How do you bill Mohs surgery stages and blocks (17311 and its add-ons)?

What counts as a stage, and what counts as a block?

A stage is one complete pass at the tumor: excise the specimen, divide it into blocks, map it, color code it, process and stain it, and read the margins. A block is one piece of that specimen embedded, cut and mounted for reading. One stage can hold one block or fifteen. That distinction decides which add-on code goes on the claim, and it is what the ranking pages get wrong most often.

The descriptor for 17311 covers the Mohs technique including removal of all gross tumor, excision of tissue specimens, mapping, color coding, microscopic examination by the surgeon, and histopathologic preparation with routine stains, for the head, neck, hands, feet, genitalia, or any location involving muscle, cartilage, bone, tendon, major nerves or vessels, first stage, a maximum of 5 tissue blocks. 17313 is the same for the trunk, arms or legs.

Which Mohs CPT code goes with which stage?

Five codes, two base and three add-on:

• 17311, base code, head, neck, hands, feet, genitalia or any site involving muscle, cartilage, bone, tendon, major nerves or vessels. First stage, 5 tissue blocks at most.
• 17312, add-on, each additional stage after the first stage, 5 tissue blocks at most. Reported with 17311.
• 17313, base code, trunk, arms or legs. First stage, 5 tissue blocks at most.
• 17314, add-on, each additional stage after the first stage, 5 tissue blocks at most. Reported with 17313.
• 17315, add-on, each additional block after the first 5 tissue blocks, any stage.

Wisconsin Physicians Service states it directly: 17312 is reported for additional stages with 17311, 17314 for additional stages with 17313, and 17315 may be used to report each block after the first 5 blocks for any single stage.

Is 17315 a stage code or a block code?

A block code, and nothing else. Several pages ranking for this question describe 17315 as the code for stages beyond the fifth or seventh, and one widely read vendor page states that beyond five blocks in a single stage no additional code exists. That is backwards. 17315 exists for exactly that situation, in any stage, the first included.

Work it through. A first stage on the nose that needed 8 blocks is 17311 once plus 17315 times 3, because blocks six, seven and eight each earn a unit. A second stage of 3 blocks adds 17312 times 1 and no 17315. Count blocks per stage, not per case: four stages of four blocks each is sixteen blocks and zero units of 17315.

How do the stages and blocks go on the claim lines?

Totaled as units on one line, never split across several. Both Novitas and WPS use the same words: multiple instances of 17312 and 17314 should not be reported on separate claim lines, and the services should be totaled and entered as a single item with the appropriate number of units.

This rule causes more rework than any other on the topic. A four-stage case on the cheek is not four lines. It is 17311 at one unit, then 17312 at three units on one line. Split it and the later instances read as duplicates, a denial that costs a rework cycle on a clinically correct claim.

A worked example: three stages, one extra block, one repair

Take a two-provider dermatology practice collecting $180,000 a month, with one Mohs surgeon running about 70 cases a month. A basal cell carcinoma on the left ala takes three stages: 7 blocks, then 4, then 2 and clear. An intermediate repair closes the defect.

The claim carries four lines: 17311 at one unit, 17315 at two units for blocks six and seven of that first stage, 17312 at two units for stages two and three on one line, then the repair on its own line. Assume the practice is contracted at the BCBS and Anthem national average of $747.97 for 17311, with 17312 at $310 and 17315 at $45 a unit. The Mohs portion is $747.97 plus $90 plus $620, or $1,457.97, before the repair.

Now drop the 17315 line, the most common error we find. The claim still pays, cleanly, at $1,367.97. Nobody appeals it because nothing was denied. At $90 a session and 16% of sessions running past five blocks, that is about 11 sessions a month, or roughly $1,000 a month and $12,000 a year on one code, with no report in the practice that would ever show it.

The commercial spread on the first stage alone is wide: Aetna averages $863.85, UnitedHealthcare $846.29, BCBS and Anthem $747.97 and Cigna $642.09, so the same case is worth a third more under one contract than another.

Can you bill the repair, a biopsy, or an E/M on the same day as Mohs?

Repairs yes, a diagnostic biopsy sometimes, an E/M rarely. These get run together as one question and have three different answers.

Repairs, grafts and flaps

The NCCI Policy Manual, Chapter III, effective January 1 2026, says it in one sentence: repairs, grafts, and flaps are separately reportable with the Mohs micrographic surgery CPT codes. No modifier is named as a condition. Two pages currently ranking for this question state that failure to append modifier 59 to the repair causes automatic denial. NCCI does not say that, and a policy built on it puts modifiers on the claim that do not belong there.

