Blood banking: grouping, Coombs and compatibility tests

26 Kerala PSC Lab Technician Gr II practice questions on Blood banking: grouping, Coombs and compatibility tests, part of Pathology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Medium

   Bovine albumin is used to enhance agglutination in some tube tests. How does it work?

   1. ABy activating complement
   2. BBy splitting IgG into fragments
   3. CBy lowering the zeta potential
   4. DBy coating cells with anti-IgG

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   Answer: C · By lowering the zeta potential

   Red cells carry a negative surface charge from sialic acid, and the cloud of cations around them creates the zeta potential that keeps the cells apart. Albumin raises the dielectric constant of the medium and lowers the zeta potential, so the cells come close enough for IgG antibodies to bridge them. It acts at the agglutination stage rather than by increasing antibody binding. LISS and PEG are other enhancement media.

   • A: Albumin does not activate complement.
   • B: Albumin does not cleave IgG molecules.
   • D: Binding IgG on the cells is the role of the AHG reagent, not albumin.

2. Question 2 · Easy

   In reverse (serum) ABO grouping, what is the patient's serum tested against?

   1. AAnti-human globulin
   2. BThe patient's own red cells
   3. CKnown A1 and B red cells
   4. DAnti-A and anti-B antisera

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   Answer: C · Known A1 and B red cells

   Reverse grouping detects the ABO antibodies in serum or plasma by mixing it with reagent A1 and B red cells. Its result must agree with the forward grouping, in which the patient's cells are tested with anti-A and anti-B. A group A person's serum, for example, agglutinates B cells only. Any mismatch is a discrepancy that must be resolved before the group is reported.

   • A: Anti-human globulin is used in Coombs tests, not in routine reverse grouping.
   • B: Serum tested against the patient's own cells is the autocontrol.
   • D: Anti-A and anti-B antisera are used in forward grouping of the patient's cells.

3. Question 3 · Medium

   In a tube test, the cell button comes off as one solid agglutinate with a clear background. How is the reaction graded?

   1. A4+
   2. B1+
   3. C2+
   4. D3+

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   Answer: A · 4+

   Tube reactions are graded after the cell button is gently shaken loose. One solid agglutinate with a clear background is 4+, several large clumps with a clear background 3+, medium clumps with a clear background 2+, and small clumps in a turbid background 1+. Recording the strength helps in spotting weak or mixed reactions and grouping discrepancies. The button is shaken gently, because rough handling can break up weak agglutinates.

   • B: 1+ is small agglutinates in a turbid background of free cells.
   • C: 2+ is medium-sized agglutinates with a clear background.
   • D: 3+ is several large agglutinates with a clear background.

4. Question 4 · Hard

   How much saline must be added to 0.2 mL of packed red cells to make a 5% cell suspension?

   1. A1.0 mL
   2. B4.0 mL
   3. C9.8 mL
   4. D3.8 mL

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   Answer: D · 3.8 mL

   A 5% suspension means 5 volumes of cells in every 100 volumes of suspension. Total volume = 0.2 ÷ 0.05 = 4.0 mL, so saline = 4.0 − 0.2 = 3.8 mL. Adding 4.0 mL of saline gives about 4.8%, and 9.8 mL would give a 2% suspension. Correct cell strength matters, because too many cells can hide weak agglutination.

   • A: 1.0 mL comes from multiplying 0.2 by 5 instead of dividing by 0.05.
   • B: 4.0 mL is the total volume; adding that much saline gives about 4.8%.
   • C: 9.8 mL of saline would make a 2% suspension.

5. Question 5 · Easy

   In which condition is the direct antiglobulin (Coombs) test usually positive?

   1. AHereditary spherocytosis
   2. BG6PD deficiency
   3. CHemolytic disease of the newborn
   4. DIron deficiency anaemia

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   Answer: C · Hemolytic disease of the newborn

   The DAT detects IgG or complement already coating red cells in the body. In HDN, maternal IgG crosses the placenta and coats the baby's red cells, so the DAT on cord cells is positive. It is also positive in autoimmune hemolytic anaemia and hemolytic transfusion reactions. Hereditary spherocytosis and G6PD deficiency cause hemolysis without antibody, so the DAT is negative.

   • A: Hereditary spherocytosis is a membrane defect; the DAT is negative.
   • B: G6PD deficiency is an enzyme defect; the DAT is negative.
   • D: Iron deficiency anaemia is not immune and does not coat red cells with antibody.

6. Question 6 · Medium

   In slide (tile) ABO grouping, results are read within about 2 minutes. Why?

   1. AAgglutinates break up on standing
   2. BThe antisera lose activity after that
   3. CDrying can be mistaken for clumping
   4. DThe red cells begin to hemolyse

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7. Question 7 · Medium

   A donor's red cells give no reaction with anti-D at immediate spin. To test for weak D, the cells are next:

   1. ATreated with papain and read at room temperature
   2. BRetested on a slide after 10 minutes
   3. CIncubated with anti-D at 37°C, then AHG added
   4. DTested with anti-A,B at 4°C

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8. Question 8 · Medium

   After a negative antiglobulin tube test, IgG-coated (check) cells are added and must agglutinate. This step shows that:

   1. AThe incubation temperature was correct
   2. BThe patient has no antibodies
   3. CThe AHG reagent was present and active
   4. DThe red cells were not haemolysed

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9. Question 9 · Easy

   Which of these is done with the indirect Coombs test rather than the direct test?

