No, i don't think that is how it works. The naltrexone will kick out any opioid
agonists that are bound to the receptor but it doesn't mean it isn't still in the blood. If it was naloxone the same thing would happen but due to the much much shorter half life the naloxone can be depleted allowing the agonist to bind once again to the receptor.
Because naltrexone has a longer half life than heroin
, it will mean that it will outlast any heroin so it won't give an opportunity for it to bind, however the urine analysis will still reflect any drug
that is still in the blood plasma.
This is just my understanding of it, i may well be wrong and if i am i would really like to know the details.
It's best to be sure the worst of the withdrawals (if any) are over or else as pointed out it will cause precipitated withdrawals which by all accounts are extremely unpleasant and could well qualify as a medical emergency.