Blood Banking Question: 1. You are needing to find 4 units of blood that are S a
ID: 80676 • Letter: B
Question
Blood Banking Question:
1. You are needing to find 4 units of blood that are S antigen negative (S-) for a patient undergoing a surgical procedure. How many units would you select to screen for the antigen (based on population percentage), in order to find the 4 antigen negative units that you need?
2. You also need to perform the QC on the anti-S reagent, and will be selecting cells to serve as a positive and negative control. From the cells below, select which would be the appropriate positive and negative control cells:
Cell # 1 s/s
Cell # 2 S/s
Cell # 3 S/S
Explanation / Answer
1) ~ 89% of caucasians have s-antigen i.e. occurrence of this antigen is 0.8 (either in heterozygous or homozygous form).
Hence, frequency of ss antigen (S negatic=ve individual will be) = 0.8 x 0.8 = 0.64
i.e. among 100 people 64 will be S negative. So for 4 units of blood i would need to screen 4 X 100/64 units of blood = 6.25 units
2) S antigen will be present only if atleat 1 copy of S (capital S; doinanant allele) is present.
positive control -> Cell #3 S/S = because all cells will surely express this antigen as no othere allele of this gene is present
negative control -> cell # s/s = because all cells will surely lack this antigen as there is no allele (S) that would express it.