Resolving the OA-23 Denial Code on Secondary and Tertiary Claims

:::info Quick answer The OA-23 denial code indicates the impact of prior payer adjudication. It appears on secondary claims when a payer reduces payment based on what the primary covered. Resolving an unexpected zero-dollar OA-23 requires fixing out-of-balance CAS segments in your EDI file so primary amounts map correctly. :::

You open an Electronic Remittance Advice (ERA) from a secondary payer, and there it sits: a zero-dollar payment attached to an OA-23 adjustment. Your first instinct might be to write off the balance. Don't pull the trigger on that adjustment just yet.

Coordination of Benefits (COB) is essentially a math test between three parties: the primary payer, the secondary payer, and your practice management software. If your claim fails the math test, you do not get paid. Claim Adjustment Reason Code (CARC) 23 isn't always a "denial" in the traditional sense—it operates as an accounting mechanism. The OA stands for Other Adjustment, and 23 officially means "The impact of prior payer(s) adjudication including payments and/or adjustments."

When you see the OA-23 denial code, the secondary payer is telling you, "We see the primary payer already handled this, and based on their math, we owe you nothing." Sometimes they are absolutely right. But frequently, your billing software just fed them garbled EDI data, forcing their adjudication system to zero out the claim to prevent an overpayment.

Here is how to read the remittance, fix the electronic looping, and push your secondary claims through for actual payment in 2026.

What Does the OA-23 Denial Code Actually Mean?

To beat this code, you have to understand the math happening behind the scenes of an 835 electronic remittance.

CARC 23 is an informational calculation code. It is used exclusively on secondary or tertiary claims. When a primary payer processes a claim, they generate an allowed amount, a paid amount, a contractual write-off (usually CO-45), and a patient responsibility amount (like PR-2 for coinsurance or PR-1 for deductible).

When that claim crosses over to the secondary payer, the secondary payer runs the gross billed charges against their own fee schedule. Then, they look at what the primary already paid. If the primary payer's payment exceeds the secondary payer's allowed amount, the secondary payer pays zero. They use OA-23 to account for the difference on the ERA so their books balance.

The Math Breakdown

Let's say you bill a total charge of $200.

• Primary Payer (Medicare): Allows $100. Pays $80. Leaves $20 as PR-2 (patient coinsurance).
• Secondary Payer (Commercial): Allows $90.

Because Medicare already paid $80, the secondary payer will only pay $10 (the difference up to their $90 allowable). But wait, what about the $20 the patient supposedly owed? The secondary payer absorbs that into their calculation. On your ERA, the secondary payer will show an OA-23 adjustment to explain why they aren't paying the full $20 you requested.

If the secondary allowable was only $75, they would pay $0, because the primary's $80 payment already covered it. That results in a strict OA-23 zero-dollar payment, and you must write off the remaining balance. But what if the math should have worked in your favor, and they still paid zero? That means your claim data was broken before it even reached them.

Why You Get Hit With OA-23 on Secondary Claims

If a secondary claim denies with OA-23 and leaves an unpaid patient responsibility balance that the secondary should have covered, you usually have a data failure. As we sit here in August 2026, we are seeing a massive uptick in OA-23 adjustments from Medicare Advantage plans acting as secondary to traditional Part B. This is largely because the 2026 Medicare Physician Fee Schedule updates created misaligned allowable rates between primary MACs and commercial secondary networks, causing automatic crossovers to fail out.

Here are the specific reasons this code triggers:

1. Out-of-Balance CAS Segments

When you drop a secondary claim via an 837 EDI file, the primary payer's adjudication data lives in Loop 2320 (Other Subscriber Information) and Loop 2430 (Line Pricing/Adjudication Information). The system has to transmit Claim Adjustment Segments (CAS) that exactly balance.

Billed Amount = Primary Paid Amount + CAS Adjustments.

If your billed amount is $150, and the primary paid $80, the CAS segments must account for the remaining $70 (usually a $50 CO-45 write-off and a $20 PR-2 coinsurance). If your practice management system pushes the $150 billed amount, reports the $80 paid, but drops the CAS segments entirely, the secondary payer's system rejects the math. It slaps an OA-23 on the claim and pays you zero because it cannot verify why the $70 is missing.

2. Missing the Primary EOB Entirely

Sometimes, a biller will simply submit a secondary claim as if it were a primary claim, leaving the COB fields entirely blank. The payer's scrubber catches the patient's coordination of benefits file, realizes they are secondary, sees no primary payment information attached, and throws an OA-23 (often paired with a PR-22 code asking for the primary EOB) to halt the payment.

3. Legitimate Maximum Allowable Reached

As outlined in the math example above, if the primary payer reimburses at a higher rate than the secondary payer's contracted rate, the secondary payer owes nothing. This is a valid OA-23, and your only action is to post the contractual adjustment.

