Physiological Systems (4A) ICD-10-PCS
The 4A body system covers Measurement and Monitoring procedures on physiological systems, with 834 ICD-10-PCS codes across 2 root operations, 830 of them billable in fiscal year 2026. Each root operation defines a distinct procedural objective, and the info icons show the official definition with a preview of the codes inside.
Root Operations in This Body System 2 operations · 834 codes
| Chars 1–3 | Root Operation | Codes | Billable |
|---|---|---|---|
| 4A0 | MeasurementDetermining the level of a physiological or physical function at a point in time | 444 | 442 |
| 4A1 | MonitoringDetermining the level of a physiological or physical function repetitively over a period of time | 390 | 388 |
The info icon shows the official root operation definition and previews the codes in that range.
About the 4A Body System
The ICD-10-PCS root operation Physiological Systems (Character 4A) encompasses procedures focused on assessing body functions rather than performing anatomical alterations. This operation includes two main activities: Measurement and Monitoring.
Within the Measurement and Monitoring classification, the Measurement component involves determining the level of a physiological or physical function at a specific point in time, such as recording a vital sign or lab value. In contrast, Monitoring refers to continuously or repeatedly assessing these functions over a period, allowing for ongoing evaluation of a patient’s condition. These distinctions are captured in ICD-10-PCS codes like 4A0 for measurement and 4A1 for monitoring. Accurate use of these codes is essential for documenting procedures that evaluate physiological systems without direct intervention, providing crucial information for clinical decision-making and patient care tracking.
Questions About This Page
How many codes are in the 4A body system?
The 4A body system contains 834 ICD-10-PCS codes in FY 2026, organized into 2 root operations. 830 of them are billable on inpatient claims.
What is a root operation?
The root operation is the third character of every ICD-10-PCS code and names the objective of the procedure. Drainage takes fluid out, Excision cuts out a portion of a body part, and Resection removes all of one. Picking the right root operation is the central skill of PCS coding, so each one above carries its official CMS definition.
Related References
Source: CMS FY 2026 ICD-10-PCS order file and code tables, effective October 1, 2025 through September 30, 2026.
