RARC N4: missing or invalid prior insurer EOB.

What X12 says RARC N4 means

Missing/Incomplete/Invalid prior Insurance Carrier(s) EOB.

RARC N4 · Start 01/01/2000, last modified 03/06/2012.

In plain terms: The earlier insurer's explanation of benefits is missing or invalid.

What it means on a remittance

A remark code (RARC) adds detail to an adjustment reason (CARC). N4 points at the secondary-payer situation: this payer needs to see what the earlier insurer decided before it can process its share. The payer picks which CARC to print beside it; X12 publishes no table pairing them.

Typical causes

• The claim was sent to the secondary payer without the primary payer's EOB or electronic remittance data.
• The primary payer's amounts or adjustment codes on the secondary claim do not match the primary's remittance.
• The EOB was attached but unreadable, incomplete or for a different claim.

These are common billing patterns, not a diagnosis of your claim. The remittance and the payer's policy decide.

What to check

1. Get the primary payer's remittance or EOB for the same claim and service lines.
2. Confirm the paid amounts and adjustment codes on the secondary claim match it.
3. Resubmit with the EOB or data in the format the secondary payer requires.
4. Read the CARC beside the remark. CARC 23 reports the impact of a prior payer's adjudication.

Correct, resubmit or appeal

Correct the coordination-of-benefits data or attach the primary EOB and resubmit as the secondary payer requires. If you believe the EOB was complete, ask the payer which element it considers missing.

Medicare dates that apply

A Medicare redetermination request must be filed within 120 calendar days from the date a party receives the notice of the initial determination; receipt is presumed 5 calendar days after the notice date unless there is evidence to the contrary (42 CFR 405.942(a)), and a late request can be considered for good cause (405.942(b)). Other payers set their own appeal windows in the contract or plan documents, so read the notice for the date. The tool above runs the date math.

Related codes

X12 does not publish a table that pairs each code with remark codes; the payer picks the remark code and prints it beside the CARC on the same remittance line. These X12 remark codes carry wording that speaks to this topic:

• N479 — Missing Explanation of Benefits (Coordination of Benefits or Medicare Secondary Payer).

Other codes: CARC 23 (impact of prior payer adjudication) and the full lookup.

Worked example (hypothetical)

A secondary claim is rejected with N4. The biller finds the primary's remittance listed a $20 patient-responsibility amount, but the secondary claim reported $0. The biller corrects the amount from the EOB and resubmits.

Sources

• X12: Remittance Advice Remark Codes — code text, usage notes and dates (list dated 10/30/2026, retrieved 2026-11-02)
• 42 CFR 405.942 — eCFR, current through 2026-10-29 (retrieved 2026-11-02)

Code descriptions are X12 copyrighted material, quoted here only for the codes on this page and linked to the owner. For the full lists, use X12. The X12 text and the Medicare rule above were read from these pages on November 2, 2026. Causes, checks and examples are general billing practice written for this page, not X12 content. Payers revise code lists, contracts and policies, so confirm anything that affects a claim or a deadline with the payer.