A physician goes on maternity leave in December. A partner has surgery and is out for eight weeks. A practice loses a provider in October and the replacement does not start until spring. In each case the schedule is full, a locum tenens agency has a physician available next week, and the question that reaches the billing office is the same: how do we bill for this person?
Medicare has a clear answer with clear limits, and most of the trouble we see comes from practices that know the first half of the rule and not the second. The substitute can be billed under the regular physician's NPI, yes. But only while the regular physician is actually absent, only for 60 continuous days, only with the right modifier on every line, and only with a record on file that names the substitute. Commercial payers have their own answers, and several of them are "no."
Key takeaways
- Medicare's fee-for-time arrangement (modifier Q6) and reciprocal billing (modifier Q5) let a substitute be billed under the absent physician's NPI, with the modifier on every line.
- The limit is 60 continuous calendar days from the first substitute service, not 60 days worked, and it applies to covering a vacancy as well as a leave.
- The regular physician must keep a record of every substitute service, including the substitute's NPI.
- Many commercial payers and Medicaid programs do not accept Q6 or Q5; ask each one before the first day and write down the answer.
- If the need may exceed 60 days, start the locum's own enrollment on the day the contract is signed.
The two Medicare arrangements
Medicare recognizes two ways for one physician to bill for another's services. Both live in Chapter 1 of the Medicare Claims Processing Manual, and both were extended and renamed by the 21st Century Cures Act, signed December 13, 2016, which also allowed physical therapists in health professional shortage areas, medically underserved areas and rural areas to use them.
| Arrangement | Modifier | Typical situation | How the substitute is paid |
|---|---|---|---|
| Fee-for-time compensation (what everyone still calls locum tenens) | Q6 on every line | Agency locum covering a leave, an illness or a vacancy | Per diem or similar fee-for-time basis by the practice or agency, not a share of collections |
| Reciprocal billing | Q5 on every line | Two physicians who cover each other's patients informally, often across practices | No payment between physicians; each bills for the other while covering |
In both arrangements the claim goes out under the regular physician's NPI as the rendering provider, with the modifier appended to each procedure code. The regular physician, or the group, is the billing party and receives the payment. The patient must be someone who arranged to see, or would normally see, the regular physician; a locum cannot be used to build a new panel under another physician's number. Within a group practice, a physician covering for a colleague in the same group does not need either arrangement at all: the covering physician simply bills under their own NPI, because the group is already paid for both.
The conditions that must all be true
- The regular physician is unavailable. Illness, leave, vacation, continuing education, military service. A physician who is in the building seeing their own patients cannot have a locum billed under their number at the same time.
- The substitute is paid on a fee-for-time basis (for Q6) and is not being brought on as a permanent member of the group under this arrangement.
- The continuous period does not exceed 60 days. The period runs from the first day the substitute sees a patient to the last, counting calendar days, not days worked. A locum who covers Mondays only for four months has still exceeded 60 continuous days. The one exception is a regular physician called to active duty in the Armed Forces, where the limit does not apply.
- The regular physician keeps a record of each service provided by the substitute, including the substitute's NPI, available to the Medicare Administrative Contractor on request.
- The substitute is eligible. Check that they have an NPI, are not on the OIG exclusion list, and have not opted out of Medicare. The regular physician is responsible for the claim.
Medicare also permits a group to use a substitute to cover for a physician who has left the group, billing under the departed physician's NPI, but the same 60-day limit applies from the first substitute service. This is the rule practices misread most often. A vacancy that lasts five months is not a locum arrangement after day 60; it is a credentialing problem that was postponed.
Counting the 60 days: a worked example
Suppose a physician begins leave on Monday, November 30, 2026, and an agency locum sees the first patient on Tuesday, December 1. Day 1 is December 1. Counting calendar days, day 60 is Friday, January 29, 2027. Every Medicare service the locum provides from December 1 through January 29 can be billed under the absent physician's NPI with Q6. A service on Monday, February 1 cannot, even though the locum only worked three days a week and the practice was closed for two holidays in between. The calendar ran; the schedule did not matter.
