system
Given the following post, respond as if you're commenting on Reddit:
human
How are false positive lateral flow tests possible?

 The rapid lateral flow tests, from my understanding, detect fragments of the virus. If no fragments of the virus are present (i.e. you are in fact negative for covid) then how can a test possibly show a false positive result?
gpt
Lateral flow tests typically use an antibody with an affinity for some target molecule (the virus fragments in this case), but there is always a non-zero probability that some other molecule will bind to and activate the antibody.  Many drugs work by fooling a protein in the body to activate as if it was in contact with it's normal target molecule, so there are molecules present that will fool an antibody.  A simple analogy is artificial sweeteners- molecules that bind to sugar receptors in your taste buds so the food tastes sweet.  In the virus tests, some other molecule might have the right shape to trigger the antibodies.  It is also possible that not all the antibodies incorporated into the test device are the correct ones- manufacturing is not perfect.

Biochemistry is inherently a probability game, and so some percentage of the time these antibody tests will trigger a positive result with no virus fragments present.  On a good test that might only be 1% or 0.1% of the time.  The tradeoff to be made with tests like this is sensitivity vs specificity.  Sensitivity is basically your detection rate for true-positive samples.  Specificity can be thought of as the detection rate for true-negative samples (how often a negative person will read a negative test).  The antibodies used in these tests are never perfect, so they usually aim for the best sensitivity they can get with reasonable specificity.  "Reasonable" is determined by the cost of a false positive- in the case of Covid, the cost would be quarantining a healthy person unnecessarily or requiring them to seek additional, more expensive tests like PCRs.