POS code 21: Inpatient Hospital.

The CMS definition

A facility, other than psychiatric, which primarily provides diagnostic, therapeutic (both surgical and nonsurgical), and rehabilitation services by, or under, the supervision of physicians to patients admitted for a variety of medical conditions.

That is the description CMS publishes for POS 21, Inpatient Hospital, in the Place of Service Code Set and in chapter 26 of the Medicare Claims Processing Manual. In the manual’s table it carries the Facility rate (F) designation for payment under the Physician Fee Schedule.

What it means in plain terms

POS 21 describes a hospital stay where the patient has been admitted. The key word is registered: the code follows the patient's status, not the room. A visit in a hospital hallway, a clinic suite or a bedside gets POS 21 if the patient is a registered inpatient.

When it is used

• A physician or practitioner sees a patient who has been admitted to a general acute-care hospital (the CMS definition excludes psychiatric facilities, which have their own code, 51).
• The patient is a registered inpatient but is seen in another part of the building. CMS section 10.6 uses the example of a patient seen in the physician's office who is actually an inpatient of the hospital: POS 21 is correct, even though it does not match the office address.

Billing implications (Medicare)

Medicare pays physician fee schedule services in POS 21 at the facility rate (the manual marks it F). The manual's registered-inpatient rule says reporting POS 21 is the minimum needed to trigger the facility payment.

If you know the exact inpatient setting, CMS allows the more specific code in place of 21. Its examples are POS 31 for a patient in a SNF receiving inpatient skilled nursing care, POS 51 for an inpatient psychiatric facility and POS 61 for a comprehensive inpatient rehabilitation facility.

The hospital itself bills on an institutional claim, not with a POS code. POS 21 belongs on the professional claim.

The general rule in section 10.6 of the manual is that the POS code “generally” reflects the actual setting where the beneficiary receives the face-to-face service, with two exceptions: a registered hospital inpatient or outpatient is reported with the hospital code regardless of where the encounter happens. The code goes in item 24B of the paper Form CMS-1500 or the matching field of the 837P.

Look-alike codes

• POS 19: Off campus-outpatient hospital. A registered outpatient, not an admitted patient.
• POS 22: On campus-outpatient hospital. Same distinction as 19, on the main campus.
• POS 23: Emergency room-hospital. An outpatient emergency visit, not an admission.
• POS 51: Inpatient psychiatric facility. The more specific code for psychiatric inpatients.

For every code in one list, with the 02 vs 10 vs 11 telehealth verdict, see the place of service code lookup.

Sources

• CMS: Place of Service Code Set — official code list and descriptions, page database updated May 2, 2024 (retrieved 2026-11-02)
• CMS: Medicare Claims Processing Manual, Chapter 26 (Rev. 12779, issued 08-09-24) — sections 10.5 and 10.6, facility and non-facility designations and special considerations (retrieved 2026-11-02)

Every definition and rule above was read from these CMS pages on November 2, 2026. Payers revise code lists, contracts and policies, so confirm anything that affects a claim with the payer. CMS itself tells billers to check each payer (Medicare, Medicaid, private plans) for how it reimburses each code.