Time Based E/M Coding: Look for "Reference Time"
Per CPT® guidelines, “When [evaluation and management] codes are ranked in sequential typical times and the actual time is between two typical times, the code with the typical time closest to the actual time is used.”
This rule applies when reporting evaluation and management services using time—rather than the key components of history, exam, and medical decision-making—as the determining factor in the level of service (e.g., if counseling and/or coordination of care comprise more than half the encounter). In such cases, you should use CPT® “reference times” to determine an appropriate evaluation and management service level.
For example, a level 3 established patient outpatient visit (99213) has a reference time of 15 minutes, and a level 4 service (99214) has a reference time of 25 minutes. When reporting a time-based evaluation and management service lasting 19 minutes, report 99213 because it has the closest reference time.