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CO-B21 Denial Code (Deactivated): Other Physician Share

CO-B21 meant the charges were reduced because part of the service or care was furnished by another physician. X12 deactivated it without naming a successor; CARC B20 now covers services partially or fully furnished by another provider.

X12 deactivated CARCB21 on October 16, 2003. Payers should no longer use it on new remittances, but it can still appear on older ERAs, corrected claims, and appeals.

Quick facts

Code
CO-B21 (CARC B21)
Status
Deactivated StoppedOctober 16, 2003 (in use since January 1, 1995).
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The reduction was a provider adjustment, not a patient balance.
Official description
The charges were reduced because the service/care was partially furnished by another physician.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-B21 meant

CARC B21 explained a reduction when care was shared. Another physician had furnished part of the service, so the payer reduced the charges for the physician whose claim carried this code. It commonly involved packaged services where pre-operative, operative, and post-operative care can be split between physicians.

What replaced it

X12 did not name a replacement, but the still-active CO-B20 covers procedures or services that were partially or fully furnished by another provider, which includes the situation B21 described. Related codes include CO-54 when multiple physicians or assistants are not covered and CO-97 when the service is included in another payment.

If you still see CO-B21

This is a long-retired code. On a similar reduction today, check whether another provider billed part of the same service and whether your claim reported the portion you actually performed, using the correct modifiers. If the split was reported wrongly, submit a corrected claim; otherwise post the reduction as a contractual adjustment.

  • CO-B20 (Procedure/service was partially or fully furnished by another provider.): Procedure or service was partially or fully furnished by another provider.
  • CO-54 (Multiple physicians/assistants are not covered in this case.): Multiple physicians or assistants are not covered in this case.
  • CO-97 (The benefit for this service is included in the payment/allowance for another service/procedure that has already been adjudicated.): The benefit is included in the payment for another service already adjudicated.

CO-B21 FAQ

When does split care cause a reduction?

Common examples are global surgical packages where one physician performs the surgery and another handles post-operative care, or services where another provider billed a component. Each physician should bill only the portion they performed.

How do I prevent this reduction?

Coordinate with the other provider and use the appropriate modifiers to indicate which portion each of you performed, following the payer's rules.