Locum tenens coverage is a practical necessity for practices when a physician is out. But billing for locum services under the absent physician's NPI — using the Q6 modifier — has strict rules that many practices don't fully understand. Get this wrong and you're looking at repayment liability.
If you're working through these issues, our medical billing services can help you address them systematically.
The Q6 modifier tells Medicare that the service was provided by a substitute physician under the locum tenens arrangement. When you use Q6, you're billing as if the regular physician provided the service — meaning the claim goes out under the regular physician's NPI, at the regular physician's contracted rate.
This is legal and appropriate — but only under specific conditions. It's not a general-purpose "someone else covered my patients" modifier.
Medicare limits Q6 billing to 60 continuous days. If the regular physician is going to be out longer — extended medical leave, sabbatical, extended vacation — you have two options: the locum becomes credentialed under their own NPI and bills independently, or a different coverage arrangement is made.
The 60 days starts from the first day the locum sees patients and runs continuously. If the regular physician returns briefly and then leaves again, the clock doesn't reset. The 60 days is intended to cover genuine temporary absences, not extended coverage situations disguised as locum arrangements.
The substitute physician must be a physician — the Q6 modifier rules don't apply to NPs or PAs covering for a physician. If an NP is covering while a physician is out, those services bill under the NP's own NPI (at the NP's rate). Trying to bill an NP's services under the absent physician's NPI using Q6 is a billing error with compliance consequences.
Keep records of every Q6 arrangement: the dates covered, which physician was the substitute, which physician was the regular physician, and confirmation that the locum was paid fee-for-service or per-diem. CMS can ask for these records in an audit. If you can't produce them, the Q6 billing is presumptively improper.
Most commercial payers have their own policies on locum billing. Some allow billing under the regular physician's NPI in similar circumstances; others require the substitute to be credentialed with them and bill under their own NPI. Don't assume that because Medicare allows Q6, your commercial payers will too.
Review each commercial contract for locum tenens provisions. If the contract is silent and you bill a locum's services under the regular physician's NPI, you may be in breach of the contract's credentialing requirements. A call to the payer's provider relations line is worth the 20 minutes it takes.
Some practices use locum billing as a workaround when they've hired a new physician who isn't yet credentialed with payers. This is sometimes called "incident-to for physicians" or a panel buy-in arrangement — and it's a gray area. CMS hasn't explicitly prohibited billing new physician services under an established physician's NPI in all circumstances, but it's risky territory without careful legal review.
The better path: start the credentialing process the moment you know you're hiring a new physician. Credentialing typically takes 60–120 days, and many payers will backdating enrollment to the hire date if applied promptly. Plan ahead and you won't need a workaround.
Medicaid programs have their own locum tenens billing policies, which often differ from Medicare's. Some state Medicaid programs don't recognize the Q6 modifier at all and require that services be billed under the rendering provider's own NPI. Check your state Medicaid program's billing manual for the specific locum policy before assuming Medicare rules apply.
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