The 2026 Guide to Locum Tenens Credentialing and Modifier Q6

:::info Quick answer Locum tenens credentialing allows practices to bill for a substitute physician while the regular doctor is absent. For Medicare, you append modifier Q6 to claims and use the absent doctor's NPI, avoiding full credentialing for up to 60 days. Commercial payers require varied, specific enrollment steps before you can bill. :::

Dr. Miller takes a 12-week medical leave. You hire a substitute physician to cover her patient panel. Patients get seen, charts get closed, and your team drops the claims. Six weeks later, a massive wave of denials hits your clearinghouse.

Why? Because your billing staff assumed a single modifier works for every payer. It doesn't. Proper locum tenens credentialing is a tightrope walk between strict Medicare limitations and complete commercial payer chaos. If you mishandle the setup, your temporary doctor is essentially working for free.

In 2026, payer algorithms are aggressively flagging locum claims that lack corresponding provider data or stretch beyond allowable timeframes. Getting this right requires understanding the exact difference between a quick modifier fix and a full enrollment slog.

The Medicare 60-Day Clock (Fee-For-Service Compensation)

Medicare officially retired the Latin phrase "locum tenens" years ago, renaming it "Fee-For-Service Compensation Arrangements." However, every billing manager, MAC rep, and auditor in the country still uses the old terminology.

The core Medicare rule remains rigid: you can bill for a substitute physician without fully credentialing them for a maximum of 60 continuous days.

The Continuous Day Trap

Medicare counts calendar days, not business days or days the substitute actually treated patients. The clock starts the first day the locum sees a Medicare patient. It stops on day 60.

If Dr. Miller goes on maternity leave on April 1, 2026, and the locum starts seeing patients on April 3, your 60-day window closes on June 1. It does not matter if the clinic was closed on weekends or if the locum only worked Tuesdays and Thursdays.

The only way to reset this 60-day clock is if the regular physician returns to work, sees patients, and then leaves again. If your regular doctor is out for 90 days, you must fully credential the substitute through PECOS to bill Medicare for days 61 through 90.

The single exception to the 60-day limit in 2026 is for regular physicians called to active duty in the Armed Forces. In that specific scenario, the substitute arrangement can extend indefinitely.

Billing Medicare: Box 24D and the Invisible Locum NPI

When you bill a Medicare locum claim, the substitute physician's NPI does not go on the CMS-1500 form. This feels wrong to most billers, but it is the exact CMS requirement.

Here is how you populate the claim:

• Item 24J (Rendering Provider): Enter the NPI of the absent regular physician.
• Item 24D (Procedures, Services, or Supplies): Append modifier Q6 to the CPT code.
• Item 33 (Billing Provider): Your group practice NPI.

By appending Q6, you are legally attesting to Medicare that a qualified substitute performed the service, and that the group is paying the substitute on a per-diem or time-based rate (not a percentage of collections). You must keep a secure internal log matching the dates of service, the patients seen, and the locum’s actual NPI. If a RAC auditor requests your records, you have to produce that log immediately.

The NP and PA Trap: Mid-Levels Cannot Be Locums

One of the most frequent denial triggers in 2026 involves Advanced Practice Providers (APPs).

Medicare strictly limits the Q6 modifier to physicians (MD, DO), podiatrists, optometrists, dentists, and chiropractors. Nurse Practitioners (NPs) and Physician Assistants (PAs) are explicitly excluded from Medicare's fee-for-service compensation arrangements.

• An NP cannot cover for an absent MD and bill Q6.
• An NP cannot cover for an absent NP and bill Q6.
• A PA cannot cover for an absent PA and bill Q6.

If your practice brings in a temporary NP to cover maternity leave, you cannot use the locum tenens shortcut for Medicare. You must submit an 855I (or use PECOS) to fully enroll that NP and reassign their benefits to your group before you can bill for their services. Since Medicare credentialing routinely takes 45 to 90 days, you must start this process long before the regular provider takes their leave.

Commercial Payer Madness: No Uniform Standard

While Medicare provides a strict (if complicated) framework, commercial payers treat substitute providers completely differently. Some mimic Medicare. Some require full credentialing from day one. Some have unique, proprietary forms you must fax to a specific department.

Assuming you can just slap a Q6 modifier on a UnitedHealthcare or Aetna claim will result in instant denials for "Provider Not Enrolled."

2026 Commercial Payer Locum Policies

Note: If your group holds a delegated credentialing contract with a payer, you can typically bypass individual payer enrollment and simply add the locum to your monthly roster. Delegated credentialing is the ultimate cheat code for fast temporary staffing.

Step-by-Step Locum Tenens Credentialing Workflow

To prevent cash flow interruptions, your revenue cycle team should treat locum onboarding with the same urgency as a permanent hire. You just need to follow a specialized track.

