Audit of Down's syndrome screening is not valid

EDITOR—Wellesley et al are unjustified in their view that serum screening for Down's syndrome is not worth while.1 They ignore evidence from previous studies that shows the substantial advantage of serum screening over screening based on maternal age alone.2^–4 Their view is based on the results from two districts, the only two out of the eight where serum screening was routinely offered (using the Double test).

In these two districts only 24% of affected pregnancies (22 of the 91) were detected antenatally by using serum screening, though the Double test has a detection rate of 58% for a false positive rate of 5%.2 This could, at least in part, be due to a low screening uptake (in which case the focus should be on why this was so), but the uptake of screening cannot be determined from the paper. It could also be due to affected pregnancies that were positive on the serum test and positive on other tests being classified as positive under the other tests.

This problem arises because the detection rate used in this paper includes women who declined screening (the performance of screening tests, the uptake of screening, and the uptake of amniocentesis should all be reported separately). Methods of detection (serum screening, scan, maternal age, etc) were also tabulated in mutually exclusive categories, which cannot be correct because there will be pregnancies positive for Down's in two or more of these categories.

Wellesley et al compare screening …