ICD-10-CM > Z00-Z99 > Z40-Z53 > Z51 > Z51.8 > Z51.81 Z51.81 Encounter for therapeutic drug level monitoring Billable/Specific Copy Code Copy Description Code Also any long-term (current) drug therapy (Z79.-) Type 1 Excludes encounter for blood-drug test for administrative or medicolegal reasons (Z02.83)