I suspect it is difficult to get the 'raw' output from a cell-phone camera so you are dependent on the phone's processing (contrast enhancement etc), which might vary between models. Not impossible (e.g. having control lines) but needs thinking about.

That's not an awful idea, and a step forward from the idea of replaceable strips on a reader that the thread suggests!

No. I'm talking about designing a paper pregnancy test that has words instead of lines.