Modifier 33 in Medical Billing: Preventive Services and Screening Colonoscopies

Quick Answers

What is modifier 33?
Modifier 33 is a CPT modifier that flags a service as preventive for commercial and private payers. It tells the payer that the primary purpose of the service was an evidence-based preventive service, so the patient’s copay, coinsurance, or deductible should be waived.

Do you use modifier 33 when a screening colonoscopy finds a polyp?
For a commercial payer, yes. When a screening colonoscopy converts to a therapeutic procedure such as a polyp removal, append modifier 33 to the therapeutic code to preserve the preventive benefit and keep the patient from getting a surprise bill.

Can you use modifier 33 with Medicare?
No. Medicare does not recognize modifier 33 for most services and may reject the claim. For a colorectal screening that becomes diagnostic or therapeutic, Medicare uses modifier PT instead.

What Is Modifier 33? Preventive Services Explained

Modifier 33 is a CPT modifier that identifies a preventive service. It tells a commercial or private payer that the primary purpose of the service was the delivery of an evidence-based preventive service, in line with recommendations such as a US Preventive Services Task Force A or B rating. When the payer sees modifier 33, it should process the claim without applying the patient’s copay, coinsurance, or deductible.

The modifier exists because many services do not have a separate preventive code. A given procedure code might be used for both a diagnostic workup and a preventive screening. Without a signal, the payer cannot tell which one it is looking at, and it may apply the patient’s regular cost-sharing. Appending modifier 33 makes the preventive intent clear, so the claim is paid under the preventive benefit instead.

That is also the simplest way to hold the modifier 33 description in mind: the primary purpose was prevention, and the modifier asks the commercial payer to treat it that way and waive the patient’s out-of-pocket cost.

One note on currency: the AMA made an editorial revision to the modifier 33 descriptor in the 2026 CPT code set. The clinical application has not changed, but it is worth confirming the exact 2026 descriptor language in the current CPT Professional Edition or with your Medicare Administrative Contractor.

When to Use Modifier 33

Reach for modifier 33 when the primary purpose of the service is prevention and the patient is covered by a commercial or private plan. It generally applies to services in these categories:

• Preventive services with a US Preventive Services Task Force A or B rating.
• Routine immunizations recommended by the Advisory Committee on Immunization Practices.
• Preventive care and screenings for children, as supported by Bright Futures and related guidelines.
• Additional preventive services for women supported under HRSA guidelines.

If a provider delivers more than one qualifying preventive service to the same non-Medicare patient on the same day, append modifier 33 to each preventive service reported. The deciding factor is always the primary purpose. If a service is being performed to monitor or treat a known condition, it is not preventive, and modifier 33 does not belong on it.

Modifier 33 and the Screening Colonoscopy

This is where modifier 33 matters most in GI billing. A colonoscopy scheduled as a preventive screening is covered with no patient cost-sharing under the ACA. The complication arises when the gastroenterologist finds something during the screening and acts on it, such as removing a polyp. At that point the procedure has converted from a pure screening into a therapeutic service.

If the claim simply reports the therapeutic colonoscopy code with no preventive signal, a commercial payer may process it as a diagnostic procedure and apply the patient’s deductible and coinsurance. The patient came in for a free screening and gets a bill. To preserve the preventive benefit, append modifier 33 to the therapeutic colonoscopy code for commercial payers, communicating that the encounter began as a screening.

Medicare is different. Medicare does not recognize modifier 33 for this scenario. Instead, Medicare uses modifier PT, which is appended to the diagnostic or therapeutic code to show that a colorectal cancer screening became a diagnostic or therapeutic service during the visit. So the same clinical event is coded two different ways depending on the payer, and choosing the wrong one is a common source of denials and patient billing complaints.

Modifier 33 vs Modifier PT: Commercial vs Medicare

Because both modifiers show up in colorectal screening work, it helps to see them side by side. The short version: modifier 33 is for commercial preventive scenarios, and modifier PT is for Medicare colorectal screenings that convert to diagnostic or therapeutic.

Factor	Modifier 33	Modifier PT
Type	CPT Level I modifier.	HCPCS Level II modifier.
Payer	Commercial and private plans.	Medicare.
What it signals	The service is an ACA preventive service, so patient cost-sharing should be waived.	A colorectal cancer screening that became a diagnostic or therapeutic procedure during the visit.
GI use	A screening colonoscopy on a commercial plan that converts to therapeutic, such as a polyp removal.	The same conversion when the patient is covered by Medicare.
Rule of thumb	Use 33 for commercial preventive scenarios.	Reserve PT for Medicare colorectal screening conversions.

When Not to Use Modifier 33

Modifier 33 is one of the more over-applied modifiers, so knowing where it does not belong is just as important:

• On services whose code already identifies them as preventive or screening, such as screening mammography or HCPCS G codes. The modifier is redundant and can cause claim errors.
• On services performed to diagnose, monitor, or treat a known illness or injury. If the primary purpose is not prevention, modifier 33 does not apply.
• On Medicare claims. Medicare does not recognize modifier 33 for most services and may reject the claim as having incomplete or missing information. Use the appropriate HCPCS code, or modifier PT for a converted colorectal screening.

It also helps to remember that appending modifier 33 does not by itself guarantee the claim will be processed as preventive. The plan still adjudicates benefits based on the patient’s coverage, so eligibility work up front is essential.

Documentation and Eligibility for Modifier 33

Clean modifier 33 claims start before the visit and depend on a clear record:

• Confirm the service qualifies as a preventive service under the ACA categories above.
• Verify the patient’s benefits ahead of the visit and confirm how the plan handles cost-sharing for that preventive service.
• Capture the clinical intent in the note, so the record shows the primary purpose was prevention.
• When the benefit is unclear, get written guidance from the payer and keep it with the visit documentation.

Common Modifier 33 Mistakes That Trigger GI Denials

Most modifier 33 problems come from a handful of recurring errors:

• Submitting modifier 33 to Medicare, which does not accept it for most services.
• Appending modifier 33 to a code that is already designated as preventive.
• Using modifier 33 on a service whose purpose was diagnostic or therapeutic from the start.
• Leaving modifier 33 off a converted screening colonoscopy on a commercial plan, which hands the patient an unexpected bill.
• Skipping eligibility verification and assuming every plan applies the preventive benefit the same way.

Frequently Asked Questions

Key Takeaways

• Modifier 33 flags a service as preventive on a commercial plan, asking the payer to waive the patient’s cost-sharing.
• The primary purpose of the service must be prevention for modifier 33 to apply.
• For a screening colonoscopy that converts to therapeutic, append modifier 33 for commercial payers and use modifier PT for Medicare.
• Do not use modifier 33 on Medicare claims, on codes already designated as preventive, or on diagnostic and treatment services.
• Eligibility verification and clear documentation of preventive intent are what keep these claims clean.