Holly - you don't have to really worry about being caught out on a blood test, unless they are purposely screening you for opiates and related substances.
If you are having ESR, liver function etc - these are specific tests done by a specific technique - you could have all manner of other substances in your blood that these sorts of tests wouldn't reveal - are you having a tox screen routinely? - probably not, so you should be fine to stoke up the vapourizer and Breathe Deeply!