1. The Pre-Hire Audit (90 Days Out)

Before signing a contract with a locum agency, demand the provider's details and run your own checks. Do not rely entirely on the staffing firm’s background check.

• Pull the provider's NPI record to ensure it is active and properly taxonomy-coded.
• Check the OIG List of Excluded Individuals/Entities (LEIE).
• Verify the provider holds an active, unrestricted medical license in your specific state. In 2026, interstate medical licensure compacts make this faster, but you still need the physical state license in hand.

2. Gather the Credentialing Packet

Even if you plan to use the Medicare Q6 route, you still need a complete file for malpractice coverage, hospital privileges, and commercial payers who refuse Q6 billing. Collect these documents immediately:

• Current CV (with month/year format and all gaps explained)
• State Medical License and DEA Certificate
• Malpractice Certificate of Insurance (COI) matching your practice address
• Board Certification documentation
• CAQH ProView ID and login (ensure the profile is attested and up-to-date)
• Copy of a government-issued ID

3. Payer-by-Payer Matrix Execution

Create a spreadsheet of your practice’s top 10 payers by volume. Cross-reference the locum’s details against each payer's specific policy.

For Medicare, set up your internal tracking log. For Aetna and UHC, determine if you need to submit advanced notification forms or trigger a full CAQH credentialing push. For state Medicaid, check if the provider needs a state-specific atypical provider identifier or standard Medicaid ID before they write a single prescription.

Modifier Q5 vs. Modifier Q6: Reciprocal Billing

Medical billers often confuse modifier Q6 (fee-for-service/locum tenens) with modifier Q5 (reciprocal billing). Getting these crossed will fail an audit.

Modifier Q5 (Reciprocal Billing): This is used when an outside physician covers for your regular physician on an occasional, informal basis. The classic example is a solo practitioner who takes a weekend off and asks a doctor from a neighboring practice to take their on-call emergencies. The absent doctor compensates the covering doctor through a reciprocal agreement (often trading weekends). This is also limited to 60 continuous days, but the covering doctor is usually from a different group entirely. You append Q5 to the CPT code.

Modifier Q6 (Locum Tenens): This is used when you officially hire a substitute provider and pay them a fee to work inside your practice while the regular doctor is away. You append Q6 to the CPT code.

Handling the Temp-to-Perm Transition

Staffing shortages in 2026 mean many practices end up hiring their locum tenens providers permanently. This creates a massive billing headache if not managed proactively.

Remember that 60-day Medicare clock? If a locum works for you for 45 days, and you decide to hire them permanently, they cannot continue billing under the regular doctor's NPI with a Q6 modifier indefinitely.

The moment you decide to convert a locum to a permanent employee, you must submit their PECOS enrollment and commercial payer credentialing applications. If you wait until day 59 to start this paperwork, you will face a blackout period where the provider cannot bill Medicare until their permanent enrollment is approved and linked to your group PTAN.

Always start the permanent credentialing paperwork the day the offer letter is signed, even if they are currently billing under Q6 as a locum.

Substitute providers are the lifeblood of practice continuity. Build a rigid credentialing workflow, respect the 60-day limit, keep meticulous internal logs, and you will capture the revenue your practice deserves while your primary doctors are away.

Frequently asked questions

Can Nurse Practitioners (NPs) or Physician Assistants (PAs) bill as locum tenens?

Not for Medicare. Medicare strictly prohibits NPs and PAs from billing under fee-for-service compensation arrangements using modifier Q6. Mid-level providers must be fully enrolled to bill Medicare. Commercial payer policies on mid-level locums vary widely.

Does the Medicare 60-day limit count business days or calendar days?

Medicare counts 60 continuous calendar days. The clock starts the first day the substitute physician sees a Medicare patient and continues running on weekends and holidays, regardless of the clinic's operating schedule.

Can I extend a locum tenens arrangement beyond 60 days for Medicare?

The only exception to the 60-day rule is if the regular physician is called to active duty in the Armed Forces. In all other scenarios, you must fully credential the substitute provider to bill for services on day 61 and beyond.

What is the difference between modifier Q5 and Q6?

Modifier Q5 is used for reciprocal billing arrangements, typically when doctors trade on-call weekends informally. Modifier Q6 is used for fee-for-service compensation (locum tenens) where a substitute is hired and paid a per-diem or time-based rate to cover an absence.

Whose NPI goes on the CMS-1500 claim form for a locum tenens service?

For Medicare, you put the absent, regular physician's NPI in Box 24J (Rendering Provider) and append modifier Q6 to the service line in Box 24D. You do not put the locum's NPI on the claim, but you must keep an internal log of it for audits.