RemitBench/Denial codes/CO-55

CO-55 denial code

Code 55 is a denial with an opinion in it. The payer is not saying the service is outside the plan. It is saying the evidence does not convince them, and that is arguable in a way most coverage denials are not.

The code list's own wording: Procedure/treatment/drug is deemed experimental/investigational by the payer.

What it means: The payer has a written policy saying this treatment is not yet proven, and it applied that policy to your line. The plan may well cover the condition and the category. It does not accept this particular treatment yet.
Who owes the money: Read the letters CO makes it your write-off, and it is the common pairing. PR puts it on the patient, which many contracts allow only if the patient agreed in writing before the treatment. Check what you had signed before you send a statement.
Is appealing worth it: Yes, more often than on any other coverage code. An experimental determination rests on a policy document and a reading of the evidence, and both can be argued. Ask for the policy by name and answer it point by point.

Get the policy, not the phone summary. Every payer that denies for experimental use has a written coverage policy with a number, a date and a list of the evidence it relied on. That document is the thing you are appealing. Writing an appeal without reading it is guessing at what you have to overcome.

Do not bill the patient on reflex. Many participating contracts bar charging for a service the plan denied unless the patient signed something beforehand that specifically named the service and the likely denial. A general financial policy is usually not enough.

Why the payer sent it

The payer's own coverage policy lists the treatment as investigational.
The treatment is accepted for other conditions but not the one you billed, so the diagnosis is doing the work. See CO-11.
A newer procedure code was used and the payer has not written a policy for it yet.
The documentation did not show the criteria the policy asks for, so the reviewer defaulted to the denial.
The payer's policy is genuinely out of date against current practice, which happens and is the strongest kind of appeal.

Remark codes you will see with it

Remark code	What it adds
N115	The decision was based on a Local Coverage Determination. On Medicare this names the document you have to answer.
N130	Read the plan document. Its own page.
N56	The procedure code billed is not correct or valid for the service or date. Worth reading, because a coding fix is faster than an appeal.

Remark code

What it adds

N115

The decision was based on a Local Coverage Determination. On Medicare this names the document you have to answer.

N130

Read the plan document. Its own page.

N56

The procedure code billed is not correct or valid for the service or date. Worth reading, because a coding fix is faster than an appeal.

Read the letters in front of the 55

CO-55: Contractual obligation. Your write-off unless you overturn it.
PR-55: Patient responsibility. Check for a signed agreement naming this service before you bill.
PI-55: Payer initiated. The patient owes nothing.

What to do next

Ask which policy was applied, by number

Payers name their coverage policies. Get the number and the version date. If the policy is older than the evidence you are relying on, that is your opening sentence.
If they cannot name a policy: that is a strong position. A determination with no written basis is hard for the payer to defend on review.
Read what the policy asks for and check your chart against it

Most of these policies list criteria. The fastest appeals are the ones where the criteria were met and the documentation did not show it.
If the chart meets the criteria: send the chart with the criteria quoted next to the matching notes. Do not make the reviewer hunt.
Check the diagnosis you billed

A treatment can be standard for one condition and investigational for another. If the diagnosis on the claim was not the one the policy covers, the fix may be a corrected claim rather than an appeal.
Decide whether the amount justifies the work

An experimental appeal is real work: policy reading, literature, a letter. It is worth it on a large line and rarely worth it on a small one. Be honest about that before you start.

Codes that get mixed up with 55

Code	How it differs from 55
50	Not medically necessary. A judgment about this patient rather than about the treatment in general. See CO-50.
96	Not covered, with the reason in the remark. The general version. See CO-96.
204	Not in the benefits at all. Plan design rather than evidence. See PR-204.
272	Coverage or program guidelines not met. Often the newer code for the same argument. See CO-272.

The line between 55 and 50 is worth holding onto. A 50 says this patient did not need it. A 55 says nobody has proved it works yet. The first is answered with the chart and the second with the literature.

When not to appeal

The plan document names the treatment as an exclusion outright. That is plan design and code 204 territory, not an evidence argument.
The patient signed a specific agreement accepting the cost, and the amount is small.
The real problem is the diagnosis or the procedure code. Send a corrected claim.

Code 55 in dental

Dental plans use this less than medical plans do, but it appears on the newer end of dentistry: some implant protocols, some laser procedures, and materials or techniques a carrier has not written a policy for yet.

The predetermination is the real defence

For anything you suspect a carrier may call experimental, a predetermination before treatment answers the question while the patient can still make a decision. Getting a 55 after the work is done leaves you choosing between a write-off and a difficult conversation.

Questions people ask about CO-55

Is a CO-55 worth appealing?

More often than most coverage denials. An experimental determination rests on a written policy and a reading of the evidence, and both can be argued. Ask the payer to name the policy and its version date first, because that document is what your appeal has to answer.

What is the difference between 55 and 50?

Code 50 says this patient did not need the treatment, which is answered with the chart. Code 55 says the treatment is not yet proven for anyone, which is answered with the literature and the payer's own policy. They need completely different appeals.

Can I bill the patient for an experimental denial?

Only if the letters say PR and your contract allows it. Many participating agreements require the patient to have signed something beforehand that named the specific service and the likely denial, and a general financial policy usually does not count.

Related codes

Back to all denial codes

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