Body & HealthHow it works
How two lines on paper add up to a measurement.
The liquid wicks through a pad and over a conjugate pad holding dried gold particles carrying antibodies, which latch onto the target molecule and show up red. At the test line, fixed antibodies capture that complex. The control line catches leftover particles regardless of whether any target was present.
Useful: A missing control line means the test is invalid, not negative.
Source: en.wikipedia.org