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Correlation and overlapping between nuchal translucency and triple test among Down syndrome-affected pregnancies
Arie Herman1, Eliezer Dreazen, Yosef Tovbin
1Department of Obstetrics and Gynecology and Genetic Institute, Assaf Harofeh Medical Center, Zerifin, and Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel. aherman@asaf.health.gov.il
Objectives:
To examine the correlation and extent of overlapping between first-trimester nuchal translucency (NT) and second-trimester triple test (TT) results in Down syndrome (DS)-affected pregnancies.
Methods:
Results of both tests were obtained in 28 cases with DS. Inter-test correlation was performed by comparing the likelihood ratios (LRs). Screen-positive rates (risk >1:380) were calculated for different age groups by adjusting age-dependent background risk and tests' LRs. Overlapping referred to ratio between screen-positive cases by both tests simultaneously and total screen-positive cases by either one or both tests.
Results:
No correlation was found between the tests' LRs (Pearson correlation test, r = 0.0487). The overlapping between the tests was 25% and 38.5% among young patients of 20 and 35 years of age, respectively. The average overlapping among patients between 25 and 35 years of age was approximately 33%. Only 3 of the 28 DS cases demonstrated LRs <1 by both tests and moreover none exhibited LRs <0.2 by both tests.
Conclusions:
The degree of overlapping of one third, between NT and TT, confirms the assumption that both tests utilized together improves DS detection. Screen- negative result, by both tests simultaneously, may reassure low-risk population and aid to reduce the number of non-indicated invasive tests.