List the four types of preauthorization or prior approval that may be used by some managed care plans.

Preauthorization or prior approval refers to any decision taken by the physician whether any health care plan, medication, etc is necessary medically or not. It is also known as precertification.
Four types of Prior Approval that may be used by some managed care plans are:
1. Formal referral
2. Direct referral
3. Verbal referral
4. Self referral