Why Health Insurance Claims Get Denied
Denials are not random, and they are not usually a judgment about whether you deserved care. Most fall into a handful of categories, and the right response is different for each one. Identifying which you are looking at is the whole first step.
Start with the reason, not the amount
The instinct on receiving a denial is to look at the dollar figure. The useful information is the reason code and the sentence next to it. Two denials for the same amount can require completely different responses, and sending the wrong kind of appeal wastes weeks of a window that is already running.
Find the reason on the Explanation of Benefits or the denial letter itself. Then match it to one of the categories below.
Administrative and coding errors
This is the largest and least dramatic category, and the best news when you find it. The service was covered, the patient was eligible, and something in the paperwork was wrong: a transposed member number, a diagnosis code that does not link to the procedure code, a missing modifier, the claim sent to the wrong payer.
These often do not need a formal appeal at all. A phone call to the plan can identify the error, and the provider's billing office can resubmit a corrected claim. That path is faster than the appeal process and does not consume your appeal rights.
What to do: call the plan first and ask specifically what would have to change for the claim to process. Then call the billing office and relay it. Ask both whether a corrected claim resubmission is possible instead of an appeal.
Not medically necessary
This is the category people find most insulting, and the one where appeals matter most. The plan is saying that based on its clinical criteria, the service was not warranted for your documented condition.
The critical thing to understand is what the reviewer actually sees. They are not evaluating you. They are comparing the documentation submitted against a written clinical policy. If the documentation does not show what the policy requires, the denial follows almost mechanically — even when the care was obviously appropriate.
What to do: request the plan's clinical criteria for that service in writing. Plans are generally required to provide the basis for the decision on request. Then work with your treating physician on a letter of medical necessity that addresses that criteria point by point, including what was tried before and why it was insufficient. A letter that argues in general terms about your suffering will lose to one that maps directly onto the policy language.
Prior authorization missing or denied
Some services require approval before they happen. If that approval was never obtained, or was requested and refused, the claim gets denied regardless of medical merit.
There are two very different sub-cases. If authorization was genuinely never sought, the question becomes whose responsibility that was — often the provider's. If authorization was sought and there is documentation of the request, a denial claiming none was obtained is a factual dispute you can win with paperwork.
What to do: ask the provider for the authorization request and any confirmation of submission. If it exists, attach it. Denials that rest on a factual error about the record are among the more straightforward to overturn.
Out-of-network
The provider is not contracted with your plan, so the claim is denied or paid at a much lower rate — and you may be billed the difference.
Federal surprise-billing protections cover important situations here, including emergency care and certain out-of-network care delivered at an in-network facility. If you went to an in-network hospital and an out-of-network anesthesiologist or radiologist was part of your care without your choosing them, that is precisely the scenario those protections address.
What to do: establish whether you chose the out-of-network provider. If you did not — emergency, or an ancillary provider assigned to you at an in-network facility — say so explicitly in writing. Also appeal on network adequacy grounds if no in-network option was reasonably available for the service you needed.
Experimental or investigational
The plan classifies the treatment as unproven. These are the hardest denials to reverse through internal appeal, because the plan is applying a policy position rather than evaluating your specific facts.
What to do: these are often where external review earns its reputation. An independent reviewer with relevant specialty expertise evaluates the clinical evidence, and their decision binds the plan. Published literature, treatment guidelines from specialty societies, and your physician's clinical reasoning carry more weight in that forum than in an internal appeal.
Timely filing
The claim arrived after the plan's filing deadline. This is usually a dispute between the provider and the plan rather than something you owe.
What to do: if the delay was the provider's, you generally should not be balance billed for it. Ask the billing office directly whether the denial was for timely filing and whether they intend to write it off.
The step almost nobody takes
Appeals succeed far more often than people expect, and the overwhelming majority of denials are never appealed at all. The distance between those two facts is the entire argument. An internal appeal costs a letter, some documentation, and postage.
Before you write anything, confirm two things: the deadline on your notice, and whether your plan is fully insured or self-funded. The second determines which rules govern everything that follows.