Thanks, DrBill and Newme. I tried another test this morning, and pretty much the same thing. I think I've slightly convinced myself that you could very very barely see a line at the 10-minute mark, which got a infinitesimally more visible over the next few minutes, but there was nothing there at all in the first several minutes beyond the 5-minute mark.

I'm going to try the dilution method over the next several hours and do another test this afternoon. I'm hoping that because the first morning piss is stronger, I'll get better results in the afternoon after drinking lots of fluids (which will be the case when I take the test).