CPT® Code 77066

Quick Reference

• Code definition: CPT 77066 reports bilateral diagnostic mammography performed to evaluate a specific breast complaint, finding, or history, with computer-aided detection (CAD) included when used; this is not a screening study.
• Key billing rule: CAD is bundled into the descriptor and is never separately reportable; one unit covers both breasts for the entire encounter (MUE = 1) [2].
• Modifier essentials: Use -26 for professional-only billing, -TC for facility-only billing, and -GG when a screening mammogram converts to diagnostic on the same date for a Medicare patient [3].
• Documentation must-have: A physician order with a documented clinical indication (symptom, finding, or relevant history) is required; pairing a screening encounter code with this code is the most commonly flagged audit trigger.
• Top confusion point: Do not append modifier -50 to 77066. Bilateral is inherent in the code descriptor, and the Medicare bilateral surgery indicator is 2, meaning the 150% adjustment does not apply.
• Payer alert: HCPCS G0204 (the former Medicare code for bilateral diagnostic mammography) was deleted effective 12/31/2017; CPT 77066 is the sole code for Medicare and all commercial payers from 1/1/2018 forward [3].
• Add-on note: HCPCS G0279 is the active Medicare add-on for diagnostic digital breast tomosynthesis (DBT) performed alongside 77066; CPT codes 77061 and 77062 are not valid for Medicare purposes.

When to Use This Code

CPT 77066 applies when a radiologist performs diagnostic mammography on both breasts with a specific clinical indication driving the encounter. Per ACR Appropriateness Criteria, supported indications include a new palpable breast mass, focal breast pain, nipple discharge, skin or nipple changes, asymmetry identified on prior imaging, short-interval follow-up of a probably benign finding (BI-RADS 3), and surveillance in patients with a personal history of breast cancer [5].

Scope boundaries: This code covers bilateral imaging regardless of which breast prompted the referral. It is clinically appropriate to image both breasts even when symptoms are unilateral; contralateral imaging provides a baseline and may reveal concurrent findings. The code includes standard craniocaudal (CC) and mediolateral oblique (MLO) views plus any additional spot-compression or magnification views obtained during the same encounter. CAD, when performed, is included and not separately reportable.

Provider and setting context: The PC/TC indicator is 1 (Diagnostic Tests for Radiology Services). An independent radiology practice billing both components bills globally (no modifier). A hospital outpatient facility bills the technical component (-TC); the interpreting radiologist bills the professional component (-26). Both settings are appropriate for this code; verify site-of-service RVU differentials when evaluating reimbursement expectations.

Distinction from screening: The critical entry point for 77066 is clinical indication. A patient with a palpable mass, nipple discharge, or abnormal prior imaging receives a diagnostic study under 77066. An asymptomatic patient presenting for routine surveillance without a specific finding receives a screening study under 77067. This distinction affects patient cost-sharing under Medicare, ICD-10-CM code selection, and payer coverage rules.

Code Differentiation Table

The most consequential differentiator is laterality. A patient with a prior mastectomy who presents for surveillance of the remaining breast should be billed as 77065, not 77066. Billing 77066 when only one breast was imaged is overcoding and is a recognized OIG audit focus area.

flowchart TD
    A[Mammography Ordered] --> B{Clinical indication present?}
    B -->|Symptom, finding, or relevant history| C{Both breasts imaged?}
    B -->|Asymptomatic routine surveillance| D[77067 Screening Bilateral]
    C -->|Yes| E[77066 Diagnostic Bilateral]
    C -->|No| F[77065 Diagnostic Unilateral]
    D --> G{DBT also performed?}
    G -->|Yes| H[Add-on 77063]
    G -->|No| I[77067 alone]
    E --> J{DBT also performed?}
    J -->|Yes, Medicare| K[Add-on G0279]
    J -->|Yes, commercial| L[Add-on 77061 or 77062 per payer policy]

Billing & Modifier Rules

Professional and technical split: The PC/TC indicator for 77066 is 1. Three billing patterns apply: global (no modifier) when an independent practice owns equipment and provides interpretation; 77066-26 when a radiologist bills professional component only; and 77066-TC when a facility bills technical component only.

Modifier -50 prohibition: Never append modifier -50 to 77066. Bilateral is inherent in the code, and the Medicare bilateral surgery indicator is 2, meaning the 150% payment adjustment is explicitly inapplicable. Submitting -50 with this code reflects a structural misunderstanding of the code and will result in claim adjustment or overpayment recovery.

Modifier -GG (Medicare-specific): When a screening mammogram (77067) is initiated and the radiologist determines additional diagnostic views are necessary during the same encounter, the claim converts to diagnostic. Bill 77066-GG to signal the conversion to Medicare [3]. The patient may then owe standard Part B cost-sharing (20% coinsurance after deductible) instead of $0. Issue an Advance Beneficiary Notice (ABN) when conversion is anticipated.

