system
Given the following post, respond as if you're commenting on Reddit:
human
Why do two lines indicate a negative result in ommunoassay urine drug tests when in most other immunoassay tests two lines indicate a positive result?
gpt
It's a competitive binding/inhibition assay for the negative result (is present = no signal), whereas in other tests a positive signal results from direct antigen-antibody binding (is present = signal). The second line, C, is an internal control of the assay for a highly expressed protein/antigen \[albumin, immunoglobulin or something\] to verify the sample and assay quality.