FS Modifier in Medical Billing: Split/Shared E/M Guide
FS modifier indicates a split or shared evaluation and management (E/M) visit performed in a facility setting by both a physician and a qualified non-physician practitioner (NPP) from the same group practice. Appending this modifier to your professional claims designates the billing provider who performed the substantive portion of the encounter—such as more than half of the total time or key medical decision-making components—ensuring strict compliance with Medicare split-visit regulations and accurate claim adjudication.
FS Modifier in Medical Billing: Split/Shared Evaluation and Management Service Guidelines
A physician and a nurse practitioner sometimes both touch the same hospital visit, one reviewing labs and setting the care plan, the other doing the physical exam and talking through discharge instructions. Medicare has a specific rule for exactly this kind of shared work, and a specific modifier to flag it. Getting the underlying “who actually did the substantive part” question right is what determines whether the claim reflects reality or just whoever happened to hit submit.
The FS modifier identifies a split or shared evaluation and management visit, one performed in part by both a physician and a non-physician practitioner (NPP) from the same group practice in a facility setting, and it must be appended to the claim regardless of which of the two clinicians ends up billing for the visit. Payment goes to whichever provider performed the “substantive portion” of the encounter, defined as more than half the total time spent, or, for most visit types, the substantive part of the medical decision-making. This guide walks through how that substantive portion determination actually works, where FS applies and where it doesn’t, and how to keep these claims defensible.
What is the FS Modifier?
FS is a HCPCS Level II modifier that stands for Split (or shared) evaluation and management visit. CMS finalized the split/shared billing policy in the 2022 Medicare Physician Fee Schedule final rule, and the modifier itself became a required part of reporting these visits starting that same year, with the underlying substantive portion definition refined further in the years since.
A split or shared visit is an E/M encounter in a facility setting, performed in part by both a physician and a qualifying NPP who belong to the same group practice, structured so that the service could legally have been billed by either clinician independently if they’d furnished it alone. Rather than billing the visit under whichever provider happens to be more convenient, or defaulting to the physician automatically, Medicare requires the claim to reflect whoever actually performed the substantive portion of the work, and FS is the flag that tells the payer this was a shared encounter in the first place.
Why the FS Modifier Matters
Before this policy took full effect, split/shared billing had a reputation for ambiguity. Physicians and NPPs frequently collaborate on hospital and facility visits, and the rules for which provider’s name should go on the claim weren’t always applied consistently. That mattered financially, since physician and NPP reimbursement rates differ, and it mattered from a compliance standpoint, since the billing provider’s identity is tied directly to their documentation, their signature, and their attestation of the work performed.
FS exists to make that attribution explicit and auditable. It tells Medicare, unambiguously, that a claim reflects shared work between two providers rather than the independent effort of just one, and it ties directly into a documentation standard that requires the record to identify both participating clinicians and to be signed by whichever one actually performed the substantive portion, and is therefore doing the billing.
Where the FS Modifier Applies: Facility Setting Only
Split/shared billing, and FS along with it, is specifically limited to facility settings. Understanding what counts as a facility setting, and what doesn’t, is the first filter for whether FS is even relevant to a given visit.
| Setting | Eligible for Split/Shared Billing (FS)? |
|---|---|
| Hospital inpatient unit | Yes |
| Hospital outpatient department | Yes |
| Emergency department | Yes |
| Skilled nursing facility | Yes |
| Physician office (non-facility) | No, this falls under incident-to billing rules instead |
| Nursing facility (NF) visits | No, not eligible for split/shared billing |
Understanding the Substantive Portion
This is the single most important concept behind the FS modifier, because it’s what actually determines which provider bills the visit. CMS currently defines the substantive portion of a split or shared visit as either of the following, and either one qualifies:
| Substantive Portion Basis | What It Requires |
|---|---|
| More than half of the total time | The billing provider spent more than 50% of the total combined time both clinicians spent on the visit |
| The substantive part of the medical decision-making (MDM) | The billing provider made or approved the management plan for the number and complexity of problems addressed, and takes responsibility for that plan, including its associated risk |
This dual-track definition has a history worth knowing. CMS originally signaled it would move to a time-only standard starting in 2023, delayed that transition multiple times, and ultimately settled, as of 2024, on allowing either time or MDM to establish the substantive portion for most visit types. That policy has remained unchanged through 2025 and into 2026, though CMS has continued to note it’s still gathering feedback and hasn’t ruled out future revisions.
