Objective: Nuchal translucency medians are used to convert nuchal translucency measurements to multiple of the median values for use in risk assessment. There has been much debate about the use of practitioner-specific medians instead of population-based medians to correct for practitioner bias. This study examined data from the California Prenatal Screening Program to evaluate the effect of individual medians on screening performance.
Methods: For this retrospective study, we identified first-trimester specimens in the California Prenatal Screening Program database for which screening results were calculated using a practitioner-specific median (n=55,286) and recalculated risk assessment using a population-based median. We looked at positive rates for Down syndrome and Trisomy 18 as well as overall screening results. We also looked at the recalculated risks for cases in which there has been a positive diagnosis of a chromosomal abnormality to determine if cases would have been missed if practitioner-specific medians had not been used.
Results: Nuchal translucency multiples of the median would be lower for the majority of nuchal translucency practitioners if a population-based median were used instead of practitioner-specific medians. Outcomes data from the California Prenatal Screening Program's database indicate that 14 positively diagnosed cases would have been missed by the screening program if population-based medians had been used.
Conclusion: The use of practitioner-specific medians corrects for practitioner bias in measuring nuchal translucency. Practitioner-specific medians improve screening performance and help meet detection rate goals.
Level of evidence: II.