Age-specific detection and false-positive rates: an aid to counseling in Down syndrome risk screening

To determine whether the serum screening test for Down syndrome provides equal detection efficacy for women of all ages, to improve the data available for patient counseling both before testing and afterward in the event of a positive result.

We examined the effect of age on Down screening by generating a set of "normal" and "Down syndrome" likelihood ratios by computer simulation. The expected false-positive and detection rates were derived for different age groups by counting the proportion of cases in which the likelihood ratio could modify the age-specific risk to be greater than the cutoff risk of one in 300 (equivalent to an incidence of 3.33 per 1000). The predictive value of a positive result was calculated using Baye's theorem.

Detection rates, false-positive rates, and predictive values were shown to be age-dependent.

Knowledge of the age dependency of Down syndrome screening results may be useful in explaining to patients that the Down screen can only detect a proportion of cases and that a negative result does not guarantee normality. This knowledge may also be helpful in minimizing psychological stress, as a positive result indicates only a small chance that the fetus will have Down syndrome.