Blood work came back negative but can't see bone in fetus nose 19 weeks does it always mean down syndrme?

No. Not always. : But an absent nasal bone at 19 weeks is very disconcerting. Studies show a very high correlation with Down syndrome (possibly as high as 90%). But of course there are false positives. Depending on how that diagnosis would affect your future decisions, an amniocentesis would be necessary to tell you definitively, but that does carry a small risk. Genetic counseling may help with this decision.