Medicare lets an independent practice bill a substitute physician's visits under the absent physician's own NPI for up to 60 days of continuous coverage, using modifier Q5 for a reciprocal arrangement or Q6 for a paid locum tenens, and any service past day 60 has to be billed another way (CMS, Medicare Claims Processing Manual, Pub. 100-04, Ch. 1, secs. 30.2.10 and 30.2.11, as accessed September 2026). Practices that plan a 3-month leave around this rule without counting days end up re-billing the final weeks, or holding claims that cannot go out under the regular physician at all.
This article provides general operational guidance on medical billing practices. It is not legal, compliance, or financial advice. Consult qualified healthcare billing counsel or a certified professional coder for your specific situation.
Medicare audit rules and Local Coverage Determinations (LCDs) vary by MAC jurisdiction and change frequently. Verify current requirements with your MAC before acting on any guidance in this article.
Credentialing and enrollment requirements vary by payer and change frequently. Verify current requirements directly with each payer.
The Short Answer
Use Q5 when another physician covers on an occasional, reciprocal basis, typically a colleague you cover for in return. Use Q6 when you pay a substitute per diem or on a similar fee-for-time basis. In both cases the regular physician must be unavailable, the patients must be ones who sought care from that regular physician, the substitute cannot cover longer than 60 continuous days, and the practice keeps a record of every service with the substitute's NPI.
If the absence is really a vacancy you need to fill, the credentialing clock matters more than the modifier; practices facing that gap can compare credentialing services on GetPracticeHelp before the 60 days run out.
What Q5 and Q6 Actually Require
The two arrangements share most of their conditions and differ mainly in how the substitute is paid. The table below maps each requirement to the source rule and to the role that usually owns it in a small practice.
| Requirement | Q5 Reciprocal Billing | Q6 Fee-for-Time (Locum Tenens) | Who Owns It |
|---|---|---|---|
| How the substitute is paid | Occasional reciprocal coverage, no per diem required | Per diem or similar fee-for-time basis | Practice administrator |
| Time limit | 60 continuous days | 60 continuous days | Billing vendor tracks the count |
| Written agreement | Not required by the manual | Not specified; a written contract is standard practice for pay terms | Practice administrator or healthcare attorney |
| Claim marker | Q5 after the procedure code, item 24d of CMS-1500 | Q6 after the procedure code, item 24d of CMS-1500 | Billing vendor |
| Records | Each service logged with the substitute's NPI, available to the MAC on request | Same | Billing vendor and front desk |
| Military exception | Physician called to active duty may exceed 60 days | Same | Practice administrator |
How the 60 days are counted
The clock starts on the first day the substitute sees the regular physician's Medicare Part B patients and ends on the last day the substitute sees them before the regular physician returns. Days with no visits do not pause it. The manual's own example: a physician leaves June 30 and returns September 4; the substitute first sees patients July 2 and last sees them September 2. That is a 63-day period, so the regular physician can bill only July 2 through August 30. Services from August 31 through September 2 must be billed by the substitute under the substitute's own billing number, or reassigned to the person or group that bills for the substitute (CMS, Pub. 100-04, Ch. 1, sec. 30.2.10). A new period can start only after the regular physician returns to work.
What counts as a covered visit service
For physicians, the covered services are not limited to office visits. They include other covered items and services furnished by the substitute, or by staff incident to the physician's services. Post-operative care that falls inside a global surgical period does not need to be flagged as substitute care. Since June 13, 2017, section 16006 of the 21st Century Cures Act has also allowed Medicare-enrolled physical therapists to use Q5 and Q6 substitutes for outpatient therapy in a Health Professional Shortage Area, a Medically Underserved Area, or a rural area.
Group Practices: The Reassignment Trap
For a group to bill the substitute's services in the regular physician's name, the substitute may not have reassigned Medicare benefits to that group through an approved CMS-855R. This is the rule that trips practices up when a locum later signs on permanently: once the reassignment is approved, the physician is a group member who bills under their own NPI, and Q5 or Q6 no longer applies.
Group claims have one more field to get right. The regular physician on whose behalf the substitute worked must be identified by NPI in block 24J of each line item. A group can also use Q6 for up to 60 days to cover a physician who has left the group while it recruits a permanent replacement (CMS, Pub. 100-04, Ch. 1, sec. 30.2.11). That provision is a bridge for a departure, not a way to bill a new hire before enrollment; the difference is laid out in the analysis of credentialing versus billing under a supervisor.
