Bioluminescent immunoassays. 
The procedures described here have been developed for only a few antigens at present.  It seems very likely that they can be extended to any other antigen of choice.  For sensitivities in the picomole range, the luciferase-antigen procedure is satisfactory.  For increased sensitivity, the glucose-6-phosphate dehydrogenase-antigen is the method of choice.  With some minor modifications the sensitivity of this assay can certainly be increased.  The lower limits of detection will ultimately be determined by the affinity of the antibody for the antigen, not by the detection system.