I work in research, not a clinical setting, and we do negative controls for
each batch of slides processed for a given antibody.  Most often, we
substitute  same species non-immune serum for the antibody, trying to
approximately match protein concentration.  Ideally, we preabsorb the
antibody with a blocking peptide.  Santa Cruz is very good about having
blocking peptides available for many of their antibodies.

Mary Ross
Med Micro and Immuno
Ohio State University