EOB Decoder & Code Glossary

 Your Quick Reference Guide to Understanding Insurance Adjustments and Denial Codes

Created by AKG Advocacy

1. What This Glossary Is For

Every Explanation of Benefits (EOB) you receive includes reason or adjustment codes — short combinations of letters and numbers (like CO-45 or PR-1) that describe why your claim was paid, adjusted, or denied.

These codes can be confusing, but once you learn to decode them, you can quickly spot errors, identify appealable denials, and take control of your claims.

💡 Tip: Use this glossary as a decoder sheet and log each code you see in your EOB tracker.
The same codes will appear again — and recognizing them saves time on future calls and appeals.

2. How Codes Are Structured

Each EOB code comes with a prefix and a number:

PrefixMeaningWhat It Refers To
COContractual ObligationThe provider must write off this amount; it cannot be billed to you.
PRPatient ResponsibilityThe amount you must pay (deductible, copay, coinsurance).
OAOther AdjustmentPayment changes due to secondary insurance or coordination of benefits.
PIPayer InitiatedThe insurer adjusted payment — possibly due to billing or documentation errors.
NNarrative Remark CodeA more detailed explanation that often follows the main code (e.g., N290).

🔍 You’ll often see more than one code on a single claim — look for PR codes first to understand what you owe, then review CO/PI codes to see what was adjusted or denied.

3. Most Common EOB Codes and What They Mean

CodeCategoryMeaningWhat You Can Do
CO-45ContractualCharge exceeds the plan’s allowed amountProvider must write this off — you don’t pay it
PR-1PatientDeductible amountYou owe this until your deductible is met
PR-2PatientCoinsurance amountYou owe this portion
PR-3PatientCopayment amountYou owe this at the time of service
CO-16ContractualMissing or invalid informationAsk your provider to correct and resubmit
CO-29ContractualFiling limit expiredAsk your provider to appeal for “timely filing exception”
CO-97ContractualBenefit not covered or paid by primary insurerCheck coordination of benefits or secondary insurance
CO-197ContractualNon-covered serviceReview your policy — may be eligible for exception or appeal
CO-109ContractualClaim not covered because service was not deemed medically necessaryFile an appeal with a Letter of Medical Necessity
CO-151ContractualPayment adjusted because the payer deems the information submitted insufficientProvider should add documentation and resubmit
PI-204Payer InitiatedService not covered under current benefit planRequest benefit clarification or policy citation
CO-18ContractualDuplicate claim or serviceProvider likely needs to confirm and refile
CO-23ContractualPayment adjusted because the charge was paid by another payerCheck coordination of benefits or secondary coverage
CO-170ContractualPayment adjusted because the service is considered experimental or investigationalAppeal with evidence and clinical studies
N290NarrativePayment adjusted based on plan maximumCheck your plan limits or file an exception request
N130NarrativeConsult plan’s provider manual for additional informationRequest written clarification from your insurer
OA-94OtherProcessed in excess of chargesOften corrected automatically; verify claim totals

4. Decoding Multi-Code Claims

Sometimes multiple codes appear together, like:

PR-1 / CO-45 / N290

This means:

  • PR-1: You owe your deductible.

  • CO-45: Provider must write off the excess over the allowed amount.

  • N290: The plan paid up to its maximum limit for this service.

🧩 Translation: You’re only responsible for the deductible. The rest should not be billed to you.

5. Codes That Often Signal Appeal Opportunities

CodeReason to AppealAction Step
CO-109“Not medically necessary”Submit a Letter of Medical Necessity from your doctor
CO-197“Non-covered service”Request policy citation; file a Coverage Exception
CO-151“Information insufficient”Ask your provider to resubmit with added documentation
CO-170“Experimental or investigational”Include peer-reviewed studies in your appeal
CO-29“Time limit expired”Request a Timely Filing Exception if you had valid delays
PI-204“Service not covered”Request full plan language; it may be appealable under Medicare or ACA

6. AKG Advocacy Pro Tips

✅ Always pair your EOB with your provider’s statement — they should match.
✅ Keep a Code Tracker Log (date, provider, code, reason, outcome).
✅ Don’t pay any amount labeled “CO” — those are contractual write-offs, not patient bills.
✅ If a denial cites “not medically necessary,” appeal immediately with supporting documentation.
✅ Save all EOBs, bills, and communications for at least 2 years.

7. Quick Reference: My EOB Code Tracker

DateProvider/ServiceCode(s)Insurer’s ExplanationMy Action/Notes

8. Additional Resources

9. Final Thoughts

Learning to read and interpret EOB codes isn’t just about decoding jargon — it’s about reclaiming control over your care and finances.
When you know what these codes mean, you can confidently question errors, file stronger appeals, and protect yourself from overpaying.

✊ AKG Advocacy Reminder: Every code tells a story. When you understand the language of insurance, you turn confusion into confidence.

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