On your explanation of benefits or remittance, CO-136 means: the primary insurer's coverage rules weren't followed before billing this (secondary) payer; usually resolved by fixing the claim path through the primary plan first.
Is there an overturn-rate benchmark for this code?
Honest answer: no. State external-review programs are limited by statute to clinical-necessity disputes, so administrative and financial denial codes like this one never reach external review and no published overturn rate exists. Anyone quoting one for this code is inventing it.
What to do next
This is usually a paperwork or billing-mechanics issue: it is often fixed by the provider correcting and resubmitting rather than by a patient appeal — call the billing office first and ask them to fix and resubmit.
Always request an itemized bill and keep everything in writing.
Now turn the number into a dispute. Knowing your percentile is step one; the letter that cites it, the visit-code check, and the deadlines are the rest. Medigami builds those from your actual bill — medigami.com