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N321 Remark Code: Last Admission Period Invalid

N321 means the last admission period was missing, incomplete, or invalid. The payer wanted the dates of the patient's most recent prior admission, which can affect readmission, qualifying-stay, or benefit-period rules.

Quick facts

Code
N321 (RARC N321)
Status
Active In use since December 2, 2004.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): A correctable data issue; the facility corrects the prior stay dates and resubmits. The patient is not billed.
Official description
Missing/incomplete/invalid last admission period.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What N321 means

Some coverage decisions depend on a patient’s previous inpatient stay. A skilled nursing benefit might require a qualifying hospital stay; a readmission policy might compare the new stay with the last one; a benefit period might reset based on time since the last admission. When the payer needs the dates of that previous admission and doesn’t receive them in usable form, it returns N321.

It is usually paired with CARC 16.

Common causes

  • The admitting facility never received the prior hospital’s admission and discharge dates.
  • Only the prior discharge date was reported, without the admission date that starts the period.
  • The prior period overlaps the current stay or is in the wrong order.
  • The period reported is from an older stay, not the most recent one.
  • The payer requires the period in a specific place (for example, as an occurrence span) and it was sent elsewhere.

What to do

  1. Get the prior facility’s discharge summary or transfer paperwork and note the admission and discharge dates.
  2. Confirm it is the most recent admission relevant to the payer’s rule.
  3. Report the period in the field or occurrence span the payer specifies.
  4. Check that it ends before the current stay begins.
  5. Submit a replacement claim with frequency code 7.

How to prevent it

At admission, especially for skilled nursing and rehab facilities, collect prior-stay dates as part of intake paperwork. Record them in structured fields that feed the claim. Where the prior facility is part of the same health system, pull the dates electronically instead of retyping them. Our eligibility guide covers other front-end data that drives facility claim denials.

Codes that may appear with N321

  • CO-16 (Claim/service lacks information or has submission/billing error(s).): Missing or invalid information; N321 names the last admission period.
  • N332 (Missing/incomplete/invalid prior hospital discharge date.): The prior hospital discharge date is missing or invalid.
  • MA40 (Missing/incomplete/invalid admission date.): The admission date for the current stay is missing or invalid.
  • N300 (Missing/incomplete/invalid occurrence span date(s).): An occurrence span date is missing or invalid.

N321 FAQ

Why would a payer need the last admission period?

Rules such as qualifying hospital stays for skilled nursing coverage, readmission reviews, and benefit periods all depend on when a patient was last admitted and discharged.

Where is it reported?

It depends on the claim type and payer. Institutional claims often use occurrence span codes and dates for prior stays; follow the payer's instructions.

What if the prior stay was at another hospital?

You still report its dates. Get them from the transferring hospital's records or discharge summary.