Contractors do require everything from the session to travel together: all procedures in the same operative session, repairs included, go on one claim. Check each payer policy on simple closures, since several commercial plans bundle simple closure into the primary procedure while paying intermediate and complex repairs separately.

A diagnostic biopsy on the same day

NCCI permits it with conditions. If a suspected skin cancer is biopsied for pathologic diagnosis before proceeding to Mohs, the biopsy and frozen section pathology may be reported separately using modifier 59, XS, XU, or 58 to distinguish it from the definitive Mohs surgery. Modifier 58 is explicitly acceptable as an alternative to 59, which almost no competing page mentions.

The limits are firm. Modifier 59 is not appropriate when the biopsy and Mohs surgery are performed on the same lesion, in the same operative session and on the same date of service, and it belongs on the biopsy and pathology lines rather than the Mohs line. Contractor guidance sets three conditions for the separate biopsy: the lesion has not been biopsied in the previous 60 days, the surgeon cannot obtain a pathology report from the referring physician with reasonable effort, or the biopsy is on an unrelated lesion.

An E/M visit on the same day

This one is contested and no MAC article settles it. The defensible position is narrow: modifier 25 is for a significant, separately identifiable problem addressed that day, and the conversation in which the surgeon decides how to close the defect is not that, it is pre-service work for the repair. An unrelated inflamed cyst is different. Across 61,400 claims we audited, same-day procedures billed with an E/M visit were denied for modifier 25 on 14% of claims. Settling coverage before the case, through eligibility and prior authorization, clears a different set of denials off the same claim.

What happens when someone else reads the slides?

It stops being Mohs. The codes assume one physician acting in two integrated and distinct capacities, surgeon and pathologist. NCCI puts it plainly: a single physician performs both the surgery and pathologic examination of the specimens.

The pathology is already inside the payment. NCCI states that the Mohs codes include skin biopsy and excision services, specifically 11102 to 11107, 11600 to 11646 and 17260 to 17286, and pathology services 88300 to 88309 and 88329 to 88332, and that reporting these in addition to the Mohs codes is inappropriate. Novitas adds the same rule: Mohs surgery includes the pathologic examination of the specimen performed by the surgeon and should not be reported separately.

When specimens go to a dermatopathologist, the answer is not to bill Mohs and let the pathologist bill separately. Novitas instructs that the service is then not considered a Mohs service and the surgeon should report excision and repair using the appropriate codes. Florida Blue puts it the same way: if either part of the procedure is delegated to another physician who reports it separately, the procedure should not be reported as Mohs micrographic surgery. That is a practice-design decision, better made deliberately than discovered in an audit. One operational requirement sits alongside it: a CLIA certification number is required on all claims submitted for Mohs surgery.

Why is Medicare auditing 17312 and 17314 units?

Because the units are wrong often enough that CMS built a national audit around them, and almost nobody writing about Mohs billing mentions it.

Recovery Audit Program approved issue 0150, Mohs Micrographic Surgery: Incorrect Coding and Incorrect Units Billed, is a complex review of professional services across all A/B MACs, approved on March 5 2019 and still listed. Reviewers verify that the physician performing the Mohs surgery is acting as both surgeon and pathologist, and determine whether the correct number of units have been billed for additional staging units for 17312 and 17314. Billing of excessive or insufficient units or a change in coding will be adjusted accordingly.

The audit corrects units in both directions, so a practice that consistently underbills stages is not safe from it. The defense is documentation: contractor guidance requires the record to show the location, number and size of the lesions, the number of stages performed, and the number of specimens per stage, with the operative note showing that the physician performed both the surgical and the pathology service.

What the Mohs note has to carry

1. Lesion location, size and diagnosis, with laterality.
2. Number of stages, stated as a number rather than implied by the narrative.
3. Number of tissue blocks in each stage, separately, since 17315 is priced on it.
4. The Mohs map with block orientation, color coding and margin status.
5. Histopathologic findings the surgeon read, per block.
6. The decision to take another stage or close, tied to margin status.
7. The repair documented as a distinct service after tumor clearance.

Contractor guidance also asks the record to show why Mohs was chosen, on the complexity, size or location of the lesion. Medical necessity lists are not universal: the Novitas article carries 142 supporting ICD-10-CM codes and the WPS article 176, and codes outside those lists are non-covered.

What do practices get wrong when billing Mohs stages and blocks?

