The image below shows a test strip used for drugs testing. Unlike the hormone in a pregnancy test, the substances tested for in this kind of drugs test are too small to bind to more than one antibody at the same time. This type of test is known as a competitive binding assay.
The components of the test contain the following:
Conjugate pad - antibodies complementary to the drug being tested for. The antibodies are attached to a coloured bead.
Test line - bound molecules of the drug being tested for.
Control line - bound antibodies complementary to the antibody from the conjugate pad.
Which of the following correctly explains the appearance of a positive result?
The drug binds to antibodies on the conjugate pad, preventing them from binding to the test line.
The antibodies from the conjugate pad bind to the test line, causing the beads to form a visible band.
Unbound antibodies from the conjugate pad bind to antibodies on the control line, forming a visible band.
The drug binds to antibodies on the conjugate pad and to antibodies on the test line.