A rejection is a bounce
You submit a claim to the clearinghouse, and the payer has an instant reason to send it back. The reason is usually format or formula based, and it has nothing to do with anything clinical. The most common version is some flavor of "we do not know this patient." Either you literally sent the claim to the wrong payer, or the payer doesnt have enough to identify the patient as a member. If, for example the clsomething is wrong with the patient demographics (member id is wrong, patient name mismatch, birthdate mismatch, patient relationship to subscriber is incorrect).
In a rejection, the payer is not reviewing the substance of the claim. Very often their system is rejecting the claim for a procedural or administrative reason, ususally within a day of submission.
Mechanically, there are two machine gatekeepers standing in front of adjudication. The clearinghouse can bounce a file back before the payer ever sees it, and the payer's own front end can bounce it after that. Neither of those is a payment decision.
This means:
- there is no explanation of benefits
- there are no appeal rights, because there is nothing to appeal
- Your clock on timely filing is running because many payers treat a rejected claim as never filed at all.
A denial is a decision
A denial means the payer took the claim through their claims process and accepted it. They have said, in effect, this was formatted correctly and we know this patient. Then, for a reason that is usually clinical, financial, or contractual, they decided not to pay it.
That decision comes back with reason codes attached. One code tells you why the claim was adjusted or denied (CARC or Claim Adjustment Reason Code), and a second layer adds detail (RARC or Remittance Advice Remark Code). This is the category people mean when they say a claim needs to be "worked."
Three ways the confusion costs you
You cannot fix what you never see. We have encountered practices whose practice management system does not receive rejected claims from the payer at all, usually because of a configuration issue at the clearinghouse. The team believes a claim is out there pending or denied. What actually happened is that the payer said, you sent me a claim with the wrong member ID, please fix it. Because the practice management system never got that message, nobody knows there is a thing they could fix, turn around, and resubmit in an afternoon.
This is worst at practices that have been through a lot of change: acquisitions, mergers, divestitures, turnover. Those organizations tend to be running multiple EHRs and PMs in parallel, that were never designed to talk to each other. Awareness is the first thing that breaks.
The fix is different. A rejection gets corrected and resent. A denial gets a decision: appeal, reconsideration, corrected claim, or rebill. There are not that many available fixes, but you have to pick the right one, because picking the wrong one means you do not get paid again and you repeat the work. Filing an appeal on something that was never adjudicated is wasted effort. Resending a claim the payer already adjudicated and refused is also wasted effort.
The clock starts regardless. Because many payers treat a rejection as never filed, your timely filing window keeps running while the claim sits in a queue nobody is watching.
CARC and RARC codes only travel with denials
Even once you are firmly in denial territory, the code is a starting point, not an answer. A "missing information" code can be a two-minute demographic correction or a full research effort taking up to 30 minutes per claim to verify the patients benefits, the payers medical necessity policy, and the prior authorization and referral the practice received.
That is why the diagnostic work is the expensive part of a denial, not the remediation. Most veteran billers, once they truly know why a claim was denied, can reason about the fix very quickly. Getting to that knowledge is the hard part.
How big is the pile
Kodiak Solutions put the initial denial rate at 11.81 percent for 2024, across more than 2,100 hospitals and roughly 300,000 physicians, up from 11.5 percent in 2023. Experian Health's State of Claims 2025 landed in the same place. Call it roughly one in eight claims denied on first submission.
Rejections sit on top of that number, and at a lot of organizations they are not measured at all.
I do not think there is malice in any of this. If you talk to people who work at the big insurance companies, they got into healthcare because they want to help people. The honest read is that there is insufficient standardization every time data gets exchanged, and the party with the most power to remove that friction is not the party incentivized to pay for removing it. The system performs as designed.
What to do about it
Measure the two separately. A rejection rate and a denial rate are different numbers with different owners and different fixes. If your reporting collapses them, you are managing an average of two unrelated problems.
Then look at where the answers actually live. A biller running down a single claim might check the clearinghouse, the payer portal, the practice management system, the lockbox, and the phone. That is a wide, tedious surface, and it is exactly the kind of work AI agents are good at. Claim Status, one of the agents in Substrate ARC, checks every channel on every claim, tirelessly, many claims at once, and hands your team a synthesized answer instead of a starting point. For NextGen RCM Services, that meant four times the volume with the same team, a 90 percent success rate, and 75 percent lower cost per claim status. Read the case study.
I want to be straight about the boundary. Substrate covers roughly 60 to 70 percent of the denials workflow today, not all of it. In denials we focus on claim status, denials caused by front-end errors, and denials caused by policy. Bundling and duplicate detection are on the roadmap and not shipped. We flag prior authorization needs, we do not obtain or submit the authorization, and we do not do autonomous coding. Medical necessity fights, peer-to-peer reviews, and novel payer disputes still need your best people. What agents buy those people is the time to take those fights on.
None of that changes if you cannot tell a bounce from a decision. Start there.
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Your team runs RCM. We give them time and leverage. See our agents work a real denial, on demand, no meeting required: watch the demo.
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