When a physician goes on leave and a substitute covers the practice, the first billing question is: whose number goes on the claim? Under Medicare’s substitute-physician rules, the answer for a hired substitute is the regular physician’s number, with modifier Q6 appended. Get the arrangement wrong — wrong clinician, wrong modifier, or past the time limit — and the claim is wrong even if the visit was well documented. This guide covers the fee-for-time arrangement, Q6 vs Q5, whose identifier belongs where, the 60-day clock, and day 61.
Terminology note: Medicare now calls these “fee-for-time compensation arrangements”; “locum tenens” is the older name for the same arrangement, still the term most practices use.
Q6 vs Q5: the payment relationship decides the modifier
Two substitute-physician arrangements exist under Medicare, and they are not interchangeable. The difference is how the substitute is paid, not the job title.
Q6 — fee-for-time compensation. The regular physician hires a substitute and pays them per-diem or fee-for-time. The substitute is an independent contractor, not an employee of the regular physician, and typically has no practice of their own. This is what most people mean by “locum tenens.”
Q5 — reciprocal billing. A colleague with their own practice covers occasionally and mutually, with no fee-for-time payment passing between them. The covering physician is repaid by reciprocation later. Reciprocal arrangements need not be in writing.
If money is paid for time, the modifier is Q6. If coverage is exchanged between colleagues with their own practices, it is Q5.
| Q6 — fee-for-time | Q5 — reciprocal | |
|---|---|---|
| Substitute | Hired substitute, usually no practice of their own | Colleague with their own practice |
| Payment | Per-diem or similar fee-for-time payment | No fee-for-time payment; coverage reciprocated later |
| Claim marker | Q6 on each procedure code | Q5 on each procedure code |
| Time limit (Medicare) | 60 continuous days, with one exception below | 60 continuous days |
The five conditions for a valid Q6 claim
Medicare pays the regular physician only when all of these hold:
- The regular physician is unavailable — illness, vacation, parental leave, continuing education, or a similar absence.
- The patient is the regular physician’s patient. Q6 covers an existing panel, not a walk-in panel billed under someone else’s number.
- The substitute is paid fee-for-time, per-diem or similar. A productivity-based payment is not the same thing.
- The 60-day limit is respected — no services to the regular physician’s Medicare patients over a continuous period longer than 60 days, subject to the active-duty exception below.
- The claim is identified and the record is kept — services marked with Q6, plus a record of each service with the substitute’s National Provider Identifier (NPI), available on request.
In short: the substitute’s NPI belongs in the coverage log, not on the Q6 claim. The claim goes out under the regular physician; the log proves who saw each patient.
Whose NPI goes where on the claim
Append Q6 to each procedure code in Box 24D of the CMS-1500 form, or the corresponding electronic-claim field, and submit under the regular physician’s NPI — in a group practice, the regular physician’s NPI in the rendering-provider field (Box 24J). The substitute’s NPI is not the rendering provider on the claim.
A clean claims management setup matters: every line the substitute furnished must carry the regular physician’s identifiers plus Q6, while a separate log ties each date back to the substitute. If your system defaults to the clinician who saw the patient, Q6 claims go out wrong by default — check that before the first claim. See our CMS-1500 form guide for the field layout, and confirm payer-specific setup with your Medicare Administrative Contractor (MAC).
The 60-day clock: continuous, not 60 working days
Under Medicare, the clock runs as follows:
- It starts the first day the substitute provides covered services to the regular physician’s Medicare patients — not the day the regular physician leaves.
- It does not pause. Weekends, holidays, and days with no patients do not stop it.
- Rotating substitutes does not reset it. A second substitute covering the same physician keeps the same period running. Swapping at day 59 buys no fresh 60 days.
- It ends when the regular physician returns to work, and a new period can begin after that return.
Illustrative example (day counts only): A substitute first sees the regular physician’s Medicare patients on Day 1; days with no patients still count. The regular physician may bill the first 60 days with Q6; from Day 61, the substitute bills under their own NPI. After the regular physician returns and later leaves again, a new period can begin.
The only Medicare extension is active duty in the Armed Forces for the regular physician. Vacation, illness, parental leave, and continuing education do not extend the limit; confirm the exception’s documentation with your MAC.
