They're primarily checking for proof that you're taking the medication.

• If levels are too low: they assume you're not using it, and if you're not using it you must be selling it.
• Levels too high: you're abusing it.
• Levels of something else (usually alcohol or cannabis): you're a junkie.

All three can get your prescription revoked, and the testing requirements change based on the weather and what your doctor had for breakfast last Tuesday.