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Abstract
To properly interpret the result of a pregnant woman’s non-invasive prenatal test (NIPT), her a priori risk must be taken into account in order to obtain her personalised a posteriori risk (PPR), which more accurately expresses her true likelihood of carrying a foetus with trisomy. Our aim was to develop a tool for laboratories and clinicians to calculate easily the PPR for genome-wide NIPT results, using diploid samples as a control group. The tool takes the a priori risk and Z-score into account. Foetal DNA percentage and coefficient of variation can be given default settings, but actual values should be used if known. We tested the tool on 209 samples from pregnant women undergoing NIPT. For Z-scores < 5, the PPR is considerably higher at a high a priori risk than at a low a priori risk, for NIPT results with the same Z-score, foetal DNA percentage and coefficient of variation. However, the PPR is effectively independent under all conditions for Z-scores above 6. A high PPR for low a priori risks can only be reached at Z-scores > 5. Our online tool can assist clinicians in understanding NIPT results and conveying their true clinical implication to pregnant women, because the PPR is crucial for individual counselling and decision-making.
Highlights
To properly interpret the result of a pregnant woman’s non-invasive prenatal test (NIPT), her a priori risk must be taken into account in order to obtain her personalised a posteriori risk (PPR), which more accurately expresses her true likelihood of carrying a foetus with trisomy
Large clinical studies including about 150,000 pregnancies have reported a sensitivity and specificity for Non-invasive prenatal testing (NIPT) of more than 99% for foetal trisomy 13 or 21, and of 98% for trisomy 18 [refs 4 and 5, reviews refs 1 and 2]
A positive NIPT result with a sensitivity and specificity of more than 99% does not mean that she has more than a 99% chance of carrying a foetus with a trisomy
Summary
To properly interpret the result of a pregnant woman’s non-invasive prenatal test (NIPT), her a priori risk must be taken into account in order to obtain her personalised a posteriori risk (PPR), which more accurately expresses her true likelihood of carrying a foetus with trisomy. A positive NIPT result with a sensitivity and specificity of more than 99% does not mean that she has more than a 99% chance of carrying a foetus with a trisomy Her true likelihood depends on her NIPT result, and on the prevalence of the anomaly in the population she belongs to[12], which is expressed as an a priori risk. Not all cell-free foetal DNA screening providers calculate a Z-score or need a priori risks, it is important for women to know their true chance of carrying a Down syndrome foetus after a positive test. In clinical counselling and decision-making following a (positive) NIPT result, the PPR is the most important factor for the parents
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