Claims · Updated 2026-09-08 · 5 min read

What do CO, PR, OA and PI mean on a dental EOB?

CO PR OA and PI are the group codes on a dental EOB. They decide who owes the money — not the number beside them. What each one means and what to do.

The two letters in front of the number decide who owes the money, and the number does not. CO is Contractual Obligation: the dentist agreed to this reduction by signing the network contract, so it is written off and the patient never sees it. PR is Patient Responsibility: deductible, coinsurance, copay, or a service the plan excludes — bill it. PI is Payer Initiated Reduction: the payer decided not to pay, and neither the contract nor the patient covers it, so it is written off and must not be billed to the patient. OA is Other Adjustment, most often the other carrier already paid under coordination of benefits. The number beside the letters is the Claim Adjustment Reason Code (CARC) and it says why the money moved, not whose money it is. As of September 2026, across 37 dental practices, most carriers print the number with no letters attached at all.

What we measured

Single code tokens on dental remittance lines, all dates, what we saw across 37 dental practices, measured September 2026. Codes seen at fewer than five practices are not shown.

Read the top two rows together. The same reason code — 45, the contracted write-off — is printed bare on 8,951 lines and with its group letters on 2,900 more. Coinsurance is printed bare on 1,179 lines and as PR2 on 318. When the letters are missing, the one fact the office needs in order to answer "can I bill the patient?" is the fact the carrier left off the line.

What the carriers' own documents say

Delta Dental publishes the crosswalk that proves the group letters, not the number, carry the answer. Its mapping table has two group columns side by side — one for a non-participating dentist and one for a participating dentist — and the same denial routinely reads PR in the first and PI in the second. Policy code 504, filed under CARC 50, states the rule inside the denial text:

"The submitted documentation does not support the payment of benefits for the procedure. Contracting providers agree to charge the patient only the amount indicated as 'Patient Pays.'"

(Delta Dental — Policy ID Mapping for CARC/RARC Health Care Policy Codes, pinned at Delta Dental of California network scope.)

Two codes in that same table carry the identical CARC — 204, not covered — and opposite group letters. FLK is PR for both par and non-par dentists:

"According to our guidelines, this service is not allowable. The patient is responsible for the amount indicated as 'Patient Pays'."

FLL is PI for both:

"According to our guidelines, the fee for this procedure is considered to be part of, and included in the fee for a completed service."

Same number on the EOB, opposite answers to who pays. MetLife states the ceiling that survives every PR code in its Preferred Dentist Program Resource Manual, a network-scope provider manual:

"Whenever a participant receives any services from a participating dentist, the plan allowance applies. When the plan allowance is not reimbursed in full or in part by the benefit plan, any difference is the responsibility of the participant up to the plan allowance. The participant is not responsible for any amount that exceeds the plan allowance."

Why it depends on the contract

The group letter is not a property of the procedure or of the denial. It is a property of the agreement between that dentist and that carrier on that date. Delta prints the same sentence as PR to a dentist outside its network and PI to a dentist inside it on codes 455, 504, 718, 2DL and FLY — a filling within six months of a crown, unsupported documentation, a bundled fee, an expired filing deadline, an alternate benefit. On codes 503, 515 and 2DX it prints PR to both.

That is why a denial code is never an answer on its own. The same three digits reaching two practices on the same day mean "collect it" at one and "write it off" at the other, and only the contract separates them. And a PR line does not release an in-network dentist from the contracted fee: MetLife's sentence above says the patient owes the plan allowance, not the office's full fee, even when the plan paid nothing.

What to do

• Read the group letters before the number. If they are missing from the line, they are in the electronic remittance behind it — ask the carrier for the group code on that line rather than guessing from the reason.
• Treat PI as a write-off, always. It is the code that most often gets billed to a patient in error, because it looks like a refusal rather than an adjustment.
• On any PR line for an in-network dentist, bill the contracted allowance, not the full fee.
• Post CO-45 as the write-off it is and do not record it as a denial reason — it rides paid lines.
• Where a carrier prints numbers with no group letters at all, keep that carrier's own legend beside the remittance. Its numbers may not be X12 codes.
• Record the par or non-par status on the plan record alongside the denial, because that is the field that decides the answer next time.

Numbers last refreshed September 2026.

All articles · Kaylie