@Sylvian, I have elevated NK cells and it was determined by an endometrial biopsy at around day 20 of my cycle. This is somewhat like a Pap smear but unfortunately a bit more painful. Quick though. My FS did the test after my first failed IVF, along with many others (14 vials of blood were taken!). It's not one of the tests that CFC requests and most FS's here wouldn't do it either. There's a select few as far as I can tell really. Not sure what state you're in but someone might be able to advise.