How do you bill G0105 and G0121 for Medicare screening colonoscopies?

G0105 or G0121: which code does this patient get?

Risk decides the code, and nothing else does. G0105 is the screening colonoscopy for a beneficiary at high risk for colorectal cancer; G0121 is the one for everyone who does not meet those criteria. Both are Medicare-only HCPCS Level II codes, and both stay screening codes regardless of what the scope finds.

The choice is binary because Medicare recognises no third category. There is no surveillance code. A patient who had an adenomatous polyp removed four years ago is high risk under the regulation and is billed G0105 on the next screening, not a diagnostic 45378. That one point settles most of the arguments a GI practice has about these codes.

The six conditions that make a Medicare patient high risk

42 CFR 410.37(a)(3) lists them, and the list is closed. A close relative, meaning a sibling, parent or child, who had colorectal cancer or an adenomatous polyp. A family history of familial adenomatous polyposis. A family history of hereditary nonpolyposis colorectal cancer. A personal history of adenomatous polyps. A personal history of colorectal cancer. Inflammatory bowel disease, including Crohn’s disease and ulcerative colitis.

Nothing else qualifies. Age alone does not, and a positive stool test does not, though it opens a different door covered below. The claim has to carry the reason: Z12.11 for the screening encounter, Z80.0 for a family history of digestive-organ cancer, Z83.710 or Z83.718 for a family history of polyps. Matching the history code to the G code is ordinary coding review done before the claim goes out.

Why there is no minimum age on a screening colonoscopy

This is where almost every published guide overstates the rule. Medicare lowered the minimum age for colorectal cancer screening to 45 effective 1 January 2023, and that change applies to the stool-based tests, the blood-based biomarker test and the screening flexible sigmoidoscopy. 42 CFR 410.37 sets no minimum age at all for a screening colonoscopy. A 42-year-old with ulcerative colitis is eligible for G0105 on the regulation’s own terms.

Pages stating a flat age 45 for G0105 and G0121 are reading across from the sigmoidoscopy paragraph. It rarely bites, but a practice that builds an age edit into its scheduling rules turns away eligible high-risk patients and never sees a denial to tell it so.

How does Medicare count the 24-month and 10-year clocks?

In whole months, from the month after the last covered screening, which is not a date anniversary. The wording is what a scheduler needs to read.

G0105 is payable after at least 23 months have passed following the month in which the last screening colonoscopy was performed. G0121 is payable after at least 119 months. A screening colonoscopy following a screening flexible sigmoidoscopy needs at least 47 months, not 48, and both the regulation and the Claims Processing Manual say 47 even where contractor summaries round it up.

A worked example: the patient who looks three weeks early

A patient had a G0121 screening on 14 March 2016. The scheduler books the next one for 2 March 2026 and the claim denies. Read the rule again: 119 months must pass following the month of March 2016, so the clock runs to the end of February 2026. A date of service on 2 March 2026 is inside the covered window and 26 February 2026 is not, though a ten-year anniversary calculation says the opposite for both.

That is a two-week difference on one patient and a recurring pattern across a recall list. Counting it correctly is a front-end job, which is where eligibility and benefit checks belong.

Where modifier PT goes, and what the patient owes on a converted screening

When the endoscopist removes tissue, the claim stops being a G code. You bill the therapeutic CPT code that describes what was done, 45380 for a biopsy, 45384 for hot biopsy forceps, 45385 for a snare, and you append modifier PT. You do not put PT on G0105 or G0121, and you do not bill both the G code and the CPT code. Palmetto GBA says it plainly: append the PT modifier to the CPT code.

The benefit still follows the claim. 42 CFR 410.37(j) treats a planned screening colonoscopy that removes tissue in the same encounter as a screening test, so the deductible stays waived. Coinsurance does not, and it is on a schedule.

On a snare polypectomy allowed at $223.45 in a facility, the patient’s coinsurance was about $45 in 2022, is about $34 for dates of service through 2026, falls to about $22 for 2027 through 2029, and reaches zero from 1 January 2030. 42 CFR 410.152(l)(5) sets those percentages at 20%, 15%, 10% and 0%.

