The NTplus method of screening for Down syndrome: achieving the 2010 targets?

Abstract

Introduction: The performance of pregnancy-associated plasma protein-A (PAPP-A) as a first trimester trisomy 21 marker is hypothesized to improve below 11 weeks, whereas beta-human chorionic gonadotrophin (hCG) is better after 14 weeks. We audited a model combining early PAPP-A (9-10 weeks) with NT (11-13 weeks and 6 days) and early triple test (>14 weeks).

Methods: A total of 1507 women with viable ongoing pregnancies were screened during 2007-2008. First-stage 'screen-positive' risk was based on combined PAPP-A and NT cut-off >or=1 : 100. Where first-stage risk was <1 : 100 or invasive testing declined, triple test was performed and a combined second-stage risk given with cut-off >or=1 : 250 being screen positive.

Results: Median age of women was 35.4 years. Sixty-four (4.2%) were 'screen positive'. Of these, 11 had a fetus with trisomy 21. Twelve pregnancies were affected with trisomy 21, giving a detection rate of 11/12 (92%) with a false-positive rate (FPR) of 3.2%. The screen-positive rate (SPR) and FPR were 1.93 and 1.44%, respectively, standardized to median maternal age 29.

Conclusions: Early PAPP-A, NT and later triple testing offers comparable detection for trisomy 21 to published data for first trimester combined testing but feasibly achieve the national target for 2010 of 90% detection of trisomy 21 for < 2% SPR.