What type of immunoassay principle is commonly used for initial drug screening in urine?

• A. Direct ELISA.
• B. Competitive immunoassay.
• C. Sandwich immunoassay.
• D. Immunohistochemistry.

Answer: (B)

Small-molecule drugs in urine are detected most effectively with a competitive immunoassay. In this approach, the drug in the sample competes with a labeled version of the drug for a limited antibody binding site. When more drug is present, less labeled drug binds, producing a weaker signal. This format suits small molecules that typically have only one epitope, so a two-antibody “sandwich” isn’t feasible. Direct ELISA relies on immobilized antigen and works better for larger targets, while sandwich immunoassays require two antibodies binding two distinct epitopes on the same molecule, which small drugs often cannot provide. Immunohistochemistry is for tissue sections, not urine screening. Hence, competitive immunoassay is the standard principle used for initial urine drug screening.