THE EFFECT OF NUCHAL TRANSLUCENCY MEASUREMENT ON 2ND-TRIMESTER BIOCHEMICAL SCREENING FOR DOWNS-SYNDROME

In this study we examined the effect of introducing first-trimester nu chal translucency measurement on the second-trimester maternal serum s creening for trisomy 21. The detection rate, false-positive rate, like lihood ratio and positive predictive value of the double marker test b efore and after introduction of nuchal translucency measurement were d etermined. The detection rate of nuchal translucency screening for tri somy 21 was 83% (5/6) with a 1.3% false-positive rate, a 63.8 likeliho od ratio and a positive predictive value of 22.7%. After the introduct ion of nuchal translucency measurement, the likelihood ratio for a pos itive result and positive predictive value of the biochemical screenin g decreased from 9.1 to 5 and 2.7% to 0.45%, respectively. Our results show that nuchal translucency measurement is not only an effective me thod of first-trimester screening for Down's syndrome but also has imp lications for the likelihood ratio and positive predictive value of se cond-trimester biochemical screening. Since the biochemical test is no w applied to a population with a decreased risk of trisomy 21, a posit ive test will be less likely to indicate Down's syndrome than when nuc hal translucency measurement was not applied.