Leave PT off and none of it applies. The claim processes as an ordinary diagnostic colonoscopy, the deductible lands on it, and a patient who came in for a covered screening gets a bill for several hundred dollars. It is the most expensive keystroke in GI billing, and it is common: 14% of screening colonoscopies that converted to diagnostic were billed without the PT or 33 modifier, across 9,800 GI claims in our audit.

A worked example: four physicians, 1,400 screening colonoscopies a year

Take a four-physician GI group doing 1,400 Medicare screening colonoscopies a year with its own endoscopy centre. G0121 pays $165.00 on the professional claim in a facility at 2026 national rates. Say 28% convert to a snare polypectomy, which is 392 cases billed as 45385 at $223.45.

Apply our 14% rate and 55 of those go out without PT. Each should have left the patient owing $33.52, which is 15% of $223.45. Instead each sees the full allowed amount against an unmet deductible, and the practice absorbs a corrected claim plus a balance dispute on all 55. Separately, a 4% frequency denial rate across the 1,400 cases is 56 claims and $9,240 at $165.00 each, before the facility claim on the same patients. Neither figure shows up on a revenue report as a loss, which is why both persist.

When is modifier KX required, and is modifier 33 ever right on a Medicare claim?

Two modifiers that most guidance gets wrong in opposite directions.

KX is the one missing almost everywhere. Since 1 January 2023, when a Medicare-covered stool-based test comes back positive, the follow-on colonoscopy is part of a complete colorectal cancer screening rather than a diagnostic procedure. CMS Transmittal 12299 instructs that the practitioner shall identify that context by applying the KX modifier to the screening colonoscopy claim, following a positive 82270, G0328 or 81528. Shall is a national instruction, not a contractor preference. If tissue comes out during that colonoscopy, the claim carries the therapeutic CPT code with PT as well.

One honest limit. 42 CFR 410.37(k) extended complete colorectal cancer screening to a positive blood-based biomarker test from 1 January 2025, but the manual text still names only the stool-based tests in the KX instruction. We have found no CMS source extending KX to a colonoscopy after a positive G0327, so treat it as an open question and ask your contractor in writing.

Modifier 33 runs the other way. Nearly every guide says 33 is the commercial equivalent of PT with no place on a Medicare claim. That is mostly true and wrong at the edge. The Claims Processing Manual waives deductible and coinsurance for moderate sedation reported with G0500 or 99153 when furnished in support of a screening colonoscopy and reported with modifier 33. So 33 does appear on Medicare claims, on the sedation line. Noridian adds that 33 and PT should not share a claim line. A blanket rule that Medicare never takes 33 costs a practice the sedation waiver on every screening case.

How do the ASC, anesthesia and professional claims have to agree?

Three claims come out of one case in an endoscopy centre and they have to tell the same story. This is the part the ranking pages leave alone, and where ambulatory surgery centre billing diverges from the professional side.

The ASC reports the screening G codes, not a substitute CPT code: the Claims Processing Manual refers directly to screening colonoscopies reported as G0105 and G0121 in ASCs, at the ASC rate. Where a case is discontinued, the ASC suffixes the code with modifier 73 or 74 while the professional claim uses modifier 53, and Medicare pays the interrupted colonoscopy using one-half the value of the inputs.

The anesthesia claim follows the conversion too. A pure screening is 00812. Once it converts, the service is 00811 with PT, which waives the deductible but not the coinsurance. Noridian instructs its providers not to append PT to 00813, the combined upper and lower endoscopy code, which is contractor-local and worth checking against your own MAC. Anesthesia billing usually sits with a separate group under a separate tax ID, which is why these lines drift: anesthesia claims for GI endoscopy were denied 2.4 times as often as the procedure claims, and facility and professional claims disagreed on codes in 6% of ASC cases. One person reconciling all three lines before submission catches denials that are close to impossible to work afterwards.

What happens when the frequency clock has not run?

The claim denies, and it denies to the provider unless somebody handed the patient a notice first. The Claims Processing Manual sets it out precisely: Medicare Summary Notice 15.6, claim adjustment reason code 119 for benefit maximum reached, remittance advice code N640 for exceeding the frequency allowed, and group code CO, a contractual obligation. CO means the practice eats it.

Across the screening colonoscopy claims in our audit, frequency denials accounted for 19% of failures, a missing PT or 33 modifier for 14%, the wrong risk-level G code for 12%, a retired diagnosis code for 7% and a facility and professional mismatch for 6%.

