The pretransfusion sample must be collected no more than how long before the transfusion?
If the patient was transfused or pregnant in the previous 3 months, the sample must be drawn within 3 days of transfusion, because new antibodies can appear quickly.
An antibody is usually considered clinically significant when it:
Antibodies active at body temperature can shorten red cell survival. Those reacting only in the cold are usually insignificant.
A patient had anti-Jka identified 5 years ago. Today's antibody screen is negative. Which red cells should be given?
Kidd antibodies often fall below detection but return quickly after exposure, causing delayed hemolysis. Antigen-negative units are given for life.
Antibody screening at the first prenatal visit should be done for:
D-positive women can have anti-c, anti-K and other antibodies that cause HDFN, so every pregnant woman is screened.
In the antiglobulin test, visible agglutination occurs because AHG:
IgG alone is too small to bridge cells. AHG binds Fc portions of IgG on different cells and links them.
Why are red cells washed at least three times before AHG reagent is added?
Free IgG in residual plasma binds and neutralizes AHG, causing false negatives. Washing does not remove bound IgG.
For a patient with sickle cell disease starting chronic transfusion, many programs give units matched for which antigens to prevent alloimmunization?
Anti-C, anti-E and anti-K are the most frequent alloantibodies in these patients, so prophylactic matching lowers alloimmunization.