SNF billing is the process of claiming Medicare payment for skilled nursing facility care, primarily under the Part A prospective payment system. Facilities bill a per-diem rate determined by PDPM classification, submitted on a 21X type of bill, and must document a qualifying three-day inpatient hospital stay, physician certification, and a daily skilled level of care.

Almost every expensive SNF billing mistake traces back to one of three things: an eligibility condition that was never verified, a level-of-care justification that the documentation does not support, or a coding element that did not match what the MDS said.

What makes SNF billing genuinely difficult is that these rules exist almost nowhere else in Medicare. Consolidated billing has no real analogue in other settings. The benefit period reset mechanic confuses even experienced hospital billers. And the payment amount is derived from a clinical assessment instrument rather than from the claim itself, which means the business office cannot fix a coding problem without going back to nursing.

This guide is organized the way a stay actually unfolds: eligibility, coverage, payment construction, claim mechanics, notices, and denials.

Part 1: Eligibility — before you can bill at all

Hathr.AI makes it easy to be able to speed up your process to determine eligibility to prevent additional work for your team and file required paperwork to keep everyone on track.

What is the Medicare 3-day rule?

The Medicare 3-day rule is the requirement that a beneficiary have a medically necessary inpatient hospital stay of at least three consecutive days before Medicare Part A will cover a subsequent skilled nursing stay. The requirement is set out at 42 CFR 409.30.

Three details cause most of the trouble:

The day of discharge does not count. Admission Monday, discharge Wednesday is two days, not three. Count admission day through the day before discharge.

Observation status does not count. This is the single most common eligibility failure. A patient can spend four nights in a hospital bed, receive hospital care, and still not have a qualifying stay, because the hospital classified the encounter as outpatient observation. The patient generally does not know this. Neither does the family. Sometimes neither does the referring discharge planner.

Verify inpatient status from the hospital record before admission, not after. A facility that admits on the assumption of a qualifying stay and discovers otherwise on day twelve has twelve days of uncovered care and a very difficult conversation ahead.

The stay must be medically necessary. A hospital admission solely for the purpose of establishing SNF eligibility does not qualify.

Note that certain Medicare Advantage plans and some Accountable Care Organization or demonstration arrangements waive the three-day requirement. Waivers are plan-specific and must be confirmed for the individual beneficiary — never assumed from the plan name.

The 30-day transfer window

Admission to the SNF must generally occur within 30 days of hospital discharge. There is a narrow exception where it would be medically inappropriate to begin skilled care within 30 days — for instance, when a condition requires a healing interval before rehabilitation can begin — but this must be documented and is scrutinized.

Skilled level of care

The beneficiary must require skilled nursing or skilled rehabilitation services on a daily basis, which as a practical matter means skilled nursing seven days a week or skilled therapy at least five days a week, and those services must as a practical matter only be available on an inpatient basis.

This is where documentation quality determines revenue. "Resident tolerated therapy well" does not establish skilled need. A note that describes the specific skilled intervention, why it required licensed personnel, the resident's response, and what remains to be achieved does. Medical review contractors deny on the documentation, not on the reality — a resident who genuinely needed skilled care but whose notes read like custodial care will produce a denial.

Physician certification

Under 42 CFR 424.20, a physician (or, where permitted, a nurse practitioner, clinical nurse specialist, or physician assistant who does not have a direct or indirect employment relationship with the facility but is working in collaboration with the physician) must certify that the beneficiary requires skilled care.

Certification is required at admission, with recertification no later than the 14th day of the stay and at intervals not exceeding 30 days thereafter. Missing or late certifications are a common and entirely preventable technical denial. Build a hard calendar control, not a reminder.

Part 2: Coverage — how long and at what beneficiary cost

What is a Medicare benefit period?

A Medicare benefit period is the unit Part A uses to measure inpatient coverage. It begins the day the beneficiary is admitted as an inpatient to a hospital or skilled nursing facility, and ends when they have been out of any hospital or SNF for 60 consecutive days. It is not a calendar year, and there is no limit on how many benefit periods a beneficiary may have in a lifetime.

