CPT Code 45378 Explained: Diagnostic Colonoscopy Billing Done Right in 2026

What Is CPT Code 45378?

CPT code 45378 is the base diagnostic colonoscopy code used when a provider performs a flexible colonoscopy to examine the colon from rectum to cecum without performing a biopsy, polypectomy, or any other therapeutic intervention. It is the foundational code in the colonoscopy endoscopy family, and per the CMS NCCI Policy Manual effective January 1, 2026, it is bundled into any surgical endoscopy code and cannot be reported separately alongside one.

Screening vs. diagnostic: For commercial insurance, 45378 with modifier 33 is the correct code for a preventive screening colonoscopy. For Medicare screening, use HCPCS codes G0105 (high risk) or G0121 (average risk) instead. Without a modifier, 45378 defaults to diagnostic.

When it converts: If a screening colonoscopy turns therapeutic (polyp removal or biopsy), replace 45378 with the appropriate higher-level CPT code and append modifier PT for Medicare or modifier 33 for commercial payers to preserve the preventive benefit.

2026 reimbursement: CPT 45378 carries a work RVU of 3.18 in the 2026 CMS Physician Fee Schedule. At the 2026 non-QP Medicare conversion factor of $33.4009, the base national payment before geographic adjustment is approximately $378 in a non-facility setting.

What CPT Code 45378 Actually Covers

The American Medical Association (AMA) defines CPT 45378 as “Colonoscopy, flexible; diagnostic, including collection of specimen(s) by brushing or washing, when performed (separate procedure).” That last parenthetical is the part billing teams need to read carefully. The “separate procedure” designation means 45378 is the standalone diagnostic exam. The moment the endoscopist performs an intervention beyond looking and washing, such as a biopsy or a polypectomy, the code changes and 45378 no longer applies.

This is the code a GI practice uses when the colonoscopy is purely diagnostic: the provider advances the scope, examines the colon, and may collect brushing or washing specimens, but does not remove tissue, cauterize a lesion, or perform any other therapeutic procedure. The documentation must clearly state that no intervention was performed and must describe the extent of the exam, ideally confirming advancement to the cecum.

One question we hear constantly from gastroenterology practice managers is why 45378 claims get denied when the colonoscopy was clearly performed. In our experience matching providers with billing partners, the denial almost always traces to one of three errors: the claim was submitted as diagnostic when the payer required a screening code, the claim was submitted without the correct modifier for a screening indication, or a therapeutic code was warranted because the operative note described a biopsy the billing team missed. The code itself is straightforward. The errors come from the context around it.

How Does Screening Differ from Diagnostic Colonoscopy Billing?

The distinction between screening and diagnostic colonoscopy is the single highest-impact coding decision in GI billing, and getting it wrong is the fastest way to generate a patient billing complaint, a payer denial, or both. The same physical procedure, a flexible colonoscopy to the cecum, is coded differently depending on why it was ordered.

A screening colonoscopy is performed on an asymptomatic patient for colorectal cancer prevention. Under the Affordable Care Act (ACA), commercial plans must cover preventive screenings at zero patient cost-sharing. For Medicare, screening colonoscopies are covered with no deductible, though a coinsurance phase-down applies when the screening converts to a therapeutic procedure (15% patient responsibility in 2026, decreasing to 0% by 2030 under the Consolidated Appropriations Act of 2023).

A diagnostic colonoscopy is performed to investigate symptoms such as rectal bleeding, abdominal pain, changes in bowel habits, abnormal imaging findings, or surveillance after a prior polyp history. Because the procedure is medically necessary rather than preventive, standard deductible and coinsurance rules apply.

Here is how the coding splits by payer type.

Scenario	Medicare Code	Commercial Code	Patient Cost-Sharing
Average-risk screening, no findings	G0121	45378 + modifier 33	$0 (preventive)
High-risk screening, no findings	G0105	45378 + modifier 33	$0 (preventive)
Screening converted to therapeutic (polyp removed)	45385 + modifier PT	45385 + modifier 33	Medicare: 15% coinsurance (2026); Commercial: $0 with modifier 33
Diagnostic (symptoms present)	45378 (no modifier)	45378 (no modifier)	Standard deductible and coinsurance

The critical rule: do not append modifier 33 or PT to a colonoscopy that was ordered for a diagnostic indication, even if the patient also happens to be due for screening. Per the American Gastroenterological Association (AGA) coding guidance, if the indication is diagnostic, the procedure is coded as diagnostic regardless of the timing. A patient presenting with rectal bleeding who is also overdue for screening still gets coded as diagnostic (45378 without a modifier) because the symptom is the reason for the procedure.

Across the billing companies we vet, the screening-versus-diagnostic decision is the single most common source of colonoscopy denials. The error runs in both directions: billing a screening code when the indication was diagnostic, and billing a diagnostic code when the procedure qualified as preventive, which shifts cost to the patient unnecessarily.

