Billing Status: YES. This is a valid, specific, and billable ICD-10-CM reference.

Inpatient Payment Groups (MS-DRG)

MS-DRGs this diagnosis helps define, as principal or secondary, per the CMS ICD-10-CM/PCS MS-DRG Definitions Manual v43.0 (FY2026), Appendix B.

A diagnosis appearing in a group's logic does not by itself determine the DRG assigned to a stay; the grouper uses the full claim.

Code Set: Valid FY2026 and FY2027. FY2026 took effect October 1, 2025.

Official Registry Overview & Definition

Encounter for therapeutic drug level monitoring is a billable ICD-10-CM diagnosis code (Z51.81).

Sequencing, inclusion, and exclusion notes published for Z51.81 in the official ICD-10-CM tabular list.

Conditions that can never be reported with this code; the two are mutually exclusive.

encounter for blood-drug test for administrative or medicolegal reasons (Z02.83)
holiday relief care (Z75.5)
follow-up examination after treatment (Z08-Z09)

Conditions not covered by this code, but which may be reported alongside it when both are present.

Additional codes that may be required to fully describe the encounter.

Official ICD-10-CM classifications closest to Z51.81 in its code family, with their registry titles.

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code.