Positive Sequential Screening Tests

Rhiannon
I am 22 weeks and 4 days. I had the second part of my sequential screening done last Thursday. I just had an ultrasound done last Wednesday(ITS A BOY💙). Ultrasound tech and Doctor checked baby and said he was perfect. His organs, brain, and everything else was great. 
Well, doctor's office called me today and said they'll need to refer me to a specialist because my sequential screening came back positive for open neural tubal defect.
This is our first baby! I'm so scared and confused as to what it means. But I faith that everything will be okay and I will continue to walk by faith and not by sight. 
But I just wanted to see from other moms, if they had this happen before. And if so is it possible for it to be a false-positive? Wouldn't they have picked it up at the ultrasound? How accurate is the test anyways?