Colonoscopy Modifiers That Affect 45378

Modifiers are where colonoscopy billing moves from code selection to claim accuracy, and a missing or incorrect modifier on a 45378 claim is one of the most preventable reasons GI practices lose revenue or generate patient billing complaints. These are the modifiers that directly affect how 45378 is processed.

Modifier 33 (Preventive Service). Appended to 45378 on commercial and Medicaid claims when the colonoscopy is a preventive screening. Modifier 33 triggers zero patient cost-sharing under ACA rules. Without it, the payer processes the claim with standard deductible and coinsurance. For a deeper look at how modifier 33 applies across GI procedures, see our full guide on modifier 33 in medical billing.

Modifier PT (Screening Converted to Diagnostic/Therapeutic). Used on Medicare claims when a colonoscopy that started as a screening is converted to a diagnostic or therapeutic procedure, such as when a polyp is found and removed. Modifier PT tells CMS to waive the deductible even though the procedure is now coded as therapeutic. Without it, the patient is billed inappropriately.

Modifier 53 (Discontinued Procedure). Appended to 45378 when the colonoscopy is started but cannot be completed, typically because the scope could not advance to the cecum due to poor prep, patient intolerance, or an obstructing lesion. CMS has specific reduced fee schedule values for 45378-53. For the full rules on when and how to use this modifier, see our guide on modifier 53 for discontinued procedures.

Modifier 59 (Distinct Procedural Service). Used in limited situations where a separate, distinct procedure is performed at the same session and NCCI edits would otherwise bundle the codes. Modifier 59 is frequently overused in GI billing and should only be appended when the documentation supports a truly separate service at a different anatomic site or during a different encounter.

Providers often come to us after receiving a wave of patient complaints about unexpected colonoscopy bills. When we help them trace the issue upstream, the root cause is almost always a missing modifier 33 on commercial screening claims or a missing modifier PT on Medicare claims that converted from screening to therapeutic. The modifier is a single field on the claim, but it determines whether the patient pays nothing or receives a bill for hundreds of dollars.

How Does NCCI Bundling Apply to CPT 45378?

Per the CMS NCCI Policy Manual effective January 1, 2026, CPT 45378 is the base code for the colonoscopy endoscopy family. A diagnostic endoscopy is always bundled into a surgical (therapeutic) endoscopy performed during the same session. This means that if a provider performs a diagnostic colonoscopy and also takes a biopsy or removes a polyp during the same encounter, only the higher-level therapeutic code is reported. Billing both 45378 and a therapeutic colonoscopy code on the same date of service triggers an automatic NCCI edit denial.

The highest-intensity procedure rule governs which code to report. If the endoscopist examines the colon (diagnostic) and also removes a polyp by snare (therapeutic), the claim reports only 45385 (colonoscopy with snare polypectomy). The work RVU for 45385 already accounts for the diagnostic component. Adding 45378 on the same claim is double-billing the base exam, and payers will deny it automatically.

The exception is when two distinct procedures are performed at genuinely separate anatomic sites or during separate encounters on the same date. In those narrow situations, modifier 59 or the appropriate XE/XS modifier may support separate reporting. However, for a single colonoscopy session where the scope is advanced once and multiple interventions occur along the way, the NCCI rules are clear: report only the most complex intervention.

The biggest issue we see providers run into with NCCI bundling is billing 45378 alongside 45380 or 45385 because the operative note describes both a “diagnostic examination” and a biopsy or polypectomy. The biopsy code already includes the diagnostic exam. Submitting both codes is the definition of an unbundling error, and it puts the practice at compliance risk beyond just the denial.

CPT 45378 vs. 45380, 45385, and Other Colonoscopy Codes

The colonoscopy CPT code family runs from 45378 through 45398, with each code representing a different level of intervention. Choosing the wrong code is the second most common colonoscopy billing error after the screening-versus-diagnostic mix-up, and it directly affects reimbursement because the work RVU and payment increase with procedural complexity.

Here is how the most commonly billed colonoscopy codes compare in 2026.

CPT Code	Description	Work RVU (2026)	Intervention	Bundles 45378?
45378	Diagnostic colonoscopy (exam only, brushing/washing)	3.18	None	Base code
45380	Colonoscopy with biopsy (single or multiple)	3.47	Biopsy	Yes
45384	Colonoscopy with hot biopsy forceps or snare removal	3.89	Hot biopsy / snare	Yes
45385	Colonoscopy with snare polypectomy	4.46	Snare polypectomy	Yes
45390	Colonoscopy with endoscopic mucosal resection	5.40	EMR	Yes

The decision tree is simple in principle: if nothing was removed, biopsied, or treated, report 45378. If something was done beyond examining and washing, move to the code that matches the most complex intervention. If both a biopsy and a polypectomy were performed at different sites during the same session, report the polypectomy code (the higher-complexity code) and do not also report the biopsy code unless payer-specific rules and documentation support it with the appropriate modifier.

