Evaluation of ntt and double-test in screening of down syndrome

Vida Shafti,1,* Negar shazdeh-ahmadi,2

1. school of medicine , Tonekabon islamic azad university , tonekabon , iran

Abstract

Introduction

Prenatal diagnosis of genetic, congenital and hereditary diseases is the first step to prevent and control these problems . to achieve this objective, various screening tests are used in the first and second trimester of pregnancy. routine screening tests in the first trimester are nuchal translucency thickness (nt) and double-test (papp-a and free β-hcg). previous studies have shown different diagnostic values for them. accordingly, this study is designed to evaluate the diagnostic value of nt and double-test in screening of down syndrome in pregnant women.

Methods

In a retrospective descriptive-analytical study, 550 pregnant women who referred to obstetrics clinics in cities of sari, ramsar, tonekabon and behshar between 2012 to 2016 were evaluated. patients’ information such as age, thickness of nt, results of double-test (papp-a and free β-hcg) were extracted from their record. the sensitivity, specificity, negative predictive value (npv), positive predictive value (ppv), false negative rate (fnr), false positive rate (fpr) and overall accuracy of each test were calculated.

Results

550 pregnant women with mean age of 27.13±5.49 years participated in the study. double test was negative in 519 patients (94.4%), suspicious in 9 patients (1.6%) and positive in 22 patients (4%). nt was positive ( more than 2.5 mm) in 15 patients (2.7%) and negative ( less than 2.5 mm) in 535 (97.3%) patients . only 4 pregnancies (0.7%) resulted to birth of children with down syndrome. the mean thickness of nt in pregnancies that resulted to down syndrome was significantly more than other patients (respectively 2.80±1.38mm versus 1.32±0.45mm, p<0.0001). sensitivity, ppv, npv,fpr, fnr and overall accuracy of nt were 50%, 97.61%, 13.3%, 99.62%, 3.73%, 50% and 97.27%, respectively, and for double test were 100%, 96.70%, 18.18%, 100%, 3.29%, 0 and 96.72%, respectively.

Conclusion

The mean thickness of nt in pregnancies that result in down syndrome was significantly higher than other pregnancies. double test had higher sensitivity and false-negative rate in compare with nt for detection of down syndrome.

Keywords

Double test , nuchal translucency thickness , down syndrome, pregnant woman.