The Biller's Guide to Substitute Physicians: Q5, Q6, and Coverage Compliance

:::info Quick answer To bill for a substitute physician, append modifier Q5 (reciprocal) or Q6 (fee-for-time) to the CPT code. Submit the absent physician’s NPI in Box 24J of the CMS-1500. Keep the substitute’s NPI on file. Medicare strictly limits these arrangements to 60 continuous days. :::

Every billing manager dreads the unexpected phone call. A provider is out for surgery, a sudden family emergency, or extended maternity leave, and the schedule is packed for the next two months. You need a substitute physician to step in immediately, but getting a new provider fully credentialed with your payer mix takes 90 to 120 days.

Revenue does not have to stop while you wait on PECOS or commercial credentialing committees. CMS and commercial payers allow you to bill for substitute physicians using the regular, absent physician's National Provider Identifier (NPI). You keep the revenue flowing through the established provider's contracts, and the patients continue to receive uninterrupted care.

But substitute physician billing is a frequent target for Recovery Audit Contractors (RACs). As Medicare steps up post-pandemic audit activity throughout 2026, dropping the wrong modifier on a claim or miscounting coverage days can trigger massive clawbacks.

The Difference Between Q5 and Q6 Modifiers

When handling coverage, billers constantly deal with the reciprocal billing Q6 modifier, even though Medicare technically delineates these arrangements into two distinct categories. Because many major commercial plans accept Q6 for both types of substitute physician coverage, the terms are often used interchangeably on the billing floor. However, for Medicare and Medicaid, you must know the difference.

Modifier Q5: Reciprocal Billing Arrangements

Reciprocal billing applies when two physicians informally agree to cover each other's practices.

For example, Dr. Smith takes a two-week vacation, and Dr. Jones covers his patients. Dr. Smith will do the same for Dr. Jones later in the year. Generally, no money changes hands between the physicians, and the covering physician is not an independent contractor. Medicare requires you to use modifier Q5 for this informal arrangement.

Modifier Q6: Fee-for-Time Compensation (Locum Tenens)

Fee-for-time compensation applies when a practice hires a substitute physician and pays them a per diem or an hourly rate to cover for an absent doctor. The industry still commonly calls this a "locum tenens" arrangement, though CMS officially updated the terminology several years ago.

Because the covering physician is acting as an independent contractor paid for their time, Medicare requires you to use modifier Q6.

While Medicare is strict about the Q5 vs. Q6 distinction, you will find that some regional Blue Cross Blue Shield or UnitedHealthcare policies still ask for Q6 regardless of how the physicians compensate each other. Always pull the 2026 payer-specific provider manual before submitting the first batch of claims.

The 60-Day Clock: Hard Limits and Resets

Whether you use Q5 or Q6, Medicare enforces a strict 60-day time limit on substitute physician billing. This is not a suggestion; it is a hard cap programmed into the MAC claim engines.

How the 60 Days are Calculated

The 60-day limit refers to 60 continuous days, not 60 cumulative days of work.

The clock starts on the very first day the substitute physician sees a Medicare patient for the absent doctor. From that calendar date, you have exactly 60 days of allowed coverage. Weekends count. Holidays count. Days when the clinic is closed count.

If the substitute physician sees a patient on September 11, 2026, day 60 hits on November 9, 2026. On November 10, any claims billed under the regular physician's NPI with a Q5 or Q6 modifier will be denied.

Breaking the Continuous Period

The only way to reset the 60-day clock is for the regular physician to return to the practice and resume treating patients.

If Dr. Smith comes back to work for one week, the substitute arrangement terminates. If Dr. Smith then needs to leave again, a brand-new 60-day period begins the next time a substitute steps in.

What happens if the regular physician cannot return after 60 days? If a doctor dies, leaves the practice permanently, or requires a medical leave lasting six months, you cannot use Q5 or Q6 indefinitely. The substitute physician must be fully credentialed and enrolled with the payers to bill under their own NPI starting on day 61.

Exception: Active duty military deployment. CMS grants an exception to the 60-day limit for regular physicians called to active duty, allowing locum coverage to extend longer.

Step-by-Step Claim Formatting for Substitute Physicians

If your claim hits the clearinghouse without the exact right setup, it will bounce back before it ever reaches the payer. Proper data entry on the CMS-1500 form (or the 837P electronic equivalent) is critical.

Loop 2310B and Box 24J

You are billing under the established, absent physician's credentials.

• CMS-1500 Block 24J: Enter the regular, absent physician's NPI here. Do not put the substitute's NPI in this box.
• Electronic 837P: The regular physician's NPI goes in Loop 2310B (Rendering Provider).

Box 24D and Modifiers

Append the Q5 or Q6 modifier to the CPT code in Box 24D. If the service requires other modifiers (like 25 for a separate E/M, or RT/LT for laterality), sequence the payment modifier first, followed by the informational Q5/Q6 modifier.

