I use the Red Sea calcium test, and it suxx! Brendan pretty much explained it in a nutshell, and he's right regarding false postives, most of the time it's user error. Check to make sure you're using the right bottle. Try to keep it in the 8-9 drop range (400-450ppm).
And yes, the nitrate test has 3 bottles and the phosphate test has 2. The other Red Sea calcium test kit thats uses powder is more accurate than the regular test kit, which is a big plus, but I don't know what ppm
it measures too