The effect of nuchal translucency measurement on second-trimester biochemical screening for Down's syndrome

In this study we examined the effect of introducing first-trimester nuchal translucency measurement on the second-trimester maternal serum screening for trisomy 21. The detection rate, false-positive rate, likelihood ratio and positive predictive value of the double marker test before and after introduction of nuchal translucency measurement were determined. The detection rate of nuchal translucency screening for trisomy 21 was 83% (5/6) with a 1.3% false-positive rate, a 63.8 likelihood ratio and a positive predictive value of 22.7%. After the introduction of nuchal translucency measurement, the likelihood ratio for a positive result and positive predictive value of the biochemical screening 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 method of first-trimester screening for Down's syndrome but also has implications for the likelihood ratio and positive predictive value of second-trimester biochemical screening. Since the biochemical test is now applied to a population with a decreased risk of trisomy 21, a positive test will be less likely to indicate Down's syndrome than when nuchal translucency measurement was not applied.