New trisomy tests have limitations: study

Cytogeneticists from a US laboratory looked at 109 maternal samples that had initially been tested using one of four brands of the non-invasive blood test and re-evaluated by standard karyotyping.

While the new blood tests had a 94.4% predictive value for trisomy 21, it rated only 59.5% predictive for trisomy 18, 44.4% for trisomy 13, and 37.9% for sex chromosome abnormalities.

The authors suggest that if a result is positive for trisomy 18, for example, the likelihood that the fetus actually has the disorder is less than 60%.

Professor Jon Hyett,