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CO-B4 Denial Code: Late Filing Penalty

CO-B4 means the payer paid the claim but applied a late filing penalty, reducing payment because the claim arrived after a deadline set by the contract or program. Unlike CO-29, the claim wasn't fully denied; part of the payment was withheld as a penalty.

Quick facts

Code
CO-B4 (CARC B4)
Status
Active In use since January 1, 1995.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The standard group. The penalty is the provider's responsibility and can't be billed to the patient.
Official description
Late filing penalty.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-B4 means

CARC B4 says late filing penalty. The payer received the claim after a deadline in its contract or program rules, but instead of denying it completely, it paid a reduced amount. The reduction is reported as B4.

Penalty structures vary. Some payers deny outright once their filing limit passes, which produces CO-29. Others use a tiered approach: full payment if the claim arrives by an early deadline, reduced payment for claims received in a later window, and denial after the final limit. B4 belongs to that middle tier. Because the rules come from contracts and program policies, the size of the penalty and the deadlines are specific to each payer.

Example: a contract pays in full for claims received within a set number of days after service, applies a percentage reduction for claims received later but within the final limit, and denies claims after that. A claim submitted in the penalty window is paid with a B4 reduction.

Common causes

  • Charges held for missing documentation, unsigned notes, or coding review.
  • Rejected claims not corrected quickly, so the accepted resubmission lands late.
  • Credentialing delays before claims could be released.
  • Secondary claims sent after a slow primary payment.
  • Deadlines misunderstood, with staff working to the final limit instead of the penalty-free window.

How to fix it

  1. Confirm the dates: date of service, date first submitted, and date the payer received the claim.
  2. Compare to the contract’s penalty terms to confirm the penalty applies and the amount is right.
  3. Appeal with proof of timely filing if an earlier submission was accepted by the payer.
  4. Request an exception if the delay was caused by the payer, retroactive eligibility, or another allowed reason.
  5. Write off the penalty if it’s valid. Don’t bill the patient.

How to prevent it

  • Track penalty windows, not just final filing limits, for each payer.
  • Submit claims within days of the service.
  • Work rejections daily. See claim rejection vs. denial.
  • Save proof of submission for every claim. See timely filing denials.
  • Monitor penalties by payer. An ERA Analyzer can total B4 reductions so the cost of late billing is visible.

Remark codes that may appear with CO-B4

  • N719 (Penalty applied based on plan requirements not being met.): A penalty was applied because plan requirements were not met.
  • N381 (Alert: Consult our contractual agreement for restrictions/billing/payment information related to these charges.): Points to the contract terms that define the penalty.
  • CO-29 (The time limit for filing has expired.): Timely filing limit expired, a full denial rather than a reduction.
  • CO-210 (Payment adjusted because pre-certification/authorization not received in a timely fashion): Authorization not received in time, a different timing penalty.
  • CO-237 (Legislated/Regulatory Penalty.): Legislated or regulatory penalty.
  • CO-45 (Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement.): Standard contractual reduction, which may appear on the same line.

CO-B4 FAQ

How is B4 different from CO-29?

CO-29 means the claim was denied because the filing limit expired. B4 means the claim was paid, but at a reduced amount, because it arrived after a deadline that triggers a penalty rather than a denial.

Who applies late filing penalties?

They come from payer contracts, program rules, or state regulations. Some payers set a penalty window between an early deadline and the final filing limit. Check your contract or provider manual.

Can I appeal a B4 penalty?

Yes, if you can prove the claim was filed on time, such as a clearinghouse acceptance report or payer acknowledgment, or if the delay qualifies for an exception under the contract.

Can I bill the patient for the penalty?

No. A late filing penalty is the provider's responsibility under CO.