Medicare’s screening colonoscopy is one of the few tests in American health care that is genuinely free at the point of care — until the doctor finds something. The moment a physician removes a polyp or other tissue during what began as a routine screening, the same procedure that cost nothing a moment earlier becomes a billed medical service, and the patient owes 15 percent coinsurance on the provider’s charge. The switch happens mid-procedure, with no separate consent step, which is why patients frequently leave the appointment surprised by a bill for a test they were told was covered in full.
How a Free Screening Becomes a Billed Procedure Mid-Exam
Medicare Part B covers screening colonoscopies at no cost when the provider accepts assignment, with no minimum age requirement and no deductible applied. The screening exists purely to check for precancerous polyps, cancer and other disease inside the colon and rectum, and as long as nothing is found and removed, the visit stays entirely within that no-cost preventive category.
The reclassification happens the instant a polyp or other tissue is removed during the same visit. Medicare’s own coverage page states that a patient who has a polyp removed during the colonoscopy pays 15 percent of the Medicare-approved amount for the provider’s services, and in a hospital outpatient setting or ambulatory surgical center, an additional 15 percent facility coinsurance applies as well. The Part B deductible is waived even in that scenario, but the coinsurance is not.
Free retirement updates: One number can cost or save hundreds a month in retirement. The free Retirement Shield newsletter surfaces the ones worth knowing. Sign up free.
The Coinsurance Number, and Why It’s Falling
That 15 percent figure is not permanent. Under a phase-down created by the Consolidated Appropriations Act of 2021, CMS guidance filed with Medicare’s billing contractors sets coinsurance at 15 percent for 2023 through 2026, dropping to 10 percent for 2027 through 2029, and reaching zero starting in 2030. The reduced rate applies regardless of which billing code is used to report the procedure once a screening becomes diagnostic or therapeutic.
The multiyear glide path exists because lawmakers treated the mid-procedure bill as an unintended consequence of how colorectal cancer screening was originally written into Medicare law, rather than a deliberate cost-sharing decision. Until the schedule reaches zero, though, the coinsurance is real money owed on a test patients were told, correctly, would be free — a gap patient advocates have nicknamed the “colonoscopy loophole,” because the point at which a screening flips to billable is determined entirely by what the doctor finds, not by anything the patient chooses.
Someone scheduling a colonoscopy in the years before 2030 has no way to know in advance whether the visit will end up in the free category or the 15-percent category, since that depends on what the physician discovers once the procedure is underway. The only real planning tool is knowing the number ahead of time, so a coinsurance bill after a polyp removal is not mistaken for a billing error.
The mechanism traces back to Section 122 of Division CC of the Consolidated Appropriations Act of 2021, which amended Section 1833(a) of the Social Security Act to create the special reduced-coinsurance rule in the first place. CMS’s guidance to billing contractors specifies that the reduced rate applies whenever a screening code such as G0104, G0105 or G0121 is submitted with a modifier showing the screening became a diagnostic or therapeutic service, regardless of which additional procedure code is billed alongside it — a technical detail that determines how the claim is processed but does not change what the patient ultimately owes.
How Often the Free Screening Is Covered
Frequency limits determine how often the no-cost screening applies in the first place. Medicare covers a screening colonoscopy once every 120 months for someone not at high risk for colorectal cancer, or 48 months after a previous flexible sigmoidoscopy, and once every 24 months for someone at high risk. A positive result from a covered stool-based or blood-based biomarker screening test also triggers a covered follow-up colonoscopy under the same screening rules. The CDC’s own screening guidance reflects the same 10-year interval for average-risk adults and confirms that most people should begin screening at age 45, the starting point the Medicare frequency clock is built around.
Those intervals matter because a colonoscopy scheduled outside the covered frequency window is treated differently from the start, regardless of what the doctor finds. A beneficiary who is unsure whether enough time has passed since a prior colonoscopy, or whether a family history of colorectal cancer qualifies for the shorter 24-month high-risk interval, can confirm eligibility before scheduling rather than discovering after the fact that the visit fell outside the no-cost screening window entirely.
Understanding both layers — the polyp-removal coinsurance and the screening-frequency rules — is what separates a bill a patient can anticipate from one that arrives as a surprise. The test itself remains one of Medicare’s most heavily promoted preventive benefits precisely because it is free when nothing is found, but the fine print about what happens when something is found is where the actual cost exposure lives.
This article was researched and drafted with the assistance of artificial intelligence.
More Financial Reading
