Role in transfusion: antigen prevalence

6 ASCP MLS practice questions on Role in transfusion: antigen prevalence, part of Blood Banking. The first 6 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 · Easy

   The K antigen is found on the red cells of about what share of white donors?

   1. A91%
   2. B1%
   3. C9%
   4. D35%

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   Answer: C · 9%

   About 9% of white donors are K+, so about 91% are K-negative. This makes K-negative blood easy to find for patients with anti-K. In contrast, k is present in more than 99.8% of people. Knowing common antigen frequencies helps the blood bank plan antigen-negative searches.

   • A: 91% is the share that is K-negative.
   • B: 1% is far below the real frequency of K.
   • D: 35% is far above the real frequency of K.

2. Question 2 · Hard

   A patient has anti-K and anti-Fya. About 91% of donors are K-negative and 34% are Fy(a-). About how many units must be screened to find 2 compatible units?

   1. A3 units
   2. B6 units
   3. C7 units
   4. D34 units

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   Answer: C · 7 units

   The chance that a unit lacks both antigens is 0.91 × 0.34 = 0.31. Units to screen = 2 ÷ 0.31 = 6.5, rounded up to 7. Frequencies are multiplied because the antigens are inherited independently. Each selected unit is then crossmatched at the antiglobulin phase.

   • A: 3 units considers only the anti-K.
   • B: 6 units rounds down and would be expected to give fewer than 2 units.
   • D: 34 units comes from using the antigen-positive frequencies.

3. Question 3 · Medium

   A patient has anti-c. c is present on about 80% of white donors. About how many units must be screened to find 4 c-negative units?

   1. A5 units
   2. B20 units
   3. C40 units
   4. D16 units

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   Answer: B · 20 units

   Only about 20% of white donors are c-negative. Units to screen = 4 ÷ 0.20 = 20. c-negative units are usually R1R1 (DCe/DCe) or r'r' blood. Patients with anti-c often also have anti-E, which makes the search harder.

   • A: 5 units comes from dividing by the c-positive frequency (80%).
   • C: 40 units would apply if only 10% of donors were c-negative.
   • D: 16 units is the number of c-positive units among 20, not the total to screen.

4. Question 4 · Medium

   A patient has anti-E. E is present on about 30% of donors. About how many ABO-compatible units must be screened to find 2 E-negative units?

   1. A4 units
   2. B7 units
   3. C2 units
   4. D3 units

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   Answer: D · 3 units

   About 70% of donors are E-negative. Units to screen = units needed ÷ frequency of antigen-negative units = 2 ÷ 0.70 = 2.9, rounded up to 3. The chosen units must also be compatible in an antiglobulin crossmatch. Always round up, because a part-unit cannot be screened.

   • A: 4 units assumes half the donors are E-negative.
   • B: 7 units comes from dividing by the E-positive frequency (30%).
   • C: Two units would be enough only if every donor were E-negative.

5. Question 5 · Hard

   A patient has anti-Jka and anti-E and needs 3 units. About 23% of donors are Jk(a-) and 70% are E-negative. About how many units must be screened?

   1. A5 units
   2. B14 units
   3. C18 units
   4. D19 units

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   Answer: D · 19 units

   The chance that a unit lacks both antigens is 0.23 × 0.70 = 0.161. Units to screen = 3 ÷ 0.161 = 18.6, rounded up to 19. The answer is always rounded up, because 18 units would be expected to give fewer than 3. Anti-Jka is evanescent, so antigen-negative units are needed even if a later screen is negative.

   • A: 5 units considers only the anti-E.
   • B: 14 units considers only the anti-Jka.
   • C: 18 units rounds down and would be expected to give fewer than 3 units.

6. Question 6 · Hard

   A 66-year-old group A, D-negative woman with anti-Fya and anti-Jkb is scheduled for hip replacement and needs 3 red cell units. Because she is D-negative, the technologist screens only units from the group A, D-negative inventory, typing each with anti-Fya and anti-Jkb before crossmatching. In this donor population, 34% of donors are Fy(a-), 26% are Jk(b-) and about 15% are D-negative, and the antigens are inherited independently. About how many group A, D-negative units must be screened to find 3 antigen-negative units?

   1. AAbout 6
   2. BAbout 34
   3. CAbout 226
   4. DAbout 11

   Answer and explanation in LearnFree account