I cannot identify a validated “IMPA score” for fentanyl. It may be a typo or a local institutional scoring system.

If you mean assessment of fentanyl/opioid withdrawal, the usual tool is the Clinical Opiate Withdrawal Scale (COWS). In people using fentanyl, many protocols wait for clear withdrawal, commonly COWS 8-12 or higher, before standard buprenorphine induction to reduce precipitated-withdrawal risk. The exact threshold depends on the protocol and clinical situation.

Did you mean COWS, or can you spell out what “IMPA” stands for?