Within each benefit period, Medicare Part A covers up to 100 days of skilled nursing care:

• Days 1–20: paid in full by Medicare
• Days 21–100: beneficiary owes a daily coinsurance amount that CMS updates annually
• Day 101 onward: no Part A coverage

Coverage is not automatic through day 100. It continues only while the beneficiary meets skilled criteria. A resident who plateaus on day 34 is no longer covered on day 35, regardless of remaining benefit days.

How a benefit period resets

Once the beneficiary has been out of any hospital or SNF for 60 consecutive days, a new benefit period begins and a fresh 100 days becomes available — but a new qualifying three-day hospital stay is also required before Part A will pay for another SNF admission.

The 60-day clock is the part people get wrong. Days spent in the facility at a custodial level of care still count as SNF days for benefit period purposes even though Medicare is not paying. A resident who converts to long-term care and remains in the building does not accrue days toward the 60-day break.

Medicare benefit period examples

Three cases that cover most of what a business office encounters:

1. A clean reset. Resident admitted to hospital 1 March, transfers to the SNF, uses 40 Medicare days, discharges home 25 April. They remain home with no hospital or SNF admission through 24 June — 60 consecutive days. On 25 June the old benefit period has ended. A hospitalization on 1 July with a qualifying three-day stay starts a new benefit period with a full 100 days available.
2. No reset, because they never left. Same resident uses 40 Medicare days, then plateaus and converts to long-term care in the same building. Six months later their condition changes and they need skilled care again. No 60-day break has occurred, because SNF residency continues to count. They have 60 days remaining in the original benefit period, not 100 — and they still need a new qualifying hospital stay.
3. The clock restarts. Resident discharges home on day 45 of the break, is hospitalized for two days on day 46, and returns home. The 60-day count restarts from that hospital discharge. It does not resume from 46.

The practical control: track the benefit-day count and the 60-day break status on the same live report, and reconcile it at admission for every readmission. Assuming a fresh 100 days is one of the more expensive assumptions in the revenue cycle.

Medicare Advantage

Roughly half of Medicare beneficiaries are now enrolled in Medicare Advantage, and MA plans are not bound to fee-for-service rules. Expect prior authorization requirements, concurrent review, plan-specific level-of-care criteria, contracted per-diem or case rates rather than PDPM, different notice obligations, and different appeal pathways.

The operational error is running MA admissions through the fee-for-service playbook. Authorization lapses are the leading cause of MA revenue loss in skilled nursing, and they are almost always administrative rather than clinical.

Part 3: How the payment amount is built

PDPM per-diem construction

Under the Patient Driven Payment Model, the daily rate is the sum of five component payments — physical therapy, occupational therapy, speech-language pathology, nursing, and non-therapy ancillary — plus a non-case-mix component. Each component is classified from MDS data. See the full PDPM explainer for the classification logic.

The critical structural point for billing: the payment is determined by the MDS, not by the claim. The business office cannot correct a payment problem by changing the claim. If the rate is wrong, the assessment is wrong, and the fix runs back through the MDS coordinator.

Variable per-diem adjustment

PDPM rates are not flat across the stay. Two components adjust over time:

• PT and OT components taper after day 20, declining incrementally through the remainder of the stay, reflecting that therapy intensity typically front-loads
• The NTA component is paid at a multiple for the first three days of the stay, reflecting concentrated pharmacy and supply costs at admission

This structure has a direct implication for the interrupted stay policy below, and it means that modeling expected revenue requires day-level rather than average calculation.

Consolidated billing

During a covered Part A stay, most services furnished to the resident are bundled into the facility's per-diem, and the facility — not the outside supplier — is responsible for payment. This is why an outside provider who treats your resident may send you a bill you did not expect. A limited set of high-cost services is excluded from the bundle and separately billable. See the consolidated billing guide for the exclusion categories and how to manage outside-provider relationships.

