Interest of foetal nasal bone measurement at first trimester trisomy 21 screening
A S Weingertner1, M Kohler, C Firtion
1Department of Obstetric & Gynaecology, Syndicat Inter-Hospitalier de la Communauté Urbaine de Strasbourg, Centre Médico, Chirurgical et Obstétrical, Schiltigheim, France.
Objectives:
The purpose of this study is to assess the feasibility of foetal nasal bone (NB) measurement during the first trimester of pregnancy, and to examine the contribution of this measurement to the prenatal screening for Down syndrome following the definition of NB threshold using ROC curves in an unselected population.
Methods:
This prospective study was carried out at our centre SIHCUS-CMCO (reference centre) from January 2002 to December 2004 on a total of 2,044 pregnant outpatients at gestational weeks 11-14. Only 1260 singleton foetuses were used for statistical analysis. In the 784 other patients, we were unable to obtain a correct image allowing a reproducible measurement. NB was measured during the same session as nuchal translucency (NT) measurement. Ten trained sonographers took part in the study. Correlation index was evaluated to shed light on a link between interest variables and NB. Screening values of NB measurement in T 21 were also calculated with NB measurement according to crown-rump length, and expressed as the best threshold of multiple of the median determined by ROC curve. Screening values of genetic ultrasound were then evaluated by adding NB measurement to maternal age and NT measurement.
Results:
Two thousand and forty-four patients were included. We indexed 30 cases of T 21, 14 cases of Trisomy 18, 10 cases of Trisomy 13 and 25 cases of other karyotype abnormalities. Feasibility of measurement was 62% of all cases. We observed a significant relation between NB and NT (p = 0.001 ), as well as between NB and crown-rump-length (p < 0.0001 ). However, size of NB was not correlated to maternal ethnic group (p = 0.314). At 0.6 multiple of the median thresholds, screening values of NB measurement in T 21 were: sensibility 32%, false positive rate 10%, positive predictive value 13.6%, and negative predictive value 96.9%. The likelihood ratio for T 21 in case of NB < or = 0.6 multiple of the median was 4.4 (2.0-9.4). Screening values for maternal age and NT measurement were: sensitivity 88%, false positive rate 23%,positive predictive value 9.7%, and negative predictive value 99.6%. Inclusion of NB measurement increased sensitivity to 100%, positive predictive value to 13.6%, and negative predictive value to 100%, and decreased false positive rate to 5%.
Conclusion:
NB measurement seemed to be a great sonographic marker for T 21. However, its low feasibility made it inadequate for routine settings in first trimester T 21 screening in an unselected population. Statistical independence with NT thickness needed to be further evaluated.
