That link is a little misleading to me
After speaking with Dr Everman at WADDL countless times just recently here is what I learned about the different types of testing....
AGID is good in the sense it will NOT give you a false positive, but it will definately give you a false negative if the goat isn't far enough along in the disease to be shedding the organism
Fecal will also never give you a false positive, but again it is pretty much usless unless your goat is showing clinical signs of Johnes...you really only do a fecal if you get a goat that tests positive on an ELISA test twice. Otherwise, if your goat is wasting away and showing all of the other signs, the organism has already been spread onto the ground and its far too late to cull that goat and save the rest of your ground.
Necropsy- well we wont even go there, you goat has either been euthanized for the disease, or died from it....
ELISA while in very rare cases you can get a false positive (ESPECIALLY when using incorrect testing methods...if you want more details on this and what labs NOT to use..... please PM me...) it has the HIGHEST sensitivity for detecting the Johnes organism.
So yes- with ANY Johnes testing, there is a chance you could test a goat and they just arent far enough along to show up in testing, so there is always that risk, which is why most herds will test yearly for Johnes.
But in my opinion from all of the research I have gathered, the correct ELISA test is the best early detection test for Johnes.