I used a D-D kit and a colorimeter and voted that option.
In your opinion what is the difference between "undetectable" and "not present at this level" ? To me "undetectable" simply means "present in levels below the threshold for the reading category". In the case of D-D/Merck, the lowest value is 0 to 0.008 (which is VERY low IMO - as in, less than 0.01 - no other test kit, including the colorimeters can read to that low of resolution). One thing about the D-D/Merck though is that it is not a linear scale.