How to Appeal an Experimental or Investigational Treatment Denial (CO-55)
Insurer denied your treatment as 'experimental' or 'investigational' (CO-55)? Learn your rights, what this code really means, and how to build a winning appeal letter backed by clinical evidence.
What does CO-55 mean?
A CO-55 denial labels a treatment, drug, device, or procedure as 'experimental,' 'investigational,' or 'not proven effective.' Insurers use this classification to exclude coverage for treatments that fall outside their approved clinical coverage policies, even when those treatments are FDA-approved, guideline-recommended, or standard of care in the medical community.
Why insurers issue CO-55 denials
Insurance companies maintain internal technology assessment bodies that evaluate treatments using proprietary criteria. A treatment can be simultaneously FDA-approved, recommended in peer-reviewed clinical guidelines, and considered 'experimental' by a specific insurer. This classification is frequently used to deny coverage for newer biologics, gene therapies, cancer immunotherapy, and precision medicine.
Appeal strategy
Document that the treatment is FDA-approved for your diagnosis, cite NCCN Guidelines, AHA/ACC guidelines, or the relevant specialty society recommendations. Show that the insurer's own clinical policy acknowledges the treatment for patients with your specific clinical profile. If the drug is prescribed per its FDA-approved label indication, cite state statutes that prohibit classifying FDA-approved drugs as experimental.