Use of the disutility ratio in prenatal screening for Down's syndrome

To assess the value of the triple test incorporating individual differences in parental evaluation of outcomes of pregnancy.

Decision analysis.

Monte Carlo simulation of triple test results in 25,000 women with a normal pregnancy and 25,000 women with a pregnancy affected by Down's syndrome.

A decision model was developed for women who were 16 weeks pregnant. Three strategies were evaluated: 1. no prenatal testing; 2. amniocentesis; and 3. the triple test followed by amniocentesis if the risk of a pregnancy with Down's syndrome, based on maternal age and the triple test results (post-test risk), was above the woman's threshold risk for amniocentesis.

The outcomes considered were: 1. birth of a child without Down's syndrome; 2. birth of a child with Down's syndrome; and 3. pregnancy loss, either spontaneously, or as a result of termination. The values of these pregnancy outcomes were expressed on a disutility scale in units of 'lost pregnancy equivalents'. The birth of a normal child brings no disutility. The disutility of the birth of a child with Down's syndrome is consequently specified by the ratio of the expected parental distress after the birth of a child with Down's syndrome to the expected parental distress after the loss of the pregnancy (disutility ratio).

The value of the triple test depends strongly on maternal age as well as on the individual evaluation of the outcome of pregnancy. The triple test is of considerable value for all women > 38 years; its value for women between 27 and 38 years depends on the disutility ratio, and it is of little value for women < 27 years.

The value of the triple test depends on the parental evaluation of outcome of pregnancy for a large group of pregnant women. The disutility ratio, as introduced in this study, might be an instrument to elicit these values for individual women in clinical practice.