Screening Tests and Diagnostic Tests

The above techniques are used for diagnosis in high-risk women. But almost all pregnant women are offered a variety of other prenatal screening tests.

Although the distinction quickly becomes complicated, in its simplest form, screening tests are offered to a population of apparently healthy persons in order to find those few at increased risk. Ideally, screening tests are easy and inexpensive to perform and interpret, and do not entail risk for the person screened. Screening tests have high rates of initial positive results and thus a large percentage of people who have positive screening tests will prove not to have the screened-for problem on follow-up diagnostic testing.

In contrast, diagnostic tests are offered to individuals known to be at increased risk of a condition in order to answer the question, "Does this person have this disease?" Diagnostic tests are generally more complicated and expensive to perform and interpret, and may entail risk. They are expected to have higher standards of sensitivity and specificity: to do a much better job at identifying all and only cases of the disorder.

The screening and diagnostic testing regimens typically offered to pregnant woman and couples at the beginning of the twenty-first century are presented in Table 1. Each begins by asking a question that assigns the woman to a risk level. It is important to realize that each screening test has its own percentage of initial positive results; thus, each additional screen raises the risk for any individual woman of getting an initial positive result at some time during pregnancy. In addition, these tests are not all done at the same time in pregnancy. For example, an African-American woman, less than thirty-five years old, would be offered carrier testing for sickle cell disease in her first trimester and would also be offered multiple marker screening in her second trimester.

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