A multiwell apparatus for the assay of microliter quantities of body fluids is provided which prevents fluid loss by lateral migration or gravity flow through a microporous membrane or ultrafilter. A sample is passed through a membrane to which an antibody has been bonded in order to harvest complementary viral antigens in the sample. The harvested viral antigen is converted to single strand DNA or RNA which then is deposited on a second membrane. The deposited single strand DNA or RNA is capable of reacting with labeled DNA or other suitable detection probe which can be detected and correlated with antigen concentration in the sample.