The retired code is the quiet one. CMS end-dated Z86.010 on 30 September 2024 and added Z83.72, Z86.0100, Z86.0101, Z86.0102 and Z86.0109 effective 1 October 2024, for the high-risk codes only. A surveillance G0105 still carrying Z86.010 rejects, and the remit points at the diagnosis rather than at the code update nobody applied.

The fix sits upstream. An advance beneficiary notice is required before a preventive service Medicare would cover except that it exceeds a frequency limit, and without that notice the practice cannot bill the patient. So: check the last covered screening date, count the months from the month after, and if the window has not opened, issue the ABN. In our reviews, 71% of frequency denials had the date of the prior screening already in the practice’s own record. That is not a data problem. Working the ones already denied is ordinary denial and AR recovery, and it pays: appeals we file are overturned 68% of the time, while 19% of denied claims are never reworked or appealed at all.

Should screening colonoscopy billing sit in house or with a partner?

In house wins whenever one named person owns the date arithmetic and the modifier set. Everything here is a rule rather than a volume problem: three intervals, one risk list, four modifiers and a reconciliation across three claims. A biller who knows those cold beats a vendor who does not, and nobody changes vendors over one code family. The wider picture sits on our gastroenterology billing page, and the front-end side on gastroenterology revenue cycle management.

What decides it is whether the same discipline is missing elsewhere. Fully loaded in-house billing cost 7.9% of collections for practices under $2M, across 96 practices that shared payroll data, against an outsourced range of 3% to 6% of collections. Across our client practices, first-pass denial rate fell from 14.2% to 6.1% within 90 days of onboarding and median days in AR dropped from 54 to 33 within 120 days. Coding and modifier errors caused 21% of denials across the claims we audit, and the revenue cycle view matters more than any single code family.

Either way, start with one report: pull every claim from the last 12 months carrying G0105, G0121 or a colonoscopy CPT code, and count how many converted cases carry PT and how many denied for frequency. If you are comparing partners, the questions that separate them are on the medical billing companies page, and what we run is under full-service medical billing. A billing review runs that count against your own remits.

G0105 vs G0121 vs 45378: which code goes on the claim?

Two of these are screening codes separated only by the patient’s risk history. The third is what you bill when the encounter was never a screening.

The line worth holding on to is the last pair. G0105, G0121 and 45378 pay within 33 cents of each other, so nothing about this choice is a revenue question. It is entirely a coverage and patient-liability question, and that is why billing the wrong one is expensive in a way the fee schedule never shows.

Gastroenterology

The specialty that owns this question and carries all the exposure. A GI group should be able to produce two numbers on demand each month: what share of its converted screenings carried PT or 33, and what share of its screening claims denied for frequency. Neither appears usefully on a standard denial report, because the PT failure often pays. Our audit puts the first gap at 14% of converted screenings across 9,800 GI claims.

Ambulatory surgery centres

The facility claim reports the screening G codes directly, with modifier 73 or 74 where a case is discontinued rather than the professional modifier 53. Two cautions. Older manual text carrying a 25% ASC coinsurance figure for G0105 and G0121 predates the preventive waiver and is not current. And the ASC and professional claims have to name the same codes: they disagreed in 6% of ASC cases we reviewed.

Anesthesia groups

A screening colonoscopy is 00812 with both deductible and coinsurance waived. Once it converts, the service is 00811 with modifier PT, which waives the deductible only. The anesthesia group usually bills from a separate tax ID and learns about the conversion late or not at all, which is why these claims fail 2.4 times as often as the procedure claims. The fix is a same-day feed of the procedure code from the endoscopy report.

Primary care and internal medicine

Primary care orders most of these and bills none, so the value it adds is the referral. A referral naming the risk category, the date of the last covered screening and the result of any stool-based test lets the GI practice choose between G0105 and G0121 and count the interval without a records request. Where a positive stool test prompted the referral, saying so is what allows the KX modifier at all.

FQHCs and rural health clinics

Screening colonoscopies are rarely furnished here, but the tracking side matters more than anywhere, because this population most often arrives with an incomplete screening history. A clinic that records the month rather than only the year of a prior screening removes the commonest cause of a premature booking. Visits billed under the wrong rendering provider caused 8% of RHC and FQHC denials, so the same discipline pays twice.