Add-on code G0279: HCPCS G0279 (Diagnostic digital breast tomosynthesis, unilateral or bilateral) is the active Medicare add-on for DBT performed alongside diagnostic mammography. Report separately in addition to 77065 or 77066. For commercial payers, CPT 77061 or 77062 may apply depending on payer policy; verify before billing.

MUE = 1: Only one unit of 77066 is payable per date of service per beneficiary [2]. Billing more than one unit will be denied.

CPT bundling instruction: Do not report 77063 in conjunction with 77066. CPT 77063 is a screening DBT add-on and is restricted to use with the screening code 77067.

Modifier -LT / -RT: These laterality modifiers are not applicable to 77066; the code is inherently bilateral. Use laterality modifiers only with the unilateral code 77065 when needed.

Documentation Essentials

The medical record supporting 77066 must include:

• A physician order specifying the clinical indication (the symptom, finding, or history that distinguishes this as a diagnostic rather than screening encounter)
• A radiology report containing: patient demographics, date of service, clinical indication, imaging technique, findings by breast and quadrant, BI-RADS assessment category, and interpreting radiologist signature
• Documentation confirming both breasts were imaged; if only one breast was imaged, 77065 applies
• MQSA certification of the imaging facility; per 42 CFR § 410.34, Medicare covers mammography only at FDA-certified facilities [1]

Audit red flags for 77066 specifically:

Pairing 77066 with Z12.31 (encounter for screening mammogram) or Z12.39 is a primary audit trigger. These are screening encounter codes and cannot support payment for a diagnostic procedure. Use the appropriate clinical code: a symptom (e.g., N63.21 for left breast lump), an imaging finding (e.g., R92.0 for microcalcification, R92.8 for other abnormal findings), or a history code (e.g., Z85.3 for personal history of breast cancer, Z80.3 for family history) [2].

Bilateral coding without bilateral imaging in the report is the second major red flag. The radiology report must explicitly document that both breasts were imaged. Absence of a physician order or referral documentation is a third known audit finding for diagnostic mammography claims.

Medical necessity: Medicare NCD 220.4 covers diagnostic mammography when ordered by a physician with documented clinical indication [1]. There is no annual frequency cap for diagnostic studies; medical necessity governs. Individual MAC LCDs specify covered ICD-10-CM codes by jurisdiction; verify the applicable LCD before submission.

Medicare, Commercial & Medicaid Payer Rules

Medicare:

Coverage is governed by NCD 220.4 [1]. Diagnostic mammography under 77066 is a covered Part B service when ordered by a physician and supported by documented medical necessity. Unlike screening mammography (77067), which carries $0 beneficiary cost-sharing as a preventive benefit, diagnostic mammography is subject to the standard Part B deductible and 20% coinsurance.

The predecessor HCPCS codes (G0204 for bilateral diagnostic, G0206 for unilateral diagnostic, G0202 for screening bilateral) were all deleted effective 12/31/2017. CPT 77066 replaced G0204 for Medicare effective 1/1/2018 [3]. Any claim submitted to Medicare using G0204 for dates of service on or after 1/1/2018 will be rejected as an invalid code.

For diagnostic DBT alongside 77066, the applicable Medicare add-on is G0279. CPT codes 77061 and 77062 carry Medicare status "Not Valid for Medicare Purposes" and will be denied if submitted [4].

MQSA facility certification is a hard Medicare coverage requirement per 42 CFR § 410.34 [1]. The facility must hold current FDA/MQSA certification; billing from a non-certified facility is a non-covered service.

Commercial payers:

Commercial payers generally follow CPT 77066 with similar PC/TC billing rules. Key divergences from Medicare:

• CPT 77061 and 77062 may be payable for diagnostic DBT (unlike Medicare); verify individual payer policy before billing
• Some payers require prior authorization for diagnostic mammography in specific clinical scenarios; check plan-level requirements
• Modifier -GG applicability varies among commercial payers; some do not recognize this modifier; verify before appending

Medicaid:

Medicaid coverage and frequency limits vary by state and managed Medicaid plan. Verify Treatment Authorization Request (TAR) requirements and covered diagnosis codes at the state plan level. Managed care organizations may apply more restrictive coverage criteria than fee-for-service Medicaid.

Common Denials & Prevention

Screening diagnosis code paired with diagnostic procedure Using Z12.31 or Z12.39 with 77066 triggers medical necessity denials because screening encounter codes cannot support payment for a diagnostic procedure. Prevention: select the specific clinical indication code reflecting why the study was ordered; a symptom, finding, or history code is always the correct pairing with 77066.

Bilateral billed, unilateral performed If the radiology report documents imaging of only one breast, 77066 is overcoded. The denial may appear as "procedure inconsistent with documentation" or surface as a post-payment audit finding. Prevention: confirm the report explicitly documents bilateral imaging before billing 77066; use 77065 whenever only one breast was imaged.

