title: All MoMs are not equal: some statistical properties associated with reporting results in the form of multiples of the median.
abstract: The statistical procedure for discriminating between a Down syndrome or neural tube defect (NTD) fetus and a normal fetus relies, to a great extent, on the reporting of maternal serum alpha-fetoprotein (MSAFP), hCG, and uE3 results in the form of multiples of the median (MoMs). Further, threshold MoMs values for MSAFP, such as 2.5 MoMs, are often used to define a reference range to identify an NTD fetus. We show that a constant threshold-MoMs cutoff for MSAFP values actually refers to different percentiles of MSAFP levels at different gestational ages and that the combining of MoMs values between centers and gestational ages, such as suggested by Wald et al. for deriving a patient-specific risk index, is highly questionable. The results presented in this paper are quite general and will apply to all situations where MoMs are used.
authors.affiliation: School of Mathematics, University of Wales, Cardiff, United Kingdom.
authors.forename: J C
authors.initials: JC
authors.lastname: Bishop
authors.forename: F D
authors.initials: FD
authors.lastname: Dunstan
authors.forename: B J
authors.initials: BJ
authors.lastname: Nix
authors.forename: T M
authors.initials: TM
authors.lastname: Reynolds
authors.forename: A
authors.initials: A
authors.lastname: Swift
mesh_terms: D004314:Down Syndrome / Q000175:diagnosis*; D005260:Female; D005315:Fetal Diseases / Q000175:diagnosis*; D005865:Gestational Age; D006801:Humans; D009436:Neural Tube Defects / Q000175:diagnosis*; D016011:Normal Distribution; D011237:Predictive Value of Tests; D011247:Pregnancy; D011296:Prenatal Diagnosis / Q000592:standards*; D012016:Reference Values; D012494:Sampling Studies; D000509:alpha-Fetoproteins / Q000032:analysis*
chemical_list: D000509:alpha-Fetoproteins