Practice question

Competitive ELISA is preferred for low molecular weight analytes present at low concentrations such as hormones insulin, triiodothyronine and steroids where sandwich assay fails due to single epitope and small size preventing dual antibody binding. Sample hormone competes with enzyme labeled hormone for limited antibody binding sites coated on plate. Signal inversely proportional to hormone level allows sensitive quantification in nanomolar range with small sample volume. Direct and indirect formats lack required sensitivity, sandwich requires two non overlapping epitopes unavailable on small hormones, making competitive format ideal.