What Is an Explanation of Benefits?
An Explanation of Benefits (EOB) is a document issued by a health insurance payer after it processes a claim. It is not a bill — it is a statement explaining how the claim was adjudicated: what was billed, what the payer allowed, what was paid, what was adjusted, and what (if anything) is the patient's responsibility.
Every claim you submit will generate an EOB or its electronic equivalent. Reading EOBs accurately is one of the most fundamental skills in medical billing — and one of the most common sources of revenue leakage when it is done poorly. Missed underpayments, unrecognized denial codes, and incorrect payment posting all trace back to inadequate EOB review.
For specialty practices — particularly those in physical therapy and mental health — EOBs often contain specialty-specific adjustment codes related to visit limits, authorization requirements, and medical necessity determinations that require careful interpretation.
EOB vs. ERA: What Is the Difference?
The terms EOB and ERA are often used interchangeably, but they refer to different formats of the same information.
| Feature | EOB (Paper/PDF) | ERA (Electronic 835) |
|---|---|---|
| Format | Paper or PDF document | Structured EDI 835 data file |
| Delivery | Mailed or faxed to provider | Transmitted via clearinghouse |
| Payment posting | Manual entry required | Auto-posted by practice management system |
| Error risk | High (manual transcription) | Low (automated import) |
| Processing speed | Days to weeks | Same day as payment |
| Preferred by | Smaller practices, some payers | Most billing companies and large practices |
Enrolling in ERA/835 delivery with your major payers is one of the fastest ways to reduce payment posting errors and accelerate reconciliation. Most clearinghouses support ERA enrollment for all major commercial payers and Medicare.
How to Read an EOB: The Key Fields
While EOB formats vary by payer, every EOB contains the same core information. Understanding each field is essential for accurate payment posting and identifying underpayments or errors.
Billed Amount
The amount your practice charged for the service. This is your fee schedule rate, not what you expect to be paid.
Allowed Amount
The maximum amount the payer will pay for the service under your contract. This is the starting point for calculating your payment.
Contractual Adjustment
The difference between your billed amount and the allowed amount. This is a write-off required by your payer contract — you cannot bill the patient for it.
Plan Paid
The amount the payer is paying directly to your practice. This is the allowed amount minus any patient responsibility (deductible, copay, coinsurance).
Patient Responsibility
The amount the patient owes — typically their deductible, copay, or coinsurance. This is what you can bill the patient.
Adjustment Reason Code (CARC)
A standardized code explaining why the payment was reduced or denied. These codes are critical for denial management and appeals.
Remark Code (RARC)
Additional detail about the adjustment, often providing more specific information about the denial reason or required documentation.
Adjustment Codes Explained
Claim Adjustment Reason Codes (CARCs) are standardized codes maintained by the Washington Publishing Company that explain why a payment was reduced or denied. Understanding the most common codes is essential for efficient denial management.
| CARC | Meaning | Action |
|---|---|---|
| CO-45 | Charge exceeds fee schedule/maximum allowable | Normal contractual adjustment — write off the difference |
| CO-4 | Service/procedure inconsistent with modifier | Review modifier usage; resubmit with correct modifier |
| CO-11 | Diagnosis inconsistent with procedure | Review diagnosis-procedure pairing; correct and resubmit |
| CO-15 | Authorization number missing or invalid | Obtain authorization; resubmit with valid auth number |
| CO-16 | Claim requires information | Review RARC for specific missing information; resubmit |
| CO-97 | Benefit included in allowance for another service | Bundling issue — review CCI edits; may need to appeal |
| PR-1 | Deductible amount | Bill patient for deductible per EOB amount |
| PR-2 | Coinsurance amount | Bill patient for coinsurance per EOB amount |
| OA-23 | Timely filing deadline exceeded | Verify submission date; appeal with proof of timely filing |
Common Denial Codes by Specialty
Denial patterns vary significantly by specialty. Knowing the most common denial codes for your specialty helps you prioritize process improvements and identify whether your billing vendor has the expertise to manage them effectively.
Mental health and behavioral health practices frequently encounter CO-96 (non-covered charge) and CO-4 (modifier inconsistency) denials related to telehealth billing and psychotherapy add-on codes. ABA practices commonly see CO-15 (authorization) and CO-11 (diagnosis inconsistency) denials. Home health agencies frequently encounter CO-B7 (provider not certified) and CO-16 (missing information) denials related to OASIS documentation.
If your billing team cannot explain your top 5 denial codes by CARC and describe the root cause for each, that is a gap in expertise. Consider browsing our directory of specialty-focused billing companies — for example, home health billing specialists or ABA billing companies — to find vendors with demonstrated expertise in your denial patterns.
Payment Reconciliation: Closing the Loop
Payment reconciliation is the process of matching each EOB or ERA payment to the corresponding claim in your practice management system and verifying that the amount posted matches the amount paid. Accurate reconciliation is essential for identifying underpayments, detecting payer errors, and maintaining clean financial records.
A complete reconciliation workflow includes three checks: verifying that the allowed amount matches your current fee schedule, confirming that the patient responsibility amount is correctly calculated and billed, and flagging any payment that is more than 5% below the expected allowed amount for manual review.
If you are working with an outsourced billing company, ask them to provide a monthly reconciliation report that shows total billed, total allowed, total collected, total adjustments, and total patient responsibility by payer. This report is the foundation of a healthy billing relationship and a key indicator of vendor performance. Use our 25-question checklist to confirm your current or prospective vendor provides this level of reporting.