Accuracy of trisomy 18 screening using the second‐trimester triple test

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Abstract
Objective To assess the accuracy of the calculated risk for trisomy 18 assigned to individual women screened with the second-trimester triple test. Methods The study was based on 382 598 women screened in the Ontario Maternal Serum Screening Programme between October 1993 and September 2000. Of the women screened, 111 cases of trisomy 18 were identified. Originally, 92 874 women were screened using a risk cut-off level method. Estimated risks of trisomy 18 were calculated by applying published population parameters for the remaining women screened using a fixed analyte cut-off method. Women were ranked according to their individual risk for trisomy 18 syndrome in decreasing order and divided into 12 groups. The mean calculated risks of having an affected pregnancy at term for each group were compared with the birth prevalence of the corresponding group after allowing for spontaneous fetal losses. Results Agreement between the mean calculated risks and the observed prevalence was seen across the entire risk range, although women identified as having high-risk pregnancies had an actual prevalence that was somewhat lower than that estimated by the screen. Conclusion The calculated risk for trisomy 18 syndrome assigned to the individual woman on the basis of the risk cut-off method accurately reflects their risk of having a term trisomy 18 syndrome pregnancy. Copyright © 2003 John Wiley & Sons, Ltd.