As stool-based colorectal cancer (CRC) screening becomes more prevalent, gastroenterologists are seeing a corresponding rise in referrals for colonoscopies following a positive result. This trend underscores the importance of understanding the nuanced billing rules for these procedures. A colonoscopy performed after a positive stool-based test is a continuation of the screening process, not a standard diagnostic exam. Navigating the specific coding and billing requirements is essential for accurate claim submission, preventing denials, and protecting patients from facing inappropriate costs.
Medicare Patients
For Medicare, bill the screening colonoscopy after a positive stool-based test using the appropriate screening G‑code:
- G0105 – Colorectal cancer screening; colonoscopy on individual at high risk
- G0121 – Colorectal cancer screening; colonoscopy on individual not meeting criteria for high risk
If pathology is found during the colonoscopy (polyps, cancer) and removed/biopsied, keep the screening G‑code framework for Medicare but apply the correct modifiers (see below); do not automatically convert to a primary diagnostic colonoscopy CPT code for Medicare FFS if the service began as screening.
Appropriate Modifiers for Medicare: KX and PT
- Modifier KX – Follow‑up colonoscopy after positive stool‑based test
Medicare requires modifier KX on the screening G code to identify a colonoscopy performed after a positive non invasive stool based test.
If KX is omitted, Medicare may return the claim as “unprocessable” or adjudicate it as diagnostic with cost sharing.
Example (Medicare, average-risk beneficiary): Bill G0121-KX
- Modifier PT – Screening colonoscopy that turns into diagnostic/therapeutic
Use modifier PT when a screening colonoscopy (including one after a positive stool test) results in diagnostic or therapeutic intervention (e.g., biopsy, polypectomy) on the same date of service.
Append PT to the diagnostic/therapeutic colonoscopy code that describes the procedure performed, not to G0121/G0105 themselves.
Medicare uses PT to recognize that the procedure started as a preventive screening and to apply the correct cost sharing waiver rules.
Example (Medicare, average risk, polypectomy performed):
Bill G0121 KX (screening colonoscopy) and 45385 PT (colonoscopy with snare polypectomy, started as screening)
Do not add modifier 33 on Medicare claims when PT already indicates screening to diagnostic conversion; PT alone is sufficient for Medicare.
Commercial and Other Payers
For commercial payers, many plans prefer using the appropriate diagnostic CPT colonoscopy code (e.g., 45378–45385 series) plus a modifier to indicate preventive intent; check payer‑specific policies.
Modifier 33 (preventive service):
- Used with commercial plans to signal a preventive service subject to zero cost‑sharing under USPSTF A/B recommendations (e.g., colorectal cancer screening).
- Often appended to the CPT colonoscopy code (45378–45385 series) when the colonoscopy is being done after a positive stool‑based test and treated as a preventive follow‑up.
Modifier PT (Medicare only unless payer says otherwise):
- Some Medicare Advantage and select commercial payers mirror CMS and accept PT for screening colonoscopy that becomes diagnostic; verify policies.
Example (commercial plan following USPSTF guidance):
- 45380‑33 (colonoscopy with biopsy after positive FIT, treated as preventive)
Relevant ICD-10 Codes
Diagnosis coding must support both the reason for the colonoscopy and its screening intent:
- Primary diagnosis – Positive stool‑based test or abnormal finding:
- Example: R19.5 (other fecal abnormalities) or payer‑specific code for “positive fecal occult blood test” / “positive stool DNA test,” as allowed.
- Secondary diagnosis – Screening intent:
- Z12.11 – Encounter for screening for malignant neoplasm of colon.
- If polyps or cancer are found and treated, add the appropriate neoplasm/polyp codes (e.g., D12.x, C18.x) as additional diagnoses per payer rules.
Documentation requirements:
The medical record should clearly document:
- That the colonoscopy is being performed as a follow‑up screening after a positive non‑invasive stool‑based colorectal cancer screening test.
- Type and date of the prior stool‑based test (e.g., Cologuard, FIT), and its positive result.
- Whether the colonoscopy was scheduled as screening and whether it became diagnostic/therapeutic (biopsy, polypectomy, control of bleeding, etc.).
- Relevant risk status (average vs high risk) to support selection of G0121 vs G0105.
Complete documentation supporting the use of KX and PT, along with correctly coding colonoscopy post positive stool-based screening tests can streamline reimbursement, reduce payer denials, and protect patients from inappropriate costs.
Sources
- https://www.cms.gov/Regulations-and-Guidance/Guidance/Transmittals/2017Downloads/R3763CP.pdf
- https://www.cms.gov/files/document/r13248cp.pdf
- https://www.cms.gov/medicare-coverage-database/view/article.aspx?articleId=56394
- https://med.noridianmedicare.com/web/jeb/topics/preventive-services/colorectal-cancer-screening
- https://learn.asge.org/AssetListing/2026-Gastroenterology-Reimbursement-and-Coding-Update-Virtual-November-2025-4408/Screening-versus-Surveillance-Colonoscopy-37712
Author
Dr. Edward Sun is Associate Medical Director at Peconic Bay Medical Center. Dr. Sun serves as Chair of the ASGE Reimbursement Committee.