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Upper and lower truncation limits are commonly applied to quantitative markers used in medical screening tests. We here examine data on 375 trisomy 18 and 522,081 unaffected singleton pregnancies, to determine if the lower truncation limit should be set below the previously specified 0.2 multiples of the median. A lower truncation limit of 0.15 would reduce the underestimation of the risk of having a trisomy 18 pregnancy in about 50% of affected pregnancies and would lead to an estimated 10 percentage point increase in the detection rate, with only a very small increase in the false-positive rate. [ABSTRACT FROM AUTHOR]

To examine the Down's syndrome screening positive rate among in vitro fertilisation (IVF) pregnancies, we measured second trimester serum marker levels in singleton IVF pregnancies (cases) and in five non-IVF pregnancies (controls) matched to each case for gestational age, age of mother, and duration of storage of the serum sample. There were 151 IVF pregnancies in which alpha fetoprotein, unconjugated oestriol (uE3), free beta-human chorionic gonadotrophin (hCG) and total hCG were measured, 104 IVF pregnancies in which free alpha-hCG was measured, and 39 IVF pregnancies in which inhibin A was measured. Median uE3 levels were 6% lower (P = 0.003), median free beta-hCG 9% higher (P = 0.024), and median total hCG 14% higher (P = 0.026) in IVF pregnancies compared with controls. The screen positive rate in the IVF pregnancies (28%) was about twice as high as that in controls (17%). High hCG levels may be explained by progesterone remaining high in IVF pregnancies. The low uE3 levels remain unexplained. In Down's syndrome screening in IVF pregnancies hCG and uE3 values should be adjusted to avoid the high screen positive rate.