Because a screening test, such as the serum test for Down's syndrome, is not a definite diagnostic test, it may be mistaken. For example, it is quite likely that although you have been given a high risk estimate from the serum test, when your amniocentesis results arrive, they shown that you do not have a baby affected by Down's syndrome. This is called having a 'false positive'.
However, in rare circumstances, you can also have a 'false negative'. This means that the serum test says you are at low risk, but in fact you do turn out to have a baby affected by Down's syndrome. Serum screening detects 8 or 9 out of every 10 babies with Down's syndrome. Serum screening detects 4 out of every 5 cases of spina bifida.
