What Is a Contractual Adjustment in Medical Billing?

A contractual adjustment in medical billing is the portion of a provider’s billed charge that must be written off based on the payer’s contracted or allowed amount. In simple terms, it is the difference between what the provider charged and what the payer recognizes as payable under the applicable contract or fee schedule.

For example, if a provider bills $200 for a service but the payer allows only $140, the $60 difference is generally posted as a contractual adjustment. On an ERA or EOB, these adjustments are commonly reported under the CO (Contractual Obligation) group code. A common example is CO-45, which is used when the billed charge exceeds the applicable fee schedule, maximum allowable amount, or contracted amount.

The important distinction is that a contractual adjustment reduces the provider’s charge. It should not be confused with patient responsibility amounts such as deductibles, copays, or coinsurance, which are typically reported separately under the PR group code.

What Does Contractual Adjustment Mean?

A contractual adjustment is the portion of a billed charge that is reduced because the payer allows a lower amount under the applicable contract or fee schedule.

For example, if a provider bills $200 and the payer allows $140, the remaining $60 may be posted as a contractual adjustment.

The key amounts are:

• Billed charge: The amount submitted on the claim
• Allowed amount: The amount recognized by the payer
• Contractual adjustment: The difference written off under the payer agreement
• Patient responsibility: Any deductible, copay, or coinsurance assigned separately to the patient

On an ERA or EOB, contractual adjustments are commonly associated with the CO (Contractual Obligation) group code. However, the group code and reason code should always be read together because patient liability depends on how the payer adjudicated that specific claim.

How Is a Contractual Adjustment Calculated?

The basic formula is:

Contractual adjustment = Billed charge − Allowed amount

For example:

• Billed charge: $200
• Allowed amount: $140
• Contractual adjustment: $60

The $60 difference is the amount reduced from the original charge based on the payer’s allowed amount.

After adjudication, the allowed amount is generally divided between the payer payment and any patient responsibility, such as a deductible, copay, or coinsurance. The contractual adjustment is separate from those patient responsibility amounts.

What Does CO-45 Mean?

CO-45 means the payer reduced the charge because it exceeded the applicable fee schedule, maximum allowable amount, or contracted or legislated fee arrangement.

When CARC 45 is reported with the CO (Contractual Obligation) group code, the reduction is generally treated as the provider’s contractual responsibility rather than patient responsibility.

For example:

• Billed charge: $200
• Allowed amount: $140
• CO-45 adjustment: $60
• Payer payment: $110
• Patient responsibility: $30

In this case, the $60 is posted as the contractual adjustment, while the remaining $140 allowed amount is split between the payer and the patient.

Contractual Adjustment vs Patient Responsibility

Contractual adjustments and patient responsibility can both appear on an ERA or EOB, but they should not be posted the same way. The adjustment group code helps show who is generally responsible for the amount.

The most important distinction for contractual adjustments is CO vs PR. CO usually reflects an amount assigned to the provider, while PR identifies an amount that may be the patient’s responsibility. The group code should always be reviewed together with the CARC and any RARC before the balance is posted.

How to Post a Contractual Adjustment

Posting a contractual adjustment is mainly about making sure the payer’s payment, the patient’s responsibility, and the write-off are all going to the right place.

1. Match the ERA or EOB to the claim

Start by confirming the patient, date of service, procedure code, and service line. Make sure you are posting the adjustment to the correct claim.

2. Check the adjustment codes

Review the group code, CARC, and any RARC on the remit. These codes explain why the payer reduced the amount and who is responsible for the adjustment.

3. Confirm what the payer paid and what the patient owes

Separate the payer payment from any deductible, copay, or coinsurance assigned to the patient.

4. Confirm the contractual adjustment

Compare the billed charge with the payer’s allowed amount. The difference may be the contractual write-off, depending on how the claim was adjudicated.

5. Post each amount to the right place

Post the contractual amount to the appropriate adjustment code in your billing system. Patient responsibility should stay separate and should only be assigned when the remittance shows it as the patient’s responsibility.

6. Check the final balance

Before closing the claim, make sure the payment, patient responsibility, and adjustments all reconcile. If the allowed amount does not match what you expected under the payer contract, review it before writing the balance off.

Contractual Adjustment Posting Mistakes to Watch For

Contractual adjustments need to be posted carefully because even small mistakes can affect patient balances, A/R, and payment reporting.

• Posting contractual amounts to the patient balance instead of the appropriate adjustment account
• Using an outdated payer fee schedule and calculating the expected allowed amount incorrectly
• Writing off a balance without confirming that the payer’s allowed amount matches the contract
• Treating every payer reduction as a contractual adjustment without checking the group and reason codes
• Posting an adjustment without reviewing whether the payer may have underpaid the claim
• Mixing contractual adjustments with bad debt, charity care, or patient discounts, which can make financial reports inaccurate

When Should a Contractual Adjustment Be Reviewed?

Not every contractual adjustment needs manual follow-up, but some should be checked before the balance is written off. Review the adjustment when:

• The allowed amount does not match the payer contract or fee schedule
• The adjustment is much higher than expected for that service
• The payer appears to have used the wrong fee schedule or effective date
• Patient responsibility looks incorrectly assigned on the ERA or EOB
• The same adjustment variance keeps appearing across multiple claims
• The payment appears lower than the contracted amount

Recurring differences can point to an outdated fee schedule, payer pricing issue, or incorrect posting setup rather than a normal contractual write-off.

Conclusion

Contractual adjustments reduce a billed charge to the amount allowed under the payer contract or applicable fee schedule. The key is to post the adjustment correctly, keep patient responsibility separate, and review any unexpected variance before the claim is closed.

Accurate posting also gives your billing team a clearer view of what was actually allowed, what the payer paid, what the patient owes, and whether the reimbursement matches the expected contract rate. That makes it easier to spot underpayments, prevent incorrect patient balances, and keep A/R reporting accurate.

If your practice is handling a high volume of ERAs and EOBs, our payment posting services can help review remits, post payments and adjustments correctly, and reconcile payer payments against the claim balance.