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 Quiz 

Answer (Question 3a)

What percentage of group B Rh positive donors would be expected to be crossmatch-compatible with Mr. FS?

According to probability theory, if p(A) is the probability that A will occur, and p(B) is the probability the B will occur, the probability of p(A, B) (i.e., that both A and B will occur) is calculated as:

p(A, B) = p(A) x p(B)

Put another way, the probability of two events happening together is derived by multiplying each probability.

In a Caucasian population the relevant antigen frequencies are as follows:

Antigen	% Positive	% Negative
C	67	33
Jka	77	23

The calculation for the percentage expected to be both C- and Jk(a-) is as follows:
0.33 X 0.23 = 0.076 or 7.6%

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Answer (Question 3b)

Given the patient's antibodies, what is the minimum number of donors that would need to be antigen typed to obtain two antigen-negative donors to crossmatch?

 

There are two ways to calculate the minimum number of donors to antigen type:

1. "Intuitive": If 7.6% of donors are C-and Jk(a-) then 7.6 in 100 are negative for both antigens. If roughly 8 in 100 are negative for both, approximately 4 in 50 would be negative for both; and approximately 2 in 25 would be negative for both. To obtain two C-and Jk(a-) donors, at least 25 would need to be tested.

2. The minimum number to antigen type can also be calculated using this formula:

# of donors (x) = # required/# antigen-negative (as a decimal)
x = 2/0.076 = 26.3

Depending on how lucky you feel, the minimum number to type is between 25 and 30, with 30 being on the safer side.

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