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Cytogeneticists at Guy's and St Thomas' Hospital Regional Genetics Centre in London have implemented a service to diagnose trisomies 13, 18 and 21 by quantitative-fluorescent PCR (QF-PCR) in amniotic fluid and chorionic villus samples. Mann et al report that of tests on 1373 samples carried out between April 2000 and April 2001, 98% gave usable results, and there were no false positives or false negatives [Mann,K. et al (2001) Lancet 358, 1057-1061]. The problem with the remaining 2% of samples was contamination by maternal cells, which was easily detected. The results of all QF-PCR tests were checked by full karyotyping (studying the chromosome structure and complement in dividing cells), which is considered the "gold standard" in this field. However, QF-PCR is substantially faster than karyotyping, taking less than 2 days rather than about two weeks, and once the technique has been set up in a laboratory it is suitable for use on a large scale. Mann et al point out that in most cases the reason for referral for prenatal chromosome analysis is increased risk of one of the three most common trisomies. They suggest that for these women QF-PCR might become the technique of choice, as the risk of there being a different chromosomal abnormality in these cases is very small; their centre is currently attempting to quantify this risk.

Comment: Mann et al, and Adinolfi et al in an accompanying commentary, also comment on the vexed question of sex chromosome abnormalities. At present the sex chromosome status is routinely reported for all samples. It would be possible to include sex chromosome analysis in the QF-PCR test, but some geneticists question the value of information about abnormalities whose significance is uncertain. Adinolfi and Sherlock suggest that parents should be able to choose whether they wish to have sex chromosome analysis included in the test.