Part 4: Claim mechanics

Bill types

• 21X — inpatient Part A SNF claims. The third digit is the frequency code: 1 for admit-through-discharge, 2 for interim first claim, 3 for interim continuing, 4 for interim last claim, 7 for replacement, 8 for void
• 22X — Part B services for residents not in a covered Part A stay
• 23X — outpatient services

SNF Part A claims are generally submitted monthly rather than at discharge, which means most stays generate a sequence of interim claims and sequencing errors are common. Claims must process in order; a rejected mid-sequence claim blocks everything after it.

Key coding elements

• HIPPS code — a five-character code derived from the MDS that communicates PDPM classification to the payer. It must match the assessment exactly
• Occurrence span code 70 — reports the qualifying hospital stay dates. Omission or mismatch against the hospital claim is a frequent rejection
• Revenue codes — accommodation and ancillary revenue codes describing services rendered
• Principal diagnosis — must support the skilled need and, under PDPM, maps to a clinical category driving PT, OT, and SLP classification. A vague or non-specific principal diagnosis both weakens the medical necessity case and can misclassify the payment
• Value codes — including beneficiary liability amounts for coinsurance days

For the Part B side — which CPT codes apply, therapy modifiers, and the 8-minute rule — see the SNF coding reference.

Interrupted stay policy

Under PDPM, if a resident is discharged from the facility and returns to the same facility by midnight of the third calendar day, the stay is treated as a continuation rather than a new admission.

Practically: the variable per-diem schedule resumes where it left off rather than resetting, no new 5-day PPS assessment is required, and the interruption is reported on the claim rather than as a discharge and readmission.

If the resident returns on day four or later, or returns to a different facility, it is a new stay — new assessment, variable per-diem restarts, and the NTA multiplier applies again.

The revenue difference between these two treatments is material, and misapplication in either direction creates exposure. Getting it wrong in the facility's favor is an overpayment; getting it wrong the other way is unclaimed revenue.

Timely filing

Medicare claims must generally be filed within one calendar year of the date of service. For SNF interim billing, this is measured against service dates rather than discharge, so a long stay can have early service dates aging toward the limit while the resident is still in the building.

Part 5: Beneficiary notices

Notice failures are among the cleanest denials to avoid and among the most commonly missed.

SNF ABN (Form CMS-10055)

The Skilled Nursing Facility Advance Beneficiary Notice informs a beneficiary that Medicare is not expected to pay for certain items or services, allowing them to decide whether to accept financial responsibility. It must be delivered before the non-covered care begins, describe the specific services and the reason for expected non-coverage in language the beneficiary can understand, and state an estimated cost. See the SNF ABN guide for delivery scenarios and completion detail.

NOMNC (Form CMS-10123)

The Notice of Medicare Non-Coverage must be delivered at least two calendar days before Medicare-covered services end. Its purpose is to give the beneficiary the opportunity to request an expedited determination from the Quality Improvement Organization.

The two-day requirement is calendar days, and the notice must be signed by the beneficiary or an authorized representative. Late delivery can extend the facility's liability.

DENC (Form CMS-10124)

If the beneficiary appeals to the QIO, the facility must supply a Detailed Explanation of Non-Coverage explaining the specific clinical basis for the coverage decision, generally by close of business the day the QIO notifies the facility of the appeal. Turnaround is same-day, which is why having the clinical rationale already assembled matters.

Part 6: The denials that cost the most

1. No qualifying hospital stay. Observation status misread as inpatient. Entirely preventable at admission.
2. Level of care not supported. Documentation reads custodial. The most expensive denial category because it is usually discovered on a large ADR sample.
3. Missing or late physician certification/recertification. A technical denial on care that was clinically appropriate.
4. HIPPS code mismatch. Claim does not agree with the transmitted MDS.
5. Occurrence span code 70 errors. Dates do not reconcile with the hospital claim.
6. Interrupted stay misapplication. Treated as new admission when it was a continuation, or the reverse.
7. MA authorization lapse. Days delivered beyond the authorized period.
8. Consolidated billing errors. Outside supplier billed Medicare directly for a bundled service.
9. Benefit period miscount. Days billed after exhaustion, or a reset applied that was not earned.
10. Notice failures. No NOMNC, late NOMNC, or unsigned notice.

