PRENATAL DIAGNOSIS Prenat Diagn 2003; 23: 389–392. Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pd.597 Centre-specific ultrasound nuchal translucency medians needed for Down syndrome screening H. Logghe 1 *, H. Cuckle 2 and I. Sehmi 2 1 Feto-maternal Medicine Unit, Clarendon Wing, Leeds General Infirmary, Leeds, UK 2 Reproductive Epidemiology, School of Medicine, University of Leeds, Leeds, UK Nuchal translucency (NT) measurements were compared between 13 centres participating in a multi-marker Down syndrome screening program. Results from 4765 women were analysed, and there were highly statistically significant between-centre differences after allowing for gestation (P < 0.0001). Examination of maternal serum marker levels, expressed in multiples of the median (MoM) for gestation, showed that this was not due to gestational errors. Regression analysis was carried out to derive an equation with a centre-specific component that could be used to express NT in MoMs. Use of this equation reduced the variance of logNT by 15% compared to a published equation. The equation can be readily modified for use in other centres. Copyright  2003 John Wiley & Sons, Ltd. KEY WORDS: nuchal translucency; multiples of the median; between-centre differences; between operators; audit INTRODUCTION Ultrasound nuchal translucency (NT) measurement at 11 to 13 weeks, gestation is increasingly used to screen for Down syndrome. The very large Fetal Medicine Foundation (FMF) prospective intervention study has demonstrated that a high detection rate can be achieved (Snijders et al., 1998). Moreover, statis- tical modelling predicts that even higher rates can be obtained by combining NT with serum mark- ers (Cuckle, 2001), and the two prospective inter- vention studies published so far are consistent with these predictions (Krantz et al., 2000; Spencer et al., 2000). Early studies reported NT in millimetre units and did not take account of the gestational age (Nicolaides et al., 1992). However, the median NT level increases with gestation, and a moderately elevated NT level at an advanced maternal age may indicate as high a Down syndrome risk as an extremely high value in a young woman. One way of overcoming this is to use exactly the same approach as with serum markers (Cuckle, 1996; Biagiotti et al., 1997). This involves expressing NT results in terms of multiples of the normal median (MoM) for the given gestation and calculating a woman’s individual risk from her NT level and age. Leeds Antenatal Screening Service (LASS) used this method and initially calculated MoMs from a published normal median equation derived from FMF data (Nico- laides et al., 1999). However, we have subsequently found that more accurate MoMs are calculated from an equation that allows for a specific centre where the NT is being measured. *Correspondence to: Dr H. Logghe, Department of Feto-Maternal Medicine, Academic Unit of Obstetrics and Gynaecology, D floor, Clarendon Wing, Leeds General Infirmary, Leeds LS2 9NS. UK. E-mail: hilde@doctors.org.uk METHODS LASS provides a national Down syndrome screening service based on a five-marker protocol: four mater- nal serum markers and ultrasound NT. Serum sam- ples are collected throughout the country and sent to Leeds for the determination of free β -human chorionic gonadotrophin (hCG), unconjugated oestriol (uE 3 ), α- fetoprotein (AFP) and pregnancy-associated plasma pro- tein A (PAPP-A). Samples taken at 10 to 13 weeks are tested for these 4 markers, but those taken at 14 weeks or later are tested for free β -hCG, uE 3 , AFP and inhibin A. Biochemical analyses are performed using a time- resolved fluorescent assay (Perkin-Elmer Life Sciences, Turku, Finland) except for inhibin, which is an enzyme immunoassay (Serotec Ltd., Oxford, UK). NT measure- ments are carried out locally at more than 25 centres throughout the country. All five markers are expressed in MoMs, where gestational age is derived from the crown-rump length (CRL), measured at the same time as the NT, using a published formula (Robinson and Flem- ing, 1975). For the serum markers, the normal medians are derived by regression and are adjusted for maternal weight, when available. For NT, a published regression equation was initially used (Nicolaides et al., 1998), but now a centre-specific equation has been adopted. RESULTS The present analysis is restricted to 4765 tests where the NT was measured by the 13 centres that had performed scans on at least 50 women for LASS. The characteristics of each centre are summarised in Table 1. All scans were performed by staff who had been formally trained in NT measurement and accredited by the Fetal Medicine Foundation. A trans-abdominal ultrasound examination was used most of the time to obtain the image. Copyright  2003 John Wiley & Sons, Ltd. Received: 20 June 2002 Revised: 17 January 2003 Accepted: 26 January 2003