As far as Marion's tests go...
Steroids are a training drug, not a racing drug. You take steroids so you can recover more quickly from training. Steroids also adds muscle mass and therefore strength and power. So a sprinter such as Marion could 'roid up during the build-up her training and stop just before she enters competition and the drugs will be out of her system but the effects will still be there.
I'm assuming we all know what adaptation and supercompensation are correct? You stress the body, the fitness level drops, the body responds, adapts, gets stronger/better, you stress it again, repeat process. Too many stress and you don't fully adapt from the inititial stress, you only get worse and worse. You become a better athlete a better runner from the rest, not the actual training.
Let's say a normal human being can perform a hard workout 12 times in 12 weeks, once every 7 days. If they run hard more than that then their body won't adapt and supercompensate, it will only get tired and fail. They rest, recover, taper and race. Now they only do enough to MAINTAIN their fitness so they can race for 4 weeks.
Ok, now let's say a 'roided human can perform a hard workout 24 times in 12 weeks, doubling their capacity for hard work. The rest, recover, taper and race. They will race faster because they were able to put in twice as much hard work.
So Marion would only have tested positive during a "suprise" out of competition drug test.
Now, EPO and blood boosting is more of a racing drug. Cheaters began using EPO because it was simpler than regular blood boosting and less dangerous. Now that they have a test for EPO it's very likely people have gone back to transfusions (Tyler Hamilton). There are two ways to do this: 1. You draw out your own blood, which means until your RBC level returns to normal you'll have to train at a reduced level (4-8 weeks). It's also very costly, probably the most costly way to cheat. It's also undetectable. 2. You use the blood of someone with the same blood type. They have recently developed a test to detect this, hence Tyler getting caught. So, it would appear the key to getting past the blood tests is to figure out the number of immature blood cells they are looking for to trip the test. From my research it says it's a "guarded secret"....though with anything I'm sure that secret has been bought so it would give cheaters a number to shoot for...like the 50% hematocrit.
You see, the big problem with the doping controls is that they are too public. If you tell them they will be busted with a hematocrit of 50% or a T:E ratio of 4:1 then they'll cheat up to that level.
Alan