What About the Substitute's NPI?

Medicare does not require the substitute physician's NPI on the actual claim form. Instead, you are legally required to keep the substitute's NPI on file in your office records.

However, watch out for commercial payers. Certain Medicaid Managed Care Organizations (MCOs) and commercial plans require you to list the substitute physician's NPI in Box 23 (Prior Authorization/Reference Number) or in Loop 2300 of the 837P. If a commercial payer denies a Q6 claim for missing provider information, check their specific NPI placement rules.

Strict Medicare Exclusions in 2026

One of the fastest ways to fail an audit is assuming reciprocal and locum rules apply to every provider type in your clinic. They do not.

Advanced Practice Providers (NPs and PAs)

Under the Social Security Act, Q5 and Q6 substitute billing arrangements apply exclusively to physicians.

Nurse Practitioners (NPs), Physician Assistants (PAs), and Clinical Nurse Specialists (CNSs) cannot use reciprocal billing or fee-for-time modifiers under Medicare.

If an NP goes on maternity leave, you cannot bring in a temp NP, bill under the absent NP's NPI, and slap a Q6 on the claim. The substitute NP must be fully credentialed and bill under their own NPI from day one. Do not attempt to bypass this; MACs routinely run NPI credential type checks against Q5/Q6 modifiers and will flag the claims as fraudulent.

Physical Therapists and Chiropractors

Physical therapists in private practice cannot use locum tenens billing. Chiropractors, however, are generally recognized as physicians under Medicare Part B for the specific purpose of manual manipulation of the spine, meaning a chiropractor can use Q5/Q6 arrangements to cover for another chiropractor.

Audit-Proofing Your Substitute Physician Workflows

When a RAC or commercial auditor requests a chart review for Q5/Q6 claims, they look for specific internal documentation. If you cannot produce it, they will recoup all payments made during the substitute period.

Required Documentation

Before the substitute sees a single patient, your compliance team must gather:

• A signed, written agreement between the regular physician and the substitute physician (or the locum agency).
• A copy of the substitute physician's active medical license in the state where services are rendered.
• Verification of the substitute's malpractice insurance coverage.
• Verification that the substitute is not on the OIG Exclusion List.

The Mandatory Coverage Log

Medicare requires the practice to maintain a continuous log of every patient seen by the substitute physician. You must cross-reference the regular NPI with the substitute NPI. Keep this log in a centralized spreadsheet, separate from the EHR, so you can export it instantly upon auditor request.

Here is a standard 2026 tracking log template:

Notice that September 13th is missing from the service dates, but the day count advanced to Day 4 on September 14th. The 60-day clock does not pause just because the clinic was closed on a Sunday.

Handling Denials and Revenue Cycle Management

Even with perfect formatting, substitute claims can hit snags. Watch your Electronic Remittance Advice (ERA) for specific denial codes.

If you see CO-16 (Claim/service lacks information) or CO-4 (The procedure code is inconsistent with the modifier used), pull the claim immediately. Check if the clearinghouse stripped the Q6 modifier during transmission, or if an inexperienced biller accidentally put the substitute's NPI in the rendering provider loop.

To keep cash flow steady, initiate credentialing for your substitute physician the moment they agree to a long-term coverage assignment. Do not wait until day 45 of the 60-day limit to start the PECOS application. Credentialing bottlenecks in 2026 are severe, and if day 61 arrives and the substitute lacks an active enrollment, you will be forced to hold claims or turn patients away.

Frequently asked questions

Can Nurse Practitioners (NPs) use the Q6 modifier for locum tenens coverage?

No. Under Medicare rules, NPs, PAs, and other non-physician practitioners cannot use reciprocal billing (Q5) or fee-for-time compensation (Q6) modifiers. Substitute mid-level providers must be credentialed and bill under their own NPIs.

Does the 60-day limit for substitute billing mean 60 working days?

No. The 60-day limit represents 60 continuous calendar days. The clock starts on the first day the substitute sees a patient and runs consecutively, including weekends, holidays, and days the clinic is closed.

How do you reset the 60-day clock for the Q6 modifier?

To reset the 60-day period, the regular, absent physician must return to the practice and resume treating patients. If they leave again after returning, a new 60-day period begins.

Whose NPI goes in Box 24J on a locum tenens claim?

The regular, absent physician's NPI goes in Box 24J of the CMS-1500 claim form (or Loop 2310B of the 837P). The substitute physician's NPI is kept on file in the practice's coverage log.

What is the difference between modifier Q5 and Q6?

For Medicare, Q5 is used for reciprocal billing (an informal coverage arrangement between two physicians). Q6 is used for fee-for-time compensation, commonly known as locum tenens, where a substitute is paid a per diem or independent contractor rate.