Mohs micrographic surgery (MMS) is one of the most precise and most frequently audited procedures in dermatology billing. Because the same physician is acting as both surgeon and pathologist, the coding rules around Mohs are unlike almost anything else in the CPT book. Getting them right protects reimbursement; getting them wrong is a fast track to denials, recoupments, or a Recovery Audit Contractor (RAC) letter.

Here’s a breakdown of what every coder, biller, and compliance-minded provider should know about Mohs.

What Is Mohs Micrographic Surgery?

Mohs surgery is a staged technique for removing skin cancer most often basal cell carcinoma and squamous cell carcinoma, one thin layer at a time. After each layer is excised, the same physician maps, color-codes, and processes the tissue, then examines it under the microscope for remaining cancer cells before deciding whether another stage is needed. Because 100% of the surgical margin is examined (rather than the small sampling typical of standard pathology), Mohs offers among the highest cure rates of any skin cancer treatment while sparing as much healthy tissue as possible.

What makes Mohs coding unique is built into the technique itself: CPT requires “the integration of an individual functioning in two separate and distinct capacities: surgeon and pathologist.” That dual role is the foundation for nearly every coding rule that follows.

Who Performs Mohs Surgery in the U.S.?

Mohs is performed almost exclusively by dermatologists with specialized training. A CMS claims-based analysis found roughly 2,240 dermatologists billing Mohs procedure codes nationally, with the overwhelming majority (about 95%) practicing in metropolitan areas. Formal fellowship training is centered around the American College of Mohs Surgery (ACMS), which represents close to 1,900 fellowship-trained Mohs surgeons who complete a rigorous one- to two-year post-residency fellowship (a minimum of 500–650 Mohs cases as primary surgeon). A smaller number of practitioners are credentialed through the American Society for Mohs Surgery (ASMS), a less formal pathway open to board-certified dermatologists with prior Mohs case experience. Regardless of pathway, only a licensed physician (MD/DO) may report the Mohs surgery codes.

The Codes

Mohs is reported using five CPT codes, split by anatomic location:

Code	Description
17311	First stage — head, neck, hands, feet, genitalia, or any site directly involving muscle, cartilage, bone, tendon, major nerves, or vessels
+17312	Each additional stage after the first, same anatomic area as 17311 (add-on)
17313	First stage — trunk, arms, or legs
+17314	Each additional stage after the first, same anatomic area as 17313 (add-on)
+17315	Each additional tissue block beyond the first 5, any stage (add-on)

Each first-stage code (17311 or 17313) includes up to five tissue blocks; 17315 only comes into play once a single stage exceeds five blocks. The add-on codes (17312, 17314, 17315) can never be billed without a corresponding primary code on the same date of service, and multiple units of 17312/17314 should be totaled and billed as a single line item rather than split across separate lines.

Routine Stains Are Bundled — Special Stains May Be Separate

Routine histopathologic staining (hematoxylin and eosin, toluidine blue) is already built into the value of 17311–17315 and can never be billed separately, regardless of how many blocks or stages are processed.

CPT code 88314 (special stain) is a different story but only under narrow circumstances. It may be reported in addition to the Mohs codes when a non-routine stain is medically necessary to interpret margins, such as for certain aggressive or unusual histologic subtypes. When 88314 is billed with Mohs, it requires modifier -59 (or the appropriate X-modifier), and the operative note must clearly document which special stain was used and why it was clinically necessary. Billing 88314 routinely or on every Mohs case rather than selectively, when justified is a well-known audit trigger.

This same logic extends to the use of permanent (paraffin-embedded) sections in addition to frozen sections. The American Academy of Dermatology’s position statement on the appropriate use of paraffin sections in conjunction with Mohs surgery outlines when sending tissue for paraffin processing is clinically appropriate for example, to confirm unexpected histology, assess for upstaging, or achieve independent confirmation on tumors with ambiguous or aggressive features. As with special stains, use of paraffin sections should be tied to medical necessity and documented accordingly, not applied as a routine, unvarying practice.

Mohs Codes Cannot Be Split Between Physicians

This is the rule at the heart of Mohs coding: 17311–17315 may only be reported when the same physician performs both the surgical excision and the pathologic interpretation. If the surgical and pathology responsibilities are divided between two different providers, for example, the surgeon excises the tissue but a separate pathologist reads the slides and bills independently, the Mohs codes cannot be used by either party. In that scenario, the surgeon reports the appropriate excision or biopsy code instead, and the pathologist may separately report the pathology service. Medicare Recovery Auditors have specifically flagged claims where slide preparation or interpretation was performed by someone other than the surgeon (or the surgeon’s own trained staff functioning under their direct supervision), this remains a known audit focus area.

