R22: Fetus with a likely chromosomal abnormality

Prenatal ultrasound imaging may identify features within a fetus that are suggestive of an underlying genetic aetiology. In such cases, the pregnant woman will usually be offered chorionic villus sampling or amniocentesis in order to obtain placental or fetal DNA on which to perform genomic testing.

Where a fetus has a likely chromosomal anomaly, the test request code is R22. This would initially activate rapid aneuploidy testing, usually via QF-PCR. Where no genetic cause is identified by this process, it is usually followed up with a chromosomal microarray. Requesting R22 will process both tests.

Where a monogenic cause for the fetal anomaly is suspected and the phenotype fulfils appropriate criteria, an R21 for rapid fetal exome sequencing can be requested via national pathways.

Advantages

• Chromosomal microarray provides detailed cytogenetic information.
• The turnaround time for a chromosomal microarray is usually two to three weeks.

Limitations

• Chromosomal microarray will not identify all genetic anomalies owing to limitations with its resolution.
• Arrays will not detect balanced chromosome rearrangements, and they have limited sensitivity for the detection of mosaicism.
• Microarray testing may identify variants of uncertain significance or incidental findings irrelevant to the testing indication.

For clinicians

• NHS England: National Genomic Test Directory