If two units of compatible blood for a patient who has anti-Jk^a are needed, then how many units should be crossmatched?

• A. 4
• B. 6
• C. 10
• D. 2

Answer: (C)

When a patient has anti-Jk^a, any donor red cells must be negative for Jk^a and you want to have a ready supply of compatible units in case more transfusions are needed. In many transfusion practices, especially in exam scenarios aimed at emphasizing preparedness, more units are crossmatched than the immediate need to ensure an adequate emergency stock. That means, even though only two units are initially required, the lab crossmatches a larger pool of units—in this case ten—to guarantee there are at least two compatible units available quickly if the situation worsens or if additional units become necessary. The key idea is to maintain a sufficient, ready-to-use set of antigen-negative units whenever a clinically significant antibody is present.