What an appeal letter has to contain, by denial code

The Five Parts

1Who and what.Patient identifier, claim number, date of service, billed amount. The reviewer has to find the claim before anything else.
2The exact code you are appealing.Quote it as the remit prints it, group code and all: CO 197, not "the authorisation denial". One letter, one code.
3One sentence saying what you want.Reprocess and pay. Not a paragraph of context.
4The evidence that answers that code.The table below. This is the part that decides it.
5A date, and a copy kept.Appeal windows are short and the burden of proving you filed on time is yours.

Upload your denial export See a finished review

Before you write

Check It Is an Appeal Before You Write One

Some denials are corrections: the claim was wrong and needs resubmitting, not arguing. Appealing those spends the window and the deadline passes while you wait.

What Each Code Actually Needs

38 codes, with the documentation that wins each one. This is the same table the product uses when it reads an export.

Denial	Attach this
CO 4 The procedure code and the modifier do not agree.	The correct modifier, supported by the documentation.
CO 5 The procedure does not match the place of service billed.	The schedule or encounter record showing where the service happened.
CO 6 The procedure does not fit the patient's age.	The corrected date of birth, or a code appropriate to the patient's age.
CO 8 The procedure does not match the provider type or specialty on file.	The correct taxonomy code, or the credentialing record for the rendering provider.
CO 11 The diagnosis does not match the procedure billed.	A recode from what the record actually documents.
CO 15 The authorisation number is missing, invalid, or does not match.	The correct authorisation number and its date range.
CO 16 Something the payer needed was missing or wrong on the claim.	A corrected claim with the missing element supplied.
OA 18 The payer treated this as an exact duplicate.	Documentation of the second, distinct service. Modifier 76, 77 or 59 as appropriate to what actually happened.
CO 21 The payer says a no-fault insurer, most often an auto policy, is liable for this injury.	The no-fault carrier's EOB, denial letter or benefits exhaustion letter. The accident details from intake: date, type, carrier and claim number. A corrected claim without the accident indicator, where the injury was not from an accident.
OA 22 Another plan should pay first.	The primary payer's EOB, submitted with the claim as secondary.
OA 23 A prior payer already adjusted this, and that adjustment carries over.	The primary payer's EOB, attached to the secondary claim.
CO 24 The patient is in a capitated or managed-care plan, so this is paid under that contract.	Current eligibility showing which plan holds the contract on the date of service.
PR 27 Coverage had ended on the date of service.	Evidence another plan was active, and a rebill to that payer.
CO 29 Filed after the payer deadline.	The payer's own receipt date for the first submission, from its claim acknowledgement or claim status. For Original Medicare this is the date that counts: the date the Medicare contractor received the claim. For a commercial or Medicare Advantage payer, a clearinghouse acceptance report carrying the original submission date. The primary payer's EOB date, where the delay was theirs. For an Original Medicare exception, the documents 42 CFR 424.44(b) calls for, for example the notice of retroactive entitlement or the written notice of the contractor's own error.
CO 31 The payer cannot find this patient as a member.	Current eligibility showing the correct member ID and plan.
PR 49 Treated as a routine or preventive service the plan does not cover.	The chart note showing a presenting complaint rather than a routine check.
CO 50 The payer decided the service was not medically necessary.	A chart note tied line-by-line to the payer's own policy criteria. Objective findings: imaging, measured range of motion, strength testing. Documented failure of conservative care, with dates.
CO 58 The payer decided the service should not have been done where it was done.	The admitting note explaining why the setting was clinically necessary. The payer policy or criteria set they applied, so the appeal answers the right standard.
PR 96 Not a covered charge under this plan.	A signed ABN, which moves the balance to patient responsibility. Whatever the RARC actually asks for, if it points at a documentation gap.
CO 97 The payer says this is already paid inside another service.	An operative or treatment note showing a separate site, session or encounter. Modifier 59, or the more specific XE, XP, XS or XU.
CO 107 The related service this one depends on was not on the claim.	Both services on one claim, or the claim number of the primary procedure.
OA 109 Wrong payer or wrong contractor for this claim.	Identification of the correct payer, and a rebill to them.
PR 119 The benefit maximum for this period has been reached.	A KX modifier where continued therapy is medically necessary and documented. Documentation supporting a medical-necessity exception.
CO 146 The diagnosis was not valid on that date of service.	The diagnosis code valid on the date of service.
CO 150 The payer says the records do not support the level of service billed.	The visit note, with the problems addressed, the data reviewed and the risk of the treatment chosen. The total time on the date of the visit, if the level was chosen by time.
CO 151 The payer says the records do not support this many units or visits.	Daily treatment notes with start and stop times, or total timed minutes. The plan of care showing the frequency was ordered and justified.
CO 167 The diagnosis billed is not covered for this service.	A covered diagnosis that is already documented in the note, if one exists. Otherwise, an appeal on the merits citing the policy.
CO 170 This payer does not pay this service when this type of provider performs it.	Documentation supporting incident-to billing, where the requirements were genuinely met on the day.
CO 181 The procedure code was not valid on that date of service.	The procedure code valid on the date of service.
CO 185 The rendering provider is not eligible to perform the service billed.	The credentialing approval letter with its effective date. The payer's own provider record showing the taxonomy.
CO 197 The payer says no prior authorisation was on file.	The authorisation number and its approval date. The retro-authorisation request and the payer's response to it. Documentation that the service was urgent or emergent, where the payer waives auth for those.
CO 198 Authorised, but more units or visits were billed than approved.	Treatment notes showing medical necessity for the units beyond the approval. A re-authorisation request covering the additional units.
PR 204 Not covered under the patient’s current benefit plan.	An ABN signed before the service was delivered.
CO 226 The payer asked you for information and did not get it, or did not get enough.	The payer's original request letter, which says what and by when. Proof of what was sent and when, including the transmission record.
CO 234 This procedure is not paid separately.	Documentation supporting a modifier, where the policy allows one.
CO 236 This procedure and another one billed the same day cannot be billed together under NCCI.	The operative or procedure note showing the two services were distinct in site, session or encounter. The NCCI edit table entry with its modifier indicator.
CO 242 The provider was out of network, or not the patient's assigned primary care provider.	The referral or authorisation, with its date range. Documentation that the care was emergent, where that applies. The network participation record for that specific plan product.
CO B7 The provider was not certified or eligible for this service on that date.	The effective date of enrolment, once corrected. An appeal covering the retroactive period the payer allows.