Day 61: a billing change, not a grace period
After 60 continuous days, the substitute must bill under their own NPI, requiring enrollment already in place. Starting enrollment on day 61 leaves a gap: services can no longer be billed under either physician.
So if an absence may exceed 60 days, start enrollment at day one. Our credentialing and enrollment service covers that step and decides whether day 61 is a smooth switch or a gap. The other clean paths are the regular physician returning (after which a new period can begin) or a genuinely new absence after that return — not rotating substitutes.
What Q6 does not cover
- Nonphysician practitioners. Under Medicare, nurse practitioners, physician assistants, and certified registered nurse anesthetists may not be billed under fee-for-time or reciprocal arrangements — physicians only, plus outpatient physical therapy in a shortage, underserved, or rural area. Some commercial plans accept Q6 more broadly; check plan by plan.
- Extra help and staffing surges. Q6 covers an absent regular physician, not additional capacity. A substitute hired because the practice is busy is not substituting for anyone.
- A new physician waiting for credentialing. Fee-for-time billing is not a stopgap for a physician without a Medicare number yet; billing their services under an existing physician’s number with Q6 misrepresents who furnished the service.
- Postoperative care inside a global period. Substitute postoperative services within the global surgical period are not marked with Q6, because the regular physician is already paid a global fee covering them.
- Same-group coverage. Where claims go in the group’s name, the performing physician is identified instead of using Q5/Q6 substitution rules. Confirm with your MAC.
Commercial payers do not all follow the Medicare rule
Commercial payers vary in whether they accept substitute billing under the absent physician’s number, how long they allow it, whether the substitute must be credentialed with them, and whether retroactive billing is allowed. Check each plan before coverage starts, and keep the same log for commercial claims: date, panel, substitute, and substitute NPI per claim line.
A pre-claim checklist
- Arrangement is fee-for-time (Q6) or reciprocal (Q5), decided by payment — modifier matches it.
- Claim carries the regular physician’s NPI with Q6 or Q5 on every line the substitute furnished.
- Coverage log is open from day one, substitute NPI matched to each date of service.
- 60-day clock tracked from the substitute’s first patient contact, day-61 switch date calendared.
- If the absence may exceed 60 days, enrollment is underway and each commercial plan’s policy checked.
Frequently asked questions
What is the difference between Q5 and Q6?
Q5 is reciprocal coverage between colleagues with their own practices, with no fee-for-time payment. Q6 is a hired substitute paid per-diem or fee-for-time. The payment relationship decides.
Whose NPI goes on a Q6 claim?
The regular physician’s, with Q6 on each code the substitute furnished. The substitute’s NPI stays in the coverage log, available on request — not as rendering provider on the claim.
When does the 60-day clock start, and do weekends pause it?
It starts the first day the substitute provides covered services to the regular physician’s Medicare patients. It is continuous: weekends and no-service days do not pause it, and another substitute covering the same physician does not restart it.
What happens on day 61?
The regular physician may no longer bill the substitute’s services with Q6 for that period. The substitute bills from day 61 under their own NPI, so enrollment must be in place beforehand. A new period can begin after the regular physician returns to work.
Can Q6 be used for a nurse practitioner or physician assistant?
Not under Medicare, which limits substitute billing to physicians plus the narrow outpatient physical-therapy exception above. Some commercial plans differ — check each plan.
Does the clock restart if the regular physician returns briefly?
The period ends with the last day the substitute provides services before the regular physician returns, and a new period can begin after that return. Document the return and confirm with your MAC; do not treat a brief return as a reset device without that confirmation.
Is there any exception to the 60-day limit?
Yes, one under Medicare: the regular physician called to active duty in the Armed Forces. Vacation, illness, parental leave, and continuing education do not extend the limit. Confirm documentation requirements with your MAC.
Conclusion
Q6 billing comes down to four decisions before coverage starts: confirm fee-for-time status, bill under the regular physician with Q6 on every line, track the 60-day clock from the substitute’s first patient contact, and have the day-61 enrollment plan ready. As a checklist, they keep coverage periods clean and audit-ready.
If a physician leave is on your calendar, request a free billing assessment and we will review your setup, clock, and day-61 plan before the first claim goes out.