Payment does not change with the substitute. The MAC prices the service as though the regular physician performed it, and noncoverage notices go out in the regular physician's name. The substitute's identity on file is there to prove on audit that the service happened.
Implementation: What Practices Actually Do
- Classify the absence first: a vacation, CME, illness, parental leave, or military call-up with a return date fits Q5 or Q6. A physician who is not coming back fits only the group's 60-day departure bridge. A new provider awaiting enrollment fits neither.
- Pick the modifier from the payment terms: if you pay the substitute a daily or hourly rate, it is Q6. If a colleague covers you and you cover them in return, it is Q5. Do not mix modifiers for the same substitute and period.
- Confirm the substitute's status: check that the substitute has an active NPI, a license in your state, and no exclusion; the screening process is covered in the guide to OIG exclusion screening for independent practices. Confirm the substitute has not reassigned benefits to your group.
- Start a day counter on day 1: log the first Medicare Part B visit date and set a hard stop at day 60 in the practice management system. Build in a 5-day warning so scheduling can move the last visits or switch billing.
- Keep the service log: for every service, record date, patient, CPT code, the substitute's NPI, and the regular physician's NPI. The MAC can request this record at any time.
- Check each commercial payer separately: Q5 and Q6 are Medicare rules. Commercial and Medicaid plans set their own locum policies, and some require the substitute to be credentialed with the plan. Get the payer's policy in writing before the first covered visit.
- Plan the handoff after day 60: if coverage continues, the substitute bills under their own number or reassigns benefits to the group, which requires enrollment work that takes weeks, so start it before the absence begins.
What Goes Wrong
- Counting only visit days: a practice that counts the 40 days the locum actually worked, instead of the calendar span from first to last visit, bills past day 60 and has to reprocess every claim after that date.
- Using Q6 for a new hire: billing an incoming physician's visits under a departed or absent colleague while the new enrollment is pending misstates who furnished the service. CMS instructs MACs to warn that a false Q5 or Q6 certification can bring civil or criminal fraud penalties or revocation of billing privileges under 42 CFR 424.535.
- Relying on the MAC to catch it: the manual tells MACs they need not track the 60-day period or validate the arrangement before payment. Problems surface later, in postpayment review, when the money has already been spent.
- No service log: without a record tying each service to the substitute's NPI, the practice cannot document the arrangement when the MAC asks.
- Assuming commercial payers follow Medicare: a Q6 claim to a plan that requires locum credentialing denies, and those denials can age past the plan's filing limit; see the guide to timely filing denials for recovery options.
What Should You Do?
Before any planned absence longer than 2 weeks, write down the start date, expected return date, and the substitute's payment terms, then decide Q5 or Q6 from those facts. Set the 60-day hard stop in your system, keep the NPI service log from the first visit, and get each commercial payer's locum policy in writing. If the absence could run past 60 days, start the substitute's own Medicare enrollment or reassignment at the same time you book them. Practices looking for outside help with coverage staffing or the billing side can start at GetPracticeHelp.
Get the full practice management guide at GetPracticeHelp, with billing benchmarks, credentialing checklists, and revenue cycle guides for independent practices.
Frequently Asked Questions
- What is the difference between modifier Q5 and Q6?
- Q5 marks services a substitute physician furnished under a reciprocal billing arrangement. Q6 marks services furnished by a substitute the regular physician pays on a per diem or similar fee-for-time basis, the arrangement formerly called locum tenens (CMS, Pub. 100-04, Ch. 1, secs. 30.2.10 and 30.2.11).
- How long can a locum tenens physician bill under the regular physician for Medicare?
- No longer than 60 continuous days, counted from the substitute's first to last day seeing the regular physician's Medicare patients. The only exception in the manual is a regular physician called to active duty in the Armed Forces.
- Does a reciprocal billing arrangement need to be in writing?
- The Medicare manual says reciprocal billing arrangements need not be in writing. The practice must still keep a record of each service with the substitute's NPI for the MAC.
- Can a group bill for a physician who has left the practice?
- Yes, for up to 60 days. A group may bill Q6 services of a temporary substitute hired to replace a physician who left the group, as long as the substitute has not reassigned benefits to the group.