I had my RMC follow up this week and they talked about this there. I'd already heard of it and only really knew it was expensive. My consultant went into detail about it and said that the blood test is quite unreliable, because we always have NK cells in us. They form the basis of our immune system and without them we wouldn't be able to fight off infections. So having the test by blood can be difficult to read because it's hard to say at what level they might be having an impact on fertility. Or even if elevated levels are affecting the uterus at all.
She said there's a newer way of testing, which involves taking a biopsy of the uterus lining and testing the levels in there instead. I don't know how commonly this is being carried out at private clinics but she seemed to think this was the way to go, if you're going to do the NK test at all.
She also said it's possible to give the NK treatments such as steroids empirically, so rather than testing , you'd just be given the drugs anyway, just in case. Some people prefer this, but I suppose you'd have to weigh up the cost of the test versus the cost of the drugs.
I'm not going for it yet, we can't justify the expense at this stage as I'm trying home insemination for a while. Perhaps if I was having a 'one last shot' at IVF, I'd go for NK testing/meds to give myself the best shot but for now I don't want to pay so much.