Keeping up with coding rules is hard for private practices that provide colorectal screening. Two codes that appear often in day-to-day work are modifier PT and modifier 33. Understanding how they apply helps teams submit clean claims and avoid preventable denials. 

In everyday medical billing across healthcare, these CPT modifiers determine how a screening is processed when it stays preventive or becomes diagnostic. Getting them right protects the revenue cycle, supports accurate reimbursements, and streamlines claims processing. 

This overview focuses on Medicare and commercial payer differences so staff can code with confidence.

Modifiers either add detail about a service or influence payment under payer fee schedules. Two bodies define them for US coding: the American Medical Association (AMA) and the Centers for Medicaid Services (CMS).

In short, modifier 33 is a CPT Level I modifier, and the PT modifier is a HCPCS Level II modifier. The AMA's preventive services coding guides explain how preventive services are identified for cost-sharing rules.

The PT modifier is a Level II code in the HCPCS system. It is used only on Medicare claims to show that a screening colonoscopy began as a preventive service under screening services for colorectal cancer screening and then changed to a diagnostic or therapeutic procedure on the date of service. 

In that situation, append PT to the diagnostic or therapeutic procedure code, not to the screening code. CMS outlines this workflow in its coverage article, and MACs such as WPS provide a concise PT fact sheet. 

Medicare waives cost-sharing for eligible Medicare beneficiaries when a Medicare patient presents for a covered screening. These services are typically reported to Medicare using HCPCS codes such as G0105 or G0121 when the exam remains preventive. 

A patient who schedules a screening expects full coverage, so using PT correctly helps the payer apply preventive policy even if a polyp is removed or another diagnostic step occurs.

In short, PT signals "started as screening, converted during the visit," which supports accurate adjudication without unexpected patient cost share.

If the endoscopist finds a polyp and removes it as a therapeutic procedure or performs another therapeutic service, report the performed procedure and append modifier PT to that CPT or HCPCS code on the Medicare claims. 

PT tells the payer the visit began as a screening, so the preventive policy should apply. Medicare has implemented a phased change to patient coinsurance when additional procedures occur during a screening. 

Use diagnosis coding that supports the performed procedure first, followed by the screening diagnosis. Correct use protects expected reimbursements and reduces avoidable bills for the Medicare patient.

For follow-on colonoscopy claims tied to prior positive tests, Medicare requires a new modifier (KX) for CRC follow-on colonoscopy claims, which practices should apply when directed by payer policy.

Report the condition that supports the procedure as the primary diagnosis code, then list the screening test diagnosis second. Use ICD-10-CM guidelines for sequencing and specificity, and confirm that the codes you choose map to the service performed. Many practices reference ICD-10 descriptions to ensure the clinical indication aligns with the documentation and the procedure reported.

Modifier 33 is a Level I current procedural terminology indicator created by the AMA to identify a preventive service that meets USPSTF grade A or B recommendations. When appended, it signals the payer to apply first-dollar coverage under ACA-compliant commercial plans, so eligible patients are not billed cost share. 

The AMA's preventive services coding guides explain how this flag supports benefit design and coverage determinations for services such as colorectal cancer screening, vaccinations, and other preventive and screening services. 

It helps to separate roles by program. Commercial insurers use modifier 33 to recognize qualified preventive encounters. Medicare, by contrast, uses the PT modifier when a screening colonoscopy begins as preventive and converts to diagnostic or therapeutic during the same visit. That distinction keeps claims clear and aligns payment policy with clinical intent.

Benefit verification still matters. Practices that confirm eligibility and document the rationale typically see cleaner adjudication, which decreases the likelihood of a denial. Used correctly, modifier 33 supports accurate processing and avoids unexpected patient balances.

The USPSTF maintains an updated list of A and B recommendations across many screening services. For colorectal cancer, the screening recommendation advises routine screening for adults ages 45 — 75 using options such as screening colonoscopy or sigmoidoscopy alongside stool-based tests. 

Practices should confirm the method and interval that best fit the patient's risk profile and benefits. Correctly identifying the service as preventive helps align cost sharing with plan rules and supports clean claim submission.

Many payers auto-adjudicate preventive benefits, but missing modifiers can still result in patient cost sharing or claim denials. Plan rules and coverage determinations drive claims processing, including items like:

Best practice is to append the correct modifier before submission. If you omit it, submit a corrected claim or file an appeal with documentation showing the visit met preventive criteria. Clear notes and eligibility checks help avoid rework and unexpected bills.

Do not append modifier 33 when PT already explains the screening-to-diagnostic conversion on Medicare claims. For Medicare, PT alone is sufficient when a screening colonoscopy is converted during an outpatient visit, and it should be placed on the performed procedure. 

Guidance in the CMS coverage article confirms PT usage on the diagnostic or therapeutic code. Reserve CPT modifiers for situations that require them under the payer's rules, and follow your eligibility checks and documentation to keep adjudication straightforward.

Follow each payer's policy and document the plan's rules in the chart. As a rule of thumb, use modifier 33 for commercial preventive service scenarios, and reserve the PT modifier for Medicare. Verify the patient's insurance benefits before the visit and confirm how the plan handles cost-sharing in an outpatient setting. 

Many plans apply first-dollar coverage to qualifying preventive encounters, while others still assess a co-pay or coinsurance based on their coverage determinations. Capture the clinical intent in the note and align it with the plan of care and eligibility results. 

When the benefit is unclear, call the payer for written guidance and keep that confirmation with the visit documentation so adjudication matches what the benefit team confirmed.

Use a simple decision path so the code matches clinical intent and payer policy. Start by confirming whether the visit was preventive at scheduling and how the claim should read after the scope findings.

AMA and CMS coordinate updates to ensure that CPT modifiers and HCPCS rules align and reduce overlap.

Getting modifier 33 and the PT modifier right keeps claims accurate, reduces denials, and protects patients from surprise bills. A clear workflow from scheduling to documentation helps private practices align clinical intent with payer rules and cleaner reimbursements. 

If you want fewer errors and simpler coding, Tebra's billing software supports documentation, eligibility checks, and claim prep so teams can focus on care instead of paperwork. Book a free, personalized demo today.