I've done the research, here's the answer:
I have only brackish water aquariums so I was questioning how accurate my readings were. I have an AP SW Test kit and I was doing tests on very low-range brackish when I first started out. What I did was I went to the LFS and took a look at the refills they have for the test solutions. The solutions are exactly the same for fresh and saltwater but the difference is the charts, but by very little.
0 NH3 will show up a little more yellowish for freshwater, everything else was the same. When testing for Nitrate compare it to your saltwater testkit, take the reading, and multiply it by 2. That is your true freshwater nitrate reading.
Try contacting the manufacture and tell them you lost your chart cards for your freshwater kit and would like to see if they could provide you with new ones. Otherwise you could go to the LFS, look at the refill kits(because they contain both cards for fresh and salt), and compare and make notes. When you need to get refills for your solution you'll end up with both sets of cards and essentially a Master Test Kit.
If you don't believe me, look for yourself... do it just in case I'm in error. I've had no problems between any of my tanks ranging from SG of 1.000 - 1.016 using the saltwater testkit.