Ranked by how often they appear in our audit of Mohs and dermatology claims: a stage past five blocks with no 17315 at 16%, a same-day E/M denied for modifier 25 at 14%, 17312 or 17314 split across separate claim lines at 13%, a repair performed but never billed at 9%, and pathology codes billed alongside the Mohs codes at 6%.

The order matters more than the percentages. The first and third errors never produce a denial, so no denial report surfaces them. They live in paid claims, which is why a Mohs practice can run a clean claim rate in the high nineties and still be underpaid on its highest value procedure. Working the ones that do deny is ordinary denial and AR recovery work, but it will not find a block code that was never billed. Two more errors sit outside the list because they are structural: billing 17311 where the trunk code 17313 applies, or the reverse, and carrying a Mohs habit into a case whose slides went to a dermatopathologist.

Should Mohs billing sit in house or with a billing partner?

In house wins when one named person reads Mohs remits line by line every week and knows what a correct four-line claim looks like. That is a real job, not a duty bolted onto a front desk, and it is the only thing that catches an underbilled block. The arithmetic is less lopsided than vendors suggest: 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. Price is not the deciding number. What the silent errors cost is.

Across 38 client practices, first-pass denial rate fell from 14.2% to 6.1% within 90 days of onboarding and clean claim rate rose from 89.6% to 97.3%. That is the visible half. The block-code half only moves when somebody reads paid claims.

Outsourcing is one option, and the wrong purchase for a practice that already has a coder who knows the Mohs edits. If you are comparing partners, ask whether they audit paid Mohs remits at line level and whether they will show underpayment reporting by code, ground covered in how to evaluate a medical billing company. Worth knowing first: 44% of practice managers could not name the fee basis in their current billing contract. Where a practice wants the whole cycle handled inside its existing system, full-service medical billing covers it, with the wider mechanics in our revenue cycle management overview. To have someone read your last quarter of Mohs claims, a free billing review starts there.

17312, 17314 or 17315: which add-on does this session need?

The three add-on codes answer different questions. Two of them ask how many times you went back to the tumor. The third asks how many pieces one of those trips was cut into. Confusing the two is the most expensive mistake on the topic.

One sentence worth memorizing: stages are counted per case, blocks are counted per stage.

Dermatology

The home specialty, carrying the whole problem. A fellowship-trained Mohs surgeon reading their own frozen sections is the only configuration the codes describe, and exposure runs both ways: the RAC audit corrects units up or down, while the silent losses sit in unbilled 17315 units and unbilled repairs. The fix is a weekly line-level read of paid Mohs remits against the operative notes, checking block counts per stage rather than stage counts per case. Everything else in dermatology billing is easier than this.

Plastic surgery

Plastic surgeons usually arrive after the Mohs, closing the defect the dermatologist left, which makes the repair the whole claim. The NCCI position that repairs, grafts and flaps are separately reportable with the Mohs codes is the sentence that protects it. Two things go wrong: the closure gets bundled by habit into somebody else's Mohs claim, and a flap performed days later hits a global period question nobody checked. Confirm the global indicator for the repair code in that year's fee schedule file rather than assuming. See plastic surgery billing.

Podiatry

Foot lesions fall under 17311 and 17312, not the trunk codes, because the descriptor names hands and feet alongside head and neck. A podiatry practice billing 17313 for a heel lesion is underpaid on every case, and the error is quiet because the claim pays. The same practices carry a separate exposure: class finding Q modifiers were missing on 16% of Medicare routine foot care claims in our audit. Podiatry billing covers the routine care side.

Otolaryngology and head and neck

Ear, nose and lip lesions are the highest-block work in the field, so this is where 17315 earns its keep and where omitting it costs most. These cases involve cartilage often enough that the muscle, cartilage, bone, tendon, major nerves or vessels clause in the 17311 descriptor matters: a lesion anywhere on the body involving those structures takes the higher-paying site code. Otolaryngology billing covers the same patients' reconstruction.

Pathology and laboratory

The instinct in a practice with an in-house lab is to bill the pathology. On a true Mohs case there is nothing to bill: 88300 to 88309 and 88329 to 88332 sit inside the Mohs payment, and the lab's role is CLIA compliance and slide quality, not a claim line. The lab does bill on the non-Mohs case, when slides go to a separate reader and the surgeon drops to excision and repair codes. Sorting which case is which before claims go out prevents a duplicate-pathology pattern that reads badly in an audit. Pathology billing covers standalone work.