If you've only been taking the opiates you've been prescribed, you don't need to worry about testing. As far as cutting down, it's difficult, but the dosing schedule you've been given doesn't sound unreasonable in exchange for a guaranteed supply of such meds. You shouldn't experience too much in the way of withdrawal, maybe a little discomfort during the adjustment period.
Take as recommended if you can, for a while at least. If you are still experiencing pain, you can talk to your doctors about it.