The general problem with bioassays of this type, whatever the indicator
parameter, is that one has to make sure that the observed change is due to
the drug in question, and not to something else in the test sample or to
some physiologic change in the cells. This necessitates careful design of
controls. I'd guess that this might be even harder for an assay based on
levels of surface markers than for one based on something like calcium flux
or membrane potential or on a percentage of cells undergoing a more-or-less
all-or-none change like membrane lysis or apoptosis.
-Howard Shapiro