   1. ATesting cord cells in HDN
   2. BAntibody screening of patient serum
   3. CDiagnosing autoimmune haemolysis
   4. DInvestigating drug-induced haemolysis

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10. Question 10 · Easy

    In the major cross-match, which two are tested together?

    1. ARecipient serum and donor red cells
    2. BDonor serum and recipient red cells
    3. CDonor serum and donor red cells
    4. DRecipient serum and recipient red cells

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11. Question 11 · Hard

    A 68-year-old man with multiple myeloma (serum total protein 11.5 g/dL; normal 6-8) types as group A on cell grouping, but his serum clumps both A1 and B cells. The next step to resolve the serum grouping is:

    1. AReport him as group AB
    2. BTest with papain-treated cells
    3. CRepeat with prewarmed serum
    4. DSaline replacement test

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12. Question 12 · Easy

    Polyspecific anti-human globulin reagent contains antibodies against which proteins?

    1. AA and B antigens
    2. BHuman IgM and IgA
    3. CHuman IgG and C3d
    4. DRabbit serum proteins

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13. Question 13 · Easy

    Why are red cells washed with saline at least three times before anti-human globulin is added?

    1. ATo break up rouleaux formed during incubation
    2. BTo elute antibody already bound to the red cells
    3. CTo remove the fibrin strands that block the reading
    4. DTo remove unbound globulins that would neutralise the reagent

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14. Question 14 · Medium

    Treating test red cells with papain will NOT help detect antibodies against which antigen?

    1. ALea (Lewis)
    2. BJka (Kidd)
    3. CD (Rh)
    4. DFya (Duffy)

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15. Question 15 · Medium

    A clotted sample kept overnight in the refrigerator gives a weak positive DAT with anti-C3d only. The most likely reason is:

    1. ACold antibodies fixed complement in the tube
    2. BContamination of the reagent with IgG
    3. CIgG alloantibody from a recent transfusion
    4. DWarm autoimmune hemolytic anemia

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16. Question 16 · Easy

    Monoclonal anti-A grouping reagent is conventionally coloured:

    1. AYellow
    2. BBlue
    3. CGreen
    4. DColourless

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17. Question 17 · Medium

    A 2-day-old baby is to be grouped. Only cell (forward) grouping is reported. Why is reverse grouping left out?

    1. AThe baby's own anti-A and anti-B are not yet formed
    2. BCord blood plasma contains an inhibitor of agglutination
    3. CNewborn red cells do not carry A or B antigens
    4. DNewborn red cells are too fragile for tube tests

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18. Question 18 · Medium

    An immediate-spin cross-match done at room temperature mainly detects:

    1. AAnti-Kell antibody
    2. BAnti-D antibody
    3. CABO incompatibility
    4. DAnti-Fya antibody

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19. Question 19 · Hard

    A patient's antibody screen and autocontrol are negative. One of four ABO-compatible donor units is incompatible at the AHG phase only, and a DAT on that unit's segment is positive. What should be done?

    1. ASelect another unit and report the donor DAT
    2. BTransfuse the unit with steroid cover
    3. CDo an antibody panel on the patient
    4. DRetype the patient's ABO group

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20. Question 20 · Medium

    In a gel (column agglutination) card, how does a negative reaction appear?

    1. ARed cells form a button at the bottom
    2. BRed cells stay in the reaction chamber
    3. CRed cells form a band at the top of the gel
    4. DRed cells spread all through the gel

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21. Question 21 · Easy

    At what temperature are serum and red cells incubated in the indirect antiglobulin test?

    1. A22°C
    2. B4°C
    3. C37°C
    4. D56°C

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22. Question 22 · Hard

    A 35-year-old woman with SLE has Hb 7.2 g/dL and a positive DAT (IgG and C3). Both screening cells, all panel cells and her autocontrol react 2+ at the AHG phase. She has never been transfused or pregnant. The most likely cause is:

    1. ADelayed hemolytic reaction
    2. BWarm autoantibody
    3. CCold agglutinin disease
    4. DAlloantibody to a common antigen

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23. Question 23 · Easy

    EDTA blood is preferred for a DAT because EDTA:

    1. APreserves IgM antibodies in the plasma
    2. BStops complement binding to cells in the tube
    3. CStops red cells from lysing on storage
    4. DStrengthens the bond of IgG to red cells

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24. Question 24 · Medium

    A donor's red cells are negative with anti-D at immediate spin but positive in the indirect antiglobulin phase (weak D). How is the unit labelled?

    1. AD positive
    2. BGroup undetermined
    3. CD negative
    4. DD variant, not for issue

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25. Question 25 · Easy

    A 25-year-old woman with a life-threatening bleed needs red cells before her blood group is known. Which units are issued?

    1. AGroup AB, Rh D-positive
    2. BGroup O, Rh D-positive
    3. CGroup O, Rh D-negative
    4. DGroup AB, Rh D-negative

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