Where AI helps in the SNF revenue cycle

Nearly every item on that denial list is a document-comparison problem, and that is precisely the work a HIPAA-compliant AI platform can carry.

• Pre-bill audit. Compare the claim against the MDS, physician certifications, and the clinical record, and flag HIPPS mismatches, missing certifications, and occurrence span discrepancies before submission
• Qualifying stay verification. Extract admission status and dates from a hospital transfer packet and confirm inpatient rather than observation status at admission — before the days are delivered, not after the denial
• Benefit period reconstruction. Read a readmission packet and prior stay history and determine whether a 60-day break actually occurred, which is the question behind most benefit-period miscounts
• Level-of-care documentation review. Read a week of skilled nursing notes and identify which days lack language supporting skilled need — while the resident is still in the building and the documentation can still be strengthened prospectively and truthfully
• ADR response assembly. Read the full requested record and draft a coverage narrative tying specific documented interventions to the applicable criteria, with citations to the page and date
• Appeal letter drafting. Produce a redetermination or reconsideration argument organized around the denial reason and grounded in the record. See the appeals guide

The practical obstacle for most facilities is that these records are large, scanned, and mixed-format. An ADR packet is not a tidy text file. Hathr.AI runs Anthropic Claude models inside AWS GovCloud under a FedRAMP High authorization boundary, reads handwritten physician orders, and processes full denial packets without truncation — which is the difference between an AI tool that helps with revenue cycle and one that cannot open the file. A signed Business Associate Agreement is in place within 24 hours on every plan.

Every output remains a draft for review by qualified staff. AI should never assert a clinical fact the record does not contain; the value is in finding, organizing, and articulating what is already documented.

Test it on a real denial

Upload a denied claim and the corresponding medical record, and ask Hathr.AI to draft the redetermination request.

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Frequently asked questions

What is the Medicare 3-day rule?
The requirement that a beneficiary have a medically necessary inpatient hospital stay of at least three consecutive days, not counting the discharge day, before Medicare Part A will cover a subsequent skilled nursing stay. Observation status does not count.

What is a Medicare benefit period?
A benefit period begins when a beneficiary is admitted as an inpatient to a hospital or skilled nursing facility and ends after they have been out of any hospital or SNF for 60 consecutive days. Each benefit period carries up to 100 days of Part A skilled nursing coverage.

When does a Medicare benefit period start over?
After 60 consecutive days out of any hospital or skilled nursing facility. Custodial days in a SNF still count as SNF days, so a resident who converts to long-term care in the same building does not accrue days toward the break.

How many days does Medicare cover in a skilled nursing facility?
Up to 100 days per benefit period. Days 1–20 are paid in full; days 21–100 carry a daily coinsurance. Coverage requires continued skilled need throughout.

What bill type is used for SNF Part A claims?
Type of bill 21X, with the third digit indicating claim frequency. Part B services for residents not in a covered Part A stay use 22X.

What is the SNF interrupted stay policy?
If a resident returns to the same facility by midnight of the third day, the stay continues rather than restarting. The variable per-diem resumes and no new 5-day assessment is required.

When must a facility issue a NOMNC?
At least two calendar days before Medicare-covered services end.

How long does a facility have to file a Medicare claim?
Generally one calendar year from the date of service.

Part of the HIPAA-Compliant AI for Skilled Nursing Facilities hub. Related: SNF Consolidated Billing · The SNF ABN · SNF CPT Codes · Appealing a Medicare Denial

This article is general regulatory information, not legal, billing, or compliance advice. Coinsurance amounts and payment rates change annually. Verify current requirements against the Medicare Benefit Policy Manual, the Medicare Claims Processing Manual, and your MAC's guidance before acting.