What Counts Toward Total Time
When time is being used to establish the substantive portion, CMS counts a fairly broad range of activities toward that total, and importantly, not all of it requires direct patient contact.
| Activity | Counts Toward Total Time? |
|---|---|
| Preparing to see the patient (e.g., reviewing prior test results) | Yes |
| Obtaining or reviewing separately obtained history | Yes |
| Performing a medically appropriate examination or evaluation | Yes |
| Counseling and educating the patient, family, or caregiver | Yes |
| Care coordination | Yes |
| Documenting clinical information in the medical record | Yes |
FS and Prolonged Services
Split/shared visits sometimes extend into prolonged service territory, additional time beyond what a given E/M code level typically covers. When a split/shared visit includes prolonged services, the same substantive portion logic applies, but it’s worth knowing that prolonged services should only be billed on a split/shared claim when time, specifically, is the basis being used to select the visit level and determine the substantive portion, not when MDM is the deciding factor. CMS has published specific guidance and a reference table covering exactly how prolonged service time should be reported in split/shared scenarios, since combining two providers’ time correctly for a prolonged service claim adds an extra layer of calculation beyond a standard-length visit.
FS Modifier Documentation Requirements
Documentation for the FS modifier should clearly identify each practitioner involved in the split or shared E/M visit and describe their individual contributions. The medical record should also support the substantive portion of the service and meet applicable payer requirements for billing the visit.
| Documentation Element | Why It Matters |
|---|---|
| Identification of both the physician and the NPP who participated in the visit | Confirms this was genuinely a shared encounter, not solo work misattributed |
| Clear documentation of who performed which elements of the visit | Supports the substantive portion determination, whether based on time or MDM |
| Total time logged by each provider, when time is the substantive portion basis | Establishes which provider's time exceeded 50% of the combined total |
| Clear MDM documentation showing who made or approved the management plan, when MDM is the basis | Confirms the billing provider genuinely performed the substantive part of the decision-making, not just some of it |
| Signature and date from the provider who performed the substantive portion | This is the individual actually billing the visit, and their attestation needs to be on the record |
| Confirmation both providers are enrolled in Medicare and have E/M services within their scope of practice | A foundational eligibility requirement for split/shared billing to apply at all |
Common FS Modifier Billing Mistakes
Common FS modifier billing mistakes include applying it when split or shared visit requirements are not met, failing to identify each practitioner’s contribution, or lacking documentation of the substantive portion. Incorrect use can result in claim denials, reduced reimbursement, and increased payer scrutiny.
| Mistake | What's Actually Happening |
|---|---|
| Applying FS to an office visit or a standard nursing facility visit | Split/shared billing, and FS, only apply in eligible facility settings; office visits fall under incident-to rules instead |
| Automatically billing under the physician regardless of who performed the substantive portion | Billing attribution has to follow the actual substantive portion determination, not a default assumption about seniority or convenience |
| Using MDM as the substantive portion basis for a critical care visit | Critical care split/shared visits require time as the substantive portion basis; MDM doesn't apply here |
| Failing to document which provider's time or MDM actually met the substantive portion threshold | Without that clarity, there's no support for whichever provider ends up billing the claim |
| Attempting to use modifier 52 (reduced services) to report partial E/M work in a split/shared context | CMS has specifically stated modifier 52 cannot be used to report partial E/M visits, including split/shared services; FS is the correct tool |
| Omitting FS from the claim because the physician is the one billing | FS is required on split/shared claims regardless of which provider, physician or NPP, ends up as the billing provider |
| Missing the face-to-face contact requirement entirely | At least one of the two providers must have had in-person contact with the patient during the visit, even if that provider isn't the one billing |
Best Practices for Billing the FS Modifier
Best practices for billing the FS modifier include verifying that the service meets current split or shared visit requirements, clearly documenting each practitioner’s involvement, and identifying who performed the substantive portion. Review payer-specific rules before claim submission to reduce denials, compliance risks, and reimbursement delays.
