If you know she has anti-E, you can probably put together a custom screen of E negative cells. That screen would only be positive if she developed another antibody. Be careful that you cover all the antigens that the FDA requires. That list used to be in the Technical Manual. I think it is D, C, E, c, e, M, N, S, s, P1, Lea, Leb, K, k, Fya, Fyb, Jka, Jkb.