Question: One of our physicians is taking maternity leave next month, and a locum physician is filling in for her. I was told that we're billing for this physician under the absent ophthalmologist's Medicare ID. I've never billed for a substitute physician before. Do I need to observe any specific coding/modifier rules for this physician's services? Codify Subscriber Answer: You'll need to know a couple of rules, and one specific modifier, in order to bill for substitute - aka locum tenens - physicians. Check out this quick advice on getting each locum claim right while your regular physicians takes extended leave: Billing: You should bill the locum's services with the ID number of the physician for whom she is subbing. Let's say Dr. X is taking leave, and Dr. Y is filling in for her. Dr. Y performs a level-three E/M service for a new (to the practice) patient. On the claim, you should report 99203 (Office or other outpatient visit for the evaluation and management of a new patient, which requires these 3 key components: a detailed history; a detailed examination; medical decision making of low complexity. ...) for the encounter under Dr. X's National Provider Identifier (NPI) number. Modifier: You should, then, attach modifier Q6 (Service furnished by a locum tenens physician) to 99203 in the above example. In fact, append Q6 whenever you code for a service that a locum provides. Time limit: You should observe a 60-day time limit for your locum, starting with his first day of service. Let's say Dr. X goes on maternity leave September 1, and you hire a locum to fill in for her. The locum performs his first service for a patient on Sept. 1, 2017. You can bill the locum's services under Dr. X's NPI number until Oct. 30, 2017.