Can I Perform Diagnostic Biopsy Prior to Mohs?

When a lesion hasn’t already been biopsied and confirmed malignant, a diagnostic skin biopsy and frozen section pathology can be performed on the same date of service, immediately before proceeding to Mohs. In that situation, modifier -59 (or the applicable X-modifier) should be appended to the biopsy and frozen section codes to distinguish them from the definitive Mohs procedure that follows. Importantly, this only applies when the biopsy is a genuinely separate diagnostic step, it is not appropriate to append modifier -59 when the biopsy and Mohs surgery are performed on the same lesion, in the same operative session, purely as part of the Mohs technique itself (i.e., the intraoperative frozen sections that are inherent to Mohs are not separately billable biopsies).

 

Can Repairs Be Billed Separately?

Unlike the pathology component, wound repair following Mohs surgery is not bundled into 17311–17315 and should be reported separately using the appropriate repair codes based on complexity and size, simple, intermediate, or complex closures (12xxx/13xxx), or more involved reconstructions such as flaps or grafts (14xxx/15xxx), as documented. Repair codes are typically reported with modifier -59 (or the corresponding X-modifier) to clearly distinguish them from the Mohs procedure itself.

Are There CLIA Requirements?

Because the histopathologic examination performed during Mohs is categorized as high-complexity testing under the Clinical Laboratory Improvement Amendments (CLIA), Mohs claims are subject to CLIA edits. A facility billing 17311–17315 must hold one of the following CLIA certificate types:

Certificate of Registration

Certificate of Compliance

Certificate of Accreditation

A CLIA Certificate of Waiver or a Certificate for Provider-Performed Microscopy (PPM) procedures is not sufficient, since Mohs pathology exceeds the scope of waived and PPM-level testing.

Every claim submitted for 17311–17315 must include the CLIA certification number reported in Item 23 of the CMS-1500 form (or the electronic equivalent). Claims missing this number will be rejected, making it one of the simplest but most consequential fields to get right on a Mohs claim.

Multiple Procedure Payment Reduction (MPPR) – What is the Financial Impact?

Mohs surgery codes are generally treated as exempt from the standard multiple procedure payment reduction meaning a physician performing several stages on the same lesion, same date of service, is typically paid in full for each stage rather than having subsequent stages reduced. MPPR issues are more likely to surface when a repair is billed alongside Mohs, particularly across different provider specialties within the same group; payer-specific reimbursement policies should always be checked, since MPPR application can vary by payer and by whether the same provider or a different-specialty provider performs the repair.

Documentation Requirements – No Exceptions

Solid documentation is what separates a payable Mohs claim from a denial. At minimum, the medical record should clearly support:

• The diagnosis and clinical rationale establishing that Mohs was the medically appropriate treatment choice for the specific lesion (location, size, histologic subtype, and recurrence risk all factor in)
• That the same physician performed both the surgical excision and the pathologic interpretation, for every stage
• The number of stages and tissue blocks processed at each stage, correlating to the codes and units billed
• Mapping/color-coding of specimens
• Medical necessity for any special stains (88314) or paraffin sections used, including the specific clinical reason
• Repair details (type, size, location) when a repair is billed separately

Legible, complete documentation isn’t just a compliance nicety it’s what Medicare and other payers require to be made available upon request, and it’s the first thing an auditor will ask for.

Why LCDs, Articles, and Commercial Payer Policies Matter

Mohs coding isn’t static. Local Coverage Determinations (LCDs) and Billing and Coding Articles issued by Medicare Administrative Contractors (MACs) are revised periodically and can differ from one jurisdiction to the next covering everything from accepted ICD-10 diagnosis codes to specific modifier instructions. Commercial payers frequently layer on their own requirements on top of Medicare’s baseline rules some, for example, require documentation of board certification or subspecialty credentialing in Mohs surgery before they’ll reimburse 17311–17315 at all, and others have their own prior authorization thresholds.

Because these policies are updated regularly and vary by payer, staying current isn’t optional. A claim coded correctly under last year’s LCD or under Medicare’s rules but not a specific commercial payer’s overlay can still be denied. Building a habit of checking the applicable LCD/Article and payer-specific policy before submitting Mohs claims is one of the most effective ways to reduce denials and stay ahead of audit risk.

This article is intended for general educational purposes and does not constitute legal or coding advice for a specific claim. Always verify current requirements against the applicable LCD/Article, CPT guidelines, and individual payer policies before billing.