Modifier -50 claim adjustment Submitting 77066-50 signals to the payer that a unilateral code is being converted to bilateral, which does not apply here. Some systems apply a payment reduction or reject the modifier entirely. Prevention: never append modifier -50 to 77066.

Legacy G-code submitted post-deletion Claims submitted to Medicare using G0204 for dates of service on or after 1/1/2018 will be rejected as invalid. Prevention: use CPT 77066 for all payers for all dates of service from 1/1/2018 forward.

Screening-to-diagnostic conversion billed without modifier -GG (Medicare) Billing 77066 without -GG when a screening encounter converted to diagnostic intraservice may generate a claim flag or compliance exposure; failure to issue an ABN creates additional risk. Prevention: bill 77066-GG when conversion occurs; issue an ABN to the patient when conversion is anticipated before or at the time of service [3].

Coding Scenarios

Scenario 1: Palpable left breast mass with bilateral diagnostic mammogram

A 52-year-old woman is referred for diagnostic mammography after her primary care physician palpates a mass in the upper outer quadrant of the left breast. The radiologist images both breasts with standard views and additional spot compression of the left breast; CAD is applied.

Correct coding: 77066 + N63.21

Why: Both breasts were imaged, so 77066 is correct over 77065. CAD is bundled and not separately reported. N63.21 (lump in the left breast, upper outer quadrant) supports medical necessity as a clinical symptom code.

Scenario 2: Post-mastectomy surveillance, remaining breast only

A 61-year-old woman with a prior right mastectomy for breast cancer presents for annual surveillance mammography. Only the remaining left breast is imaged.

Correct coding: 77065 + Z85.3

Why: Only one breast is present and imaged; 77066 would be overcoding. Z85.3 (personal history of malignant neoplasm of breast) supports medical necessity for ongoing diagnostic surveillance. This is one of the most common overcoding errors auditors identify for this code family.

Scenario 3: Screening converted to diagnostic, Medicare patient

A Medicare beneficiary presents for her annual screening mammogram. During the study, the radiologist identifies suspicious microcalcifications and performs additional diagnostic views of both breasts before the patient leaves.

Correct coding: 77066-GG + R92.0

Why: The encounter converted from screening to diagnostic intraservice; modifier -GG signals the conversion to Medicare [3]. Do not bill 77067 alongside 77066; the encounter converts entirely. R92.0 (mammographic microcalcification) reflects the finding that triggered conversion. The patient now owes Part B cost-sharing; issue an ABN.

Scenario 4: Hospital outpatient PC/TC split with diagnostic DBT add-on (Medicare)

A hospital outpatient radiology department performs bilateral diagnostic mammography with DBT on a Medicare patient presenting for follow-up of an abnormal prior study. The radiologist provides the interpretation separately.

Correct coding (facility): 77066-TC + G0279 + R92.8

Correct coding (radiologist): 77066-26 + G0279 + R92.8

Why: PC/TC indicator = 1 permits split billing. G0279 is the correct Medicare add-on for diagnostic DBT (not 77062, which is not valid for Medicare). R92.8 (other abnormal findings on diagnostic imaging) captures the follow-up indication.

Related Codes

• 77065 — Diagnostic mammography, unilateral, CAD included; use when only one breast is imaged
• 77067 — Screening mammography, bilateral, CAD included; asymptomatic patients, $0 Medicare cost-sharing
• 77063 — Screening DBT, bilateral (add-on); pairs with 77067 only, never with 77066
• G0279 — Diagnostic DBT, unilateral or bilateral (add-on); Medicare add-on to 77065 or 77066
• G0204 — Predecessor HCPCS code for bilateral diagnostic mammography; deleted 12/31/2017, replaced by 77066
• 76641 — Breast ultrasound, unilateral, complete; commonly ordered alongside 77066 when mammography findings are inconclusive
• 76642 — Breast ultrasound, unilateral, limited; targeted ultrasound frequently paired with 77066 for focused evaluation
• R92.0 — Mammographic microcalcification; high-frequency indication code for follow-up diagnostic studies
• Z85.3 — Personal history of malignant neoplasm of breast; supports medical necessity for surveillance diagnostic imaging

Sources

1. CMS Medicare Coverage Database — NCD 220.4 Mammography — National Coverage Determination governing Medicare coverage for all mammography services; includes MQSA certification requirement under 42 CFR § 410.34
2. CMS NCCI — Medically Unlikely Edits (MUEs) — MUE tables confirming MUE = 1 for CPT 77066; updated quarterly
3. CMS Medicare Claims Processing Manual, Chapter 18 — Preventive and Screening Services — Covers screening-to-diagnostic conversion rules, modifier -GG, G-code deletion transition from G0204 to CPT 77066
4. CMS NCCI — Procedure-to-Procedure (PTP) Edits — PTP edit pairs for 77066 bundling rules; updated quarterly
5. CMS Physician Fee Schedule Look-Up Tool — RVUs, PC/TC indicators, bilateral surgery indicators, global days, and payment status for CPT 77066; updated annually