I haven't had it done personally but I have seen it done a number of times ( between 5-10) as I am a midwife.
I've never seen anyone deliver soon after a negative result, but its not 100% guarentee, I think its 95% or something. Come to think of it I don't think I've seen a positive result from it but I haven't seen heaps.
Where you in an appropriate hospital to have a 34 weeker?