DENIAL CODE · CO-55

CO-55: Experimental / Investigational

CO-55 means the payer classifies the service, drug, or device as experimental or investigational under its medical policy, so it won't cover it — even when it's the appropriate care.

Official (X12/WPC): "Procedure/treatment/drug is deemed experimental/investigational by the payer."
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Why payers issue CO-55

  • The payer's medical policy labels the service investigational
  • Supporting evidence wasn't submitted with the claim
  • Coding suggested an unproven indication
  • The policy lags current standards of care

Is it recoverable? The hardest to win but highest-dollar — a well-evidenced appeal with peer-reviewed literature, FDA status, and guideline support overturns a real share.

How to appeal an Experimental / Investigational denial →

Common questions

What does CO-55 mean?

CO-55 means the payer classifies the service, drug, or device as experimental or investigational under its medical policy, so it won't cover it — even when it's the appropriate care. Procedure/treatment/drug is deemed experimental/investigational by the payer.

How do I appeal or fix a CO-55 denial?

The hardest to win but highest-dollar — a well-evidenced appeal with peer-reviewed literature, FDA status, and guideline support overturns a real share. Common causes: the payer's medical policy labels the service investigational; supporting evidence wasn't submitted with the claim; coding suggested an unproven indication; the policy lags current standards of care.

Is a CO-55 denial worth appealing?

The hardest to win but highest-dollar — a well-evidenced appeal with peer-reviewed literature, FDA status, and guideline support overturns a real share. You only pay on what's actually recovered, so there's no cost to working the ones that are winnable.

Other denial codes
CO-197 · Precertification / Authorization Absent →CO-50 · Not Medically Necessary →CO-16 · Missing or Invalid Information →CO-45 · Charge Exceeds Fee Schedule →CO-97 · Bundled into Another Service →CO-29 · Timely Filing Expired →CO-96 · Non-Covered Charge →OA-18 · Duplicate Claim →OA-22 · Coordination of Benefits →CO-11 · Diagnosis Inconsistent with Procedure →CO-151 · Too Many Services (Frequency/MUE) →CO-B7 · Provider Not Eligible →CO-27 · Coverage Terminated →CO-204 · Not Covered Under Current Plan →CO-109 · Not Covered by This Payer →CO-119 · Benefit Maximum Reached →CO-4 · Modifier Inconsistent or Missing →CO-15 · Authorization Number Missing or Invalid →OA-23 · Prior Payer Adjudication Impact →CO-24 · Charges Covered Under Capitation →CO-26 · Expenses Incurred Prior to Coverage →CO-54 · Assistant Surgeon Not Covered →CO-58 · Inappropriate Place of Service →CO-59 · Multiple/Concurrent Procedure Reduction →CO-107 · Related/Qualifying Claim Not Identified →CO-167 · Diagnosis Not Covered →CO-170 · Denied for This Provider Type →CO-181 · Procedure Code Invalid on Date of Service →CO-182 · Procedure Modifier Invalid on Date of Service →CO-183 · Referring Provider Not Eligible to Refer →CO-185 · Rendering Provider Not Eligible →CO-198 · Precertification / Authorization Exceeded →CO-226 · Requested Provider Information Not Provided →CO-231 · Mutually Exclusive Procedures →CO-234 · Procedure Not Paid Separately →CO-236 · Procedure/Modifier Combination Not Compatible →CO-242 · Out-of-Network / Non-Network Provider →CO-252 · Documentation Required to Adjudicate →CO-253 · Sequestration Reduction →CO-256 · Not Payable Per Managed-Care Contract →CO-288 · Referral Absent →CO-B15 · Qualifying Service Not Received →CO-B16 · New Patient Qualifications Not Met →CO-A1 · Claim/Service Denied (Remark Required) →

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