Causes and Fixes at a Glance

Step-by-Step Resolution: Balancing and Appealing OA-23

When an OA-23 denial costs you actual revenue, you cannot just click "rebill" and hope the secondary payer suddenly understands the math. You have to spoon-feed them the correct data.

Step 1: Pull the Primary 835 or EOB

First, isolate the original payment. Pull the primary payer's ERA or paper EOB. Note the exact allowed amount, paid amount, and the specific patient responsibility codes (PR-1, PR-2, PR-3). You need these numbers to verify the secondary's math.

Step 2: Check Your Clearinghouse Output

Log into your clearinghouse portal and look at the raw 837 file that was sent to the secondary payer. Did the primary payment data actually attach? Look at Loop 2320. If you only see the gross billed amount and no CAS codes, your practice management software failed to map the COB data. You will need to manually enter the primary EOB details into the secondary claim screen before resending.

Step 3: Verify the Allowed Amounts

Compare the primary allowed amount to the secondary fee schedule. If you are billing a Medicaid plan as secondary to Medicare, remember that state Medicaid programs strictly enforce "lesser of" logic. If Medicare paid $80 and the Medicaid allowable is $60, Medicaid will pay zero. Do not waste time appealing a mathematically accurate OA-23 adjustment.

Step 4: Submit a Corrected Claim or Paper Drop

If the CAS segments were missing or unbalanced, correct the loops in your software and transmit a replacement claim (Frequency Code 7). If the secondary payer's EDI gateway is notoriously bad at reading Loop 2320—a persistent issue with certain regional Blue Cross plans in 2026—drop the claim to paper on a CMS-1500. Fill out Boxes 9a-9d, put the primary paid amount in Box 29, staple the primary EOB to the back, and mail it.

Appeal Timeline for COB Disputes

Stick to a rigid timeline when dealing with secondary payer math errors:

• Day 1: ERA Received. You spot the zero-dollar payment with CARC 23.
• Day 2-3: ERA Reconciliation. Pull the primary 835 and check the secondary 837 CAS segments in your clearinghouse.
• Day 5: Re-batching. If the CAS segments were broken, correct the loops and transmit a replacement electronic claim.
• Day 30: Paper Secondary Drop. If the replacement EDI claim fails again due to payer gateway errors, drop it to paper with the primary EOB attached.
• Day 45-60: Formal Appeal. If the secondary payer processed the math wrong (e.g., they ignored the PR-2 coinsurance entirely), file a formal written appeal with a clear spreadsheet breaking down the primary allowed vs. secondary allowed amounts.

Prevention: Cleaning Up Your COB Workflows for 2026

Secondary claim denials are almost always preventable. The key is stopping bad data before it leaves your office.

Set up hard stops in your clearinghouse scrubber for secondary claims. Any claim batched to a secondary payer must flag for review if the primary paid amount is missing or if the CAS segments do not equal the total billed amount minus the primary paid amount.

Next, audit your automatic crossover claims. Medicare Part B automatic crossovers are convenient, but supplemental plans frequently fail to update their coordination rules for current-year fee schedules. If you notice a high volume of OA-23 zero-pays coming back on automatic crossovers, turn off the crossover for that specific payer and batch the secondary claims out of your own system where you control the CAS segment math.

Finally, train your payment posters. An OA-23 is not a license to immediately adjust off a patient balance. Posters must verify whether the secondary payer actually covered the patient responsibility or if a bad EDI file simply tricked the payer into issuing a zero-dollar remittance.

Frequently asked questions

Can I bill the patient after receiving an OA-23 adjustment?

Only if the secondary payer's remittance advice explicitly leaves a Patient Responsibility (PR) balance. If the OA-23 adjustment absorbs the remaining balance without assigning a PR code, billing the patient violates your network contract.

Why did the secondary payer deny with OA-23 when the primary left a patient deductible?

This usually happens because the secondary payer's system did not read the PR-1 (deductible) code from your claim file. If your software dropped the CAS segments, the secondary payer assumes the primary paid in full and zeroes out the claim.

How do I fix a missing COB segment on an electronic claim?

You must ensure your practice management system is configured to map primary remittance data into Loop 2320 and Loop 2430 of the 837 file. If automatic mapping fails, you will need to manually key the primary allowed, paid, and adjustment amounts into the secondary claim screen.

Does an OA-23 adjustment mean my claim was flagged as a duplicate?

No. Duplicate claims are typically denied with CARC 18. OA-23 specifically refers to the mathematical impact of a prior payer's processing. However, if a claim crosses over automatically and you also bill it manually, the second version may hit various COB or duplicate snags.

What is the difference between CARC 22 and CARC 23?

CARC 22 indicates that another payer's explanation of benefits is required to process the claim (often asking for the primary EOB). CARC 23 indicates that the primary EOB was received, and the current payer's reimbursement has been mathematically adjusted based on what the primary already paid.