In reply to ffati:
A "fat burner" is very unlikely to give a false positive on a drugs test - they're mostly caffeine (literal German translation - "in coffee") and some capsaicin (the "hot" chemical in chilli) to make you sweat and make it feel like it's doing something.
CLA is conjugated linoleic acid - it's the oil/fat found in flax seeds
In other words, they're all "natural" chemicals that you could get by eating/drinking conventional foodstffs and thus shouldn't give a false positive.
(As an aside, and from personal experience, taking pseudofed before a drugs test can give a false positive - apparently pseudofed is often used by cheating sports persons as a masking agent, and hence the cheapo drugs tests used by employers are designed to give a positive result by default when they detect pseudofed)