87400 vs 87804

My office has billed with 87400 with 2 units for 2 years with no problems. Just recently insurance companies have either only paid for 1 unit or nothing at all. I have been researching and found the code 87804 and people are saying to use 25 and 59 mod with 2 units. Can anyone help clear this up. What exactly is the correct way to bill for the Flu Test A and B?
Thanks