Every EOR carries CAS (Claim Adjustment Segment) codes explaining why the payer paid less than you billed. Reading them correctly is the core skill of California workers'-comp billing — the code tells you whether to accept the adjustment, fix and rebill, or fight it with a Second Review. The codes come in groups: CO (contractual obligation, the provider eats it), PR (patient responsibility), and OA (other adjustment). Here's how to read the ones that matter for medical-legal, and where Mindbill decodes them for you.
CO-45 (charge exceeds fee schedule) means the payer applied a fee-schedule cap — for med-legal, that should be the California Medical-Legal Fee Schedule under CCR §9795. Action: if the reduction matches the MLFS allowance, accept it; if it cuts below the schedule, file a Second Review. CO-97 (procedure bundled into another) claims one code is already included in another that was paid. On medical-legal codes this is almost always wrong — the ML codes and their modifiers are distinct, billable line items — so file a Second Review, with modifier 25/59/XS where a distinct-service argument applies.
CO-50 (non-covered / not medically necessary) is the payer refusing the service on medical grounds. Action: pull the UR/RFA authorization and file a Second Review on procedural grounds; if the denial is genuinely medical rather than procedural, it heads toward Independent Medical Review (IMR/IBMR), not IBR. CO-16 (missing information) means the bill lacked a required attestation or supporting document — most often the LC §4062.3 sender declaration on an MLPRR line. CO-16 is NOT a Second Review trigger: you fix the missing piece and rebill, no SR required.
PR-242 is dressed up as patient responsibility but is really the payer applying a PPO/MPN network discount. On a medical-legal bill this is flatly illegal: CCR §9789.30(d) bars PPO/MPN reductions on MLFS reimbursement. This is the most lucrative dispute in the book — file a Second Review with a rebuttal citing §9789.30(d) and attach the controlling Maximus precedent (IBR-2019-04482) that overturned the identical reduction. It's settled law, which makes these disputes highly winnable, and Mindbill estimates recovery at the historical 62% Second Review collection rate.
OA-100 (bill not on file) means the payer received the file but can't locate the bill — usually a routing or matching problem, not a substantive dispute. Action: resubmit with the original tracking ID; it's frequently resolved by clearinghouse re-routing, which is one reason Mindbill's multi-clearinghouse routing (Carisk, Jopari, Data Dimensions) matters. A persistent OA-100 with no EOR can also become an audit-complaint basis once the bill passes 60 days.
You rarely need to read raw X12 or download the PDF EOR. Every bill detail page has an EOR Summary panel that decodes the CAS codes inline, line by line, against the MLFS allowance. The Recovery Insight card reads the CAS code(s) present and auto-suggests the right playbook — pre-selecting the §9789.30(d) rebuttal for a PR-242, the distinct-service argument for a CO-97, or the fix-and-rebill path for a CO-16 — so the adjustment code on the EOR turns directly into the correct next action.
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