Well the simple answer is "don't take drugs, and you'll pass the drug tests".
EPO use - for many years there was no test, and the current EPO test has only limited effectiveness, as EPO is cleared from the body within a few days and but if I am informed correctly the EPO test is getting more and more precise all the time.
There is no possible test for blood boosting using ones own blood (a la Puerto)
for steroids and such, see Balco. they were using top secret designer steroids that the agencies never knew about. I suspect that there are many others like that, no masking agent needed if you don't know that the original steroid exists.
Swimming Workout of the Day: http://forum.slowtwitch.com/...ost=5784860#5784860;
Favourite Swim Sets: http://forum.slowtwitch.com/...m.cgi?;post=5004659;
<oh, I wish I had a GoPro Hero4>