Portable arsenic test kit for drinking water

Does anyone have experience with getting false positive results from arsenic test kits?

I tested the water with a portable kit and the results showed 0.025 - 0.05 mg/l of arsenic.
I sent the sample to the lab and their test showed 0 mg/l.

What could be the reason for the difference in the results?  

Which arsenic test kit do you use? 
Can you recommend one which gives the most reliable results?