In our experience matching providers with billing partners, the practices that get this right consistently are the ones whose billing teams read the operative note line by line before selecting the code, rather than defaulting to 45378 because “it was a colonoscopy.” Defaulting to the base code when a therapeutic code is warranted leaves money on the table. Defaulting to a therapeutic code when no intervention occurred creates a compliance problem.

How to Bill an Incomplete Colonoscopy

An incomplete colonoscopy occurs when the scope cannot be advanced to the cecum due to poor bowel preparation, patient intolerance, an obstructing lesion, or another clinical reason. Per CMS guidance, an incomplete colonoscopy is still reported using CPT 45378 (or the applicable G-code for Medicare screening), but modifier 53 must be appended to indicate that the procedure was discontinued.

CMS has established specific reduced fee schedule values for 45378-53 in the Medicare Physician Fee Schedule database. The reimbursement is lower than the full procedure because the work was not completed, but the code still reflects that a colonoscopy was initiated and documented. The operative note must state why the procedure was discontinued and how far the scope advanced, as this documentation supports the use of modifier 53 on audit.

A common billing error is coding an incomplete colonoscopy as a flexible sigmoidoscopy (45330 series) on the theory that the scope only reached the sigmoid or descending colon. Per CMS, if the procedure was initiated as a colonoscopy, it is reported as a colonoscopy with modifier 53, not reclassified as a sigmoidoscopy. The procedure intent determines the code family, not the anatomic endpoint.

For a full walkthrough of when and how to use modifier 53 across GI procedures, including colonoscopy, see our guide on modifier 53 for discontinued procedures.

Common CPT 45378 Denials and How to Prevent Them

Colonoscopy denials follow a predictable pattern, and every one of them is preventable with the right pre-submission process. These are the denial reasons that surface most frequently across the GI practices we work with.

Submitting 45378 for a Medicare screening colonoscopy. Medicare requires G0105 or G0121 for screening. Filing 45378 without modifier 33 or PT on a Medicare screening results in the claim being processed as diagnostic, shifting cost to the patient and triggering complaints.

Missing modifier 33 on a commercial screening claim. Without modifier 33, the commercial payer applies standard deductible and coinsurance instead of the zero cost-sharing mandated by ACA preventive coverage rules. The claim may pay, but the patient receives a bill they should not have.

Missing modifier PT on a Medicare screening that converted to therapeutic. If a polyp is found and removed during a Medicare screening, the therapeutic code must carry modifier PT. Without it, the patient loses the deductible waiver and is billed inappropriately.

Billing 45378 alongside a therapeutic colonoscopy code. NCCI bundling edits deny 45378 when it appears on the same claim as 45380, 45385, or another surgical endoscopy code for the same session. The therapeutic code already includes the diagnostic component.

Insufficient documentation of medical necessity. A diagnostic 45378 claim requires a valid diagnostic indication. If the operative note does not clearly state the symptom or clinical finding that prompted the colonoscopy, the payer may deny for lack of medical necessity.

Coding an incomplete colonoscopy without modifier 53. Submitting 45378 without modifier 53 for a procedure that did not reach the cecum can trigger an audit or a request for records, because the documentation will not support a complete procedure.

Which ICD-10 Codes Support CPT 45378?

Every 45378 claim needs an ICD-10-CM diagnosis code that supports the medical necessity of the procedure. The correct diagnosis depends on whether the colonoscopy is screening or diagnostic, and using the wrong one is a fast path to a denial or an audit flag.

For screening colonoscopies, report Z12.11 (encounter for screening for malignant neoplasm of colon) as the primary diagnosis. If the patient has a personal history of polyps, add Z86.010 (personal history of colonic polyps) as a secondary code.

For diagnostic colonoscopies, the ICD-10 code must reflect the symptom or condition that prompted the procedure. Common diagnostic pairings with 45378 include K92.1 (melena), R19.5 (other fecal abnormalities), K63.5 (polyp of colon), R10.9 (unspecified abdominal pain), K57.30 (diverticulosis of large intestine without perforation or abscess), and R76.0 (raised antibody titer, used for positive stool-based screening follow-up). The diagnosis code must match the clinical indication documented in the operative note. A mismatch between the procedure code and the diagnosis code is an automatic audit trigger.

Frequently Asked Questions

Next Steps

Need help choosing a GI billing partner? Read our guide on how to find the right gastroenterology medical billing service for the criteria that matter most when evaluating partners.

Using modifier 33 on screening claims? See our full breakdown of modifier 33 in medical billing for every scenario where it applies across GI procedures.

Dealing with incomplete procedures? Our guide on modifier 53 for discontinued procedures covers the documentation and billing rules for colonoscopies that cannot be completed.

Ready to stop losing revenue on colonoscopy billing errors? Get matched with vetted GI billing companies that code colonoscopies correctly and catch the denials before they age out.