Test reliability

A prenatal test must be sufficiently reliable to permit decisions
to be made once results are available. Some conditions can be
diagnosed with certainty, others cannot, and it is important that
couples understand the accuracy and limitations of any tests
being undertaken. Chromosomal analysis usually provides
results that are easily interpreted. Occasionally there may be
difficulties, because of mosaicism or the detection of an
unusual abnormality. In some cases, an abnormality other than
the one being tested for will be identified, for example a sex
chromosomal abnormality may be detected in a pregnancy
being tested for Down syndrome. For many mendelian
disorders biochemical tests or direct mutation analysis is
possible. The biochemical abnormality or the presence of a
mutation in an affected person or obligate carrier in the family
needs to be confirmed prior to prenatal testing. Once this has
been done, prenatal diagnosis or exclusion of these conditions
is highly accurate. In other inherited disorders, neither
biochemical analysis nor direct mutation testing is possible.
DNA analysis using linked markers may enable a quantified risk
to be given rather than an absolute result.