Answer: You would assign 99354 (prolonged physician service in the office or other outpatient setting requiring direct [face-to-face] patient contact beyond the usual service [e.g., prolonged care and treatment of an acute asthmatic patient in an outpatient setting]; first hour [list separately in addition to code for office or other outpatient evaluation and management service]).
This is determined by subtracting the time that CPT indicates is typical for the level of office visit you provided from the time you actually spent in face-to-face contact with the patient. The CPT manual says a 99213 typically involves 15 minutes of face-to-face time. Subtracting 15 from 70 leaves 55 minutes. Prolonged services code 99354 may be used to report prolonged service of 30 minutes to 74 minutes and should be submitted with the office visit code.
If the prolonged service exceeded 74 minutes, also submit 99355 (... each additional 30 minutes [list separately in addition to code for prolonged physician service]).