CPT 87804 with modifier X{EPSU}

This question was posted by someone in Dermatology forum and was suggested to re-post here.

The original question is:

When coding the flu A and B test - we use 87804 and 87804-59 mod?? I have noticed that medicare is denying the second test?? Is this because we are not using the "X" modifiers - if so which one should be used??

CPT 87804 with modifier X{EPSU}