Classifications are our own reading of each reason code, not the payer's.

How Long You Have

We are not going to print a number here, and you should distrust sites that do. An appeal window is set by your contract with that payer. It is not the same as the timely filing limit for an original claim, it differs by plan and by product line within the same payer, and it changes.

Look in this order:

Your payer contract. It governs, and it is the only source that binds them.
The provider manual for that plan, which usually restates it.
The remittance advice itself. Many payers print the appeal window on the remit that carried the denial.
Your state's prompt-pay statute, which sometimes sets a floor the contract cannot go under.

Original Medicare is the exception. CMS sets the first-level window for every Part A and Part B claim: see the Medicare redetermination form.

Your Questions Answered

What has to be in a medical claim appeal letter?

The claim identifiers, the exact denial reason code being appealed, one sentence saying what you want, the evidence that answers that specific code, and a date. The evidence is the part that decides it. An appeal that argues without attaching what the code asks for is refused again.

Does a longer appeal letter work better?

No. The reviewer is checking whether the missing thing is now present. A page of argument with no document attached loses to three lines with the authorisation number and its approval date.

How long do I have to appeal a denial?

It depends on your contract with that payer, and it is not the same as the timely filing limit for an original claim. Check the payer contract first, then the provider manual, then the remittance advice itself, which often prints the appeal window. Do not rely on a number you read on a website, including this one.

Should every denial be appealed?

No. Some are corrections, not appeals: the claim was wrong and needs resubmitting. Appealing those burns the appeal window and the deadline passes while you wait. Some are contractual write-downs you agreed to and there is nothing to win.

All denial codes

Start Your Free Denial Review

4 columns. No patient data. One page back.

Payer, denial code, count, billed amount. You get back which denials were winnable and what they were worth.