But we're not talking about a blood test now, are we?
OP wanted to pass a piss test for his sport, which is not very hard to pass if you stick to orals.
okay, I'll give you the basics.
you take 17aa oral steroids for 4-8 weeks, daily, letting them build up into your system. steroid metabolites can be hidden in fat cells, because they are fat soluble, they are not water soluble.
the typical steroid test will test for these various metabolites.
now, metabolism.
After a drug is swallowed, it is absorbed by the digestive system and enters the hepatic portal system. It is carried through the portal vein into the liver before it reaches the rest of the body. this is first pass metabolism.
one filtered via the liver, the waste will be sent into the blood stream, then to the kidneys to be removed. (still following me here hoss?)
metabolites will be found in the blood stream, and urine, as the blood is filtered via the kidneys, and as you use fat for fuel, the metabolites that are released, will be processed, and eventually pissed out.
more on fat tissue being a safety deposit box for the steroids you take.
http://www.ncbi.nlm.nih.gov/pubmed/3160722
steroids don't just evaporate from your blood.