Fig. 3

Sample challenge NIPT-02: reported z-scores and fetal fractions from 21 participants in the educational exercise. These sample lots were collected from women with a confirmed fetus with trisomy 21 (Down syndrome). For each laboratory, the reported fetal fraction is shown on the horizontal scale and the reported chromosome z-score on the vertical axis. The vertical line at a 4% fetal fraction indicates a level at which some laboratories will consider the level to be too low for reliable testing. The horizontal line at a z-score of 3 is a commonly used cut-off to classify results as screen positive (at or above 3) or screen negative (below 3) for Down syndrome. Open symbols show responses from international laboratories and filled symbols show responses from laboratories located in the US. Each symbol indicates a different lot, where those laboratories received an aliquot from the same enrolled pregnancy. All successfully identified this sample from a fetus affected with Down syndrome (trisomy 21). The regression lines show the known relationship of increasing z-scores with increasing fetal fraction, but the slope for the 21 observations is not statistically significant (p = 0.18). The z-scores are generally higher for the US laboratories but so are the corresponding intended fetal fractions.