- Confirm facility setting eligibility before anything else: If the visit happened in a physician’s office or was a standard nursing facility visit, FS and split/shared billing don’t apply at all.
- Decide upfront, or as close to real-time as possible, whether time or MDM will be used to establish the substantive portion, except for critical care, where time is mandatory. This keeps documentation focused rather than reconstructed after the fact.
- Log time consistently for both providers when time is the basis: Vague or missing time documentation from either clinician makes the 50 percent threshold impossible to verify.
- Document MDM ownership explicitly when MDM is the basis: State clearly who made or approved the management plan and who is taking responsibility for its associated risk.
- Confirm both providers are enrolled in Medicare with E/M in scope, before assuming a specific case qualifies for split/shared billing at all.
- Train staff on the incident-to versus split/shared distinction: These two billing frameworks solve a similar problem in different settings, and applying the wrong one is one of the more common structural errors in this area.
- Review a sample of split/shared claims periodically against the underlying documentation, checking that the billing provider genuinely matches whoever performed the substantive portion under the correct basis.
Compliance Considerations
FS sits at the intersection of two things payers scrutinize closely: physician versus NPP reimbursement differentials, and the accuracy of medical record attestation. Because the modifier directly determines which provider’s name, NPI, and signature end up on the claim, incorrect substantive portion determinations aren’t just a documentation nuance, they represent billing under the wrong provider entirely.
CMS’s own guidance is explicit that documentation needs to clearly support whichever basis, time or MDM, is being used to justify the billing provider’s role. A pattern of split/shared claims that consistently bill under the physician regardless of actual time or MDM distribution is exactly the kind of utilization pattern that draws scrutiny, since it suggests the substantive portion determination isn’t genuinely being made case by case. The safest practice is treating that determination as a real, documented decision on every shared visit, not a default setting.
Frequently Asked Questions About the FS Modifier
What does the FS modifier mean in medical billing?
FS indicates a split or shared evaluation and management visit, performed in part by both a physician and a non-physician practitioner from the same group practice in a facility setting, with billing attributed to whichever provider performed the substantive portion of the visit.
Who bills for a split or shared visit?
Whichever provider, physician or NPP, performed the substantive portion of the visit, defined as more than half of the total combined time, or the substantive part of the medical decision-making, except for critical care visits, where time is the only allowed basis.
Does FS apply to office visits?
No, split/shared billing and FS apply only in facility settings, such as hospital inpatient units, hospital outpatient departments, emergency departments, and skilled nursing facilities. Office visits fall under separate incident-to billing rules.
Does FS apply to nursing facility visits?
No, standard nursing facility visits are not eligible for split/shared billing, even though a skilled nursing facility itself is a recognized facility setting for other types of split/shared E/M visits.
Does the billing provider have to be the one who saw the patient in person?
No, at least one of the two participating providers must have had face-to-face contact with the patient during the visit, but that doesn’t have to be the same provider who performed the substantive portion and bills the claim.
How is the substantive portion determined for critical care visits specifically?
For split or shared critical care services, the substantive portion is based on cumulative time across both providers only. Medical decision-making cannot be used as the basis for critical care split/shared visits.
Can modifier 52 be used instead of FS to indicate partial E/M work in a shared visit?
No, CMS has specifically stated that modifier 52 (reduced services) cannot be used to report partial E/M visits, including any portion of a split or shared visit. FS is the required modifier for this purpose.
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