We are currently validating the Echo and have noticed the lack of homozygous cells for rule-outs between all Echo panels. How are others handling this situation if homozygous rule-outs are needed?

This is an old topic; however, I worked with an Echo and Immucor reagents/cells and did not have any particular difficulty with rule-outs, but this was years after this post.  Hopefully that means if there was an issue that it has been resolved.

Erythrocytes cannot be either homozygous or heterozygous (or hemizygous for that matter).  The terms homozygous, heterozygous and hemizygous should only be used when referring to genes, and, while antigens are (ultimately) derived from genes, many, such as A, B, H, I, i, Lea and Leb are not proteins, and cannot, therefore, be direct gene products (although even the "protein" antigens go through post-translational modification and so also cannot, strictly speaking, be direct gene products).  On top of this, of course, the mature erythrocyte has exuded all nuclear material.

Antigens, therefore, should only be referred to a single or double "dose".

While Malcolm is correct, I also agree with OneMore. I have worked with Immucor instrumentation for 17+ years, and always ruled out with a "double dose" cell, and never had issues.