An Explanation of Benefits is covered in short codes, and the codes are where the real information hides. The most important one to understand is CO-45, because it marks money you are not supposed to be charged. Here is what it and its neighbors mean.
The two parts of a code
Most EOB adjustment codes come in two pieces:
- A group code - two letters saying who absorbs the amount.
COmeans "contractual obligation" (the provider eats it).PRmeans "patient responsibility" (you owe it).OAmeans "other adjustment." - A reason code - a number explaining why.
45means the charge exceeded the plan's allowed amount.
So CO-45 reads as: "the amount over what we allow, and the provider has agreed to write it off." That group code, CO, is the key. A CO amount is not yours.
The codes worth knowing
| Code | Plain meaning | Who pays |
|---|---|---|
CO-45 | Charge above the plan's allowed amount; provider writes it off | Provider, not you |
PR-1 | Applied to your deductible | You |
PR-2 | Coinsurance (your percentage share) | You |
PR-3 | Your copay | You |
CO-97 | Service is bundled into another paid service | Provider, not you |
CO-18 | Duplicate claim or service | Provider (flag it) |
PR-204 | Service not covered by your plan | You, unless appealed |
CO-50 | Insurer says it was not medically necessary | Provider, but appealable |
The pattern to internalize: a PR code is a bill to you; a CO code is a write-off the provider accepted. If a CO amount somehow reappears on the hospital's statement as something you owe, that is an error worth challenging.
Why CO-45 is the one to watch
On a typical EOB line, a $2,850 charge might carry a CO-45 adjustment of $1,930, leaving a much smaller "you may owe" figure. The provider agreed, in its contract with your insurer, to accept the allowed amount and forgive the rest. If the hospital then bills you for the forgiven portion anyway, it is trying to collect money it already wrote off. This is a close cousin of balance billing.
That is why comparing your two documents matters so much. I walk through the exact comparison in EOB vs bill: the one comparison that catches overcharges. And to line the EOB's codes up against the actual charges, you will want the itemized bill open too - see how to read an itemized hospital bill line by line.
When a code signals a denial
Some codes are not adjustments but denials - the insurer declined to pay at all. CO-50 (not medically necessary) and PR-204 (not covered) are the common ones. A denial is not the end of the road; it starts an appeal clock. If you are holding a denial, the next step is understanding your deadline before anything else.
What to do next
- Find every
COcode on your EOB - those amounts are the provider's to absorb. - Confirm none of them reappear as charges on the hospital statement.
- Add up the
PRamounts; that sum should match what the hospital asks you to pay. - If you cannot make sense of a code, call the number on your EOB and ask them to explain it in plain terms. You are entitled to that explanation.
Once you can read the group code, an EOB stops being a wall of numbers and becomes a map of what you owe and what you do not.