POS to use on Observation patients


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Can others tell me the POS you are using for a scenario such as in the example below: NOTE, this is for private payers and Medicaid

When patients are placed in an observation status after coming in through the emergency room our cardiologists are performing certain diagnostic tests in order to determine whether the patients need to be admitted or discharged. We have been billing the interpretation of these tests with a POS 22 and getting a denial from private payers and Medicaid that the tests were not pre-authorized. These are emergent patients, with the tests often performed after hours and/or on the weekends, so we have not received pre-authorization. They are not required if we performed the tests with an emergency room POS (POS 23). Should we be billing these tests with a POS 23 since the patients are coming through the ER?