Now suppose the regular physician returns on January 18 for two full weeks of clinic, then goes out again on February 1 for a second surgery. The first continuous period ended on January 17. A new 60-day period can begin with the first substitute service on or after February 1, because the regular physician genuinely resumed practice. What does not work is a physician who comes in for a single half-day on January 29 so the clock can "reset." The rule contemplates a real return to work, and we would not defend the half-day version in an audit. Put the day-60 date on the calendar the day the locum contract is signed, with a reminder at day 45.
What happens on day 61
Claims for a substitute beyond 60 continuous days under the absent physician's NPI are not payable, and if they were paid, they are overpayments. In our experience the practice usually discovers this during an audit or when a new billing manager asks why one physician's volume doubled during a leave that should have shown zero. The remedy is to refund the overpaid claims through the normal overpayment process and, if the substitute is still there, to have already started their own enrollment. Services the locum provided after day 60 that were never billed cannot be billed under the absent physician, and cannot be billed under the locum's own NPI until enrollment is effective, which is the real cost of starting late.
Commercial payers and Medicaid
Medicare's rule is not universal. Many commercial payers do not recognize Q6 or Q5 and expect the rendering provider on the claim to be the person who actually saw the patient, which means the locum must be credentialed and linked to the group before their claims will pay. Some payers accept the Medicare approach for a defined period if notified in advance; some accept it silently; some deny every line. State Medicaid programs vary in the same way.
Before the locum's first day, call each of your top payers, ask the specific question ("Do you accept modifier Q6 for a substitute physician billing under our absent physician's NPI, and for how long?"), and write the answer, the date and the representative's name in the payer file. Then set a claim rule for each answer: payers that accept Q6 get the modifier; payers that require notification get a letter before the first claim and the modifier after; payers that refuse have their claims held until the locum's own enrollment is effective, or the visit is scheduled with another provider. Do not rely on a general assumption that "payers follow Medicare"; for this rule, many do not, and a payer that pays Q6 claims silently for a year can still recoup them.
When to credential the locum
If the coverage need will last more than 60 days, or the substitute is likely to be offered a permanent position, start enrollment on the day the contract is signed. The work is the same as for any new provider: a Medicare enrollment or reassignment through PECOS (Medicare allows an effective date up to 30 days before the application is received, which covers a short gap), a CAQH profile that is current and attested, and credentialing applications to each commercial payer, which routinely take 90 to 150 days. Locum agencies usually carry the physician's malpractice coverage and can supply the certificate, license copies and work history quickly; ask for the packet before the first day. Hospital privileges, if needed, are a separate and often slower track. Our article on common credentialing delays covers the timeline in detail.
Practices sometimes resist credentialing a locum because it costs time and the person may leave. We think the arithmetic favors doing it anyway for anything beyond a few weeks: the enrollment work is measured in hours, and the alternative is either unbillable visits or claims that do not comply. If the locum leaves before the applications finish, withdraw them; nothing is lost but the hours.
Questions we hear
Can a nurse practitioner or physician assistant be a locum under this rule?
Medicare's fee-for-time and reciprocal billing provisions apply to physicians, and to physical therapists in the areas the Cures Act specified. For non-physician practitioners, the usual answer is to enroll them directly, and to check each commercial payer's policy separately.
Does the 60-day clock stop on weekends or weeks the locum does not work?
No. It is a continuous calendar period from the first substitute service to the last. Gaps in the schedule do not pause it.
What if the absence is open-ended?
Plan as if it will exceed 60 days and begin the locum's own enrollment immediately. If the regular physician returns early, the applications can be withdrawn. Our credentialing and provider enrollment team handles locum enrollments routinely; the common mistake is starting in week seven rather than week one.
What to do this week
- Put the written locum agreement on file with start date, expected end date and the fee-for-time terms, and calculate the day-60 date.
- Collect the substitute's NPI, license, DEA, malpractice certificate and OIG exclusion check into the credentialing file.
- Set the substitute up in the practice management system as a non-billing provider linked to the regular physician, so the record of services is generated automatically.
- Add a claim scrubber rule that appends Q6 (or Q5) to every Medicare line where the substitute is the scheduled provider.
- Call each top payer, document its answer, and set the hold or notification rule for its claims.
- Decide now whether enrollment must begin, and if the answer is "maybe," begin it.
