Prenatal tests
The genetic testing services are currently being reorganised by NHS England to form the NHS Genomic Medicine Service. A letter has been sent to prenatal referring centres clarifying referral pathways and can also be viewed here.
All prenatal samples should be sent with a 4ml maternal peripheral blood sample, collected in an EDTA tube, in order to enable maternal genotype exclusion in the rapid (QF-PCR) test. If the sample is being referred for microarray, please also send paternal blood in EDTA if available.
Sample requirements
Amniotic fluid
10 - 20ml collected aseptically into a sterile universal (for array testing (see below) minimum volume 15ml). Send as quickly as possible to the Cytogenetics Department, either same day, or for next day delivery Monday to Thursday. Avoid posting samples on Fridays, as they will be delayed over a weekend. Please use hospital transport or taxi and advise the department if samples are arriving in this way.
Chorionic Villus (CV)
10 - 20mg collected aseptically into a sterile universal with heparinised culture medium (available from the Cytogenetics Department). For microarray testing (see below) the minimum amount is 15mg. Send immediately to the Cytogenetics Department after sampling; the success of the test depends on short transportation times. On Fridays, CVs must arrive by 1pm to allow for the sample to be processed the same day.
Fetal blood samples (from ongoing pregnancies)
Please send 1~2ml blood sample in a lithium heparin (green cap) tube if karyotyping is required, or in EDTA if microarray is required. These referrals will usually be reported in 7-10 days.
Acceptance and rejection criteria
All amniotic fluid samples of greater than 1ml will be accepted and testing will be attempted. Small or blood stained samples may impact on the success and quality parameters of the tests. For chorionic villus samples where no identifiable fronds are present; the sample will be rejected. Small samples may impact on the success and quality of the parameters of the tests.
Preliminary tests - QF-PCR & FISH
Rapid testing for the common aneuploidies in amniotic fluids and CV samples is normally undertaken by QF-PCR in collaboration with the Department of Molecular Genetics. If this test is normal and microarray is not being carried out, this will be the final result.
Please note that blood stained amniotic fluid samples may not be suitable for a rapid aneuploidy screen test, although rapid tests will be attempted whenever possible.
FISH testing for prenatal samples is used in certain situations:
- To confirm aneuploidy detected by the QF-PCR test on amniotic fluids
- To test for a selected group of micro-deletion syndromes, for example DiGeorge syndrome, see constitutional FISH probe list (by clinical request). Please note that microarray will also detect DiGeorge syndrome.
Long term culture karyotyping
This will only be undertaken for samples where the rapid test result is abnormal. A result will usually be reported in 14 days of sample receipt. If microarray testing shows a normal result, karyotyping of the long term cultures will not be undertaken.
Microarray testing of amniotic fluid and chorionic villus samples
Please note that this service requires additional specific patient consent to be taken, prior to the sample being referred.
Parental blood samples
If a prenatal sample is referred for array testing (see below for referral categories) please send 4ml maternal and paternal peripheral blood samples in EDTA tubes. These may be used to aid the interpretation of prenatal microarray results.
Samples referred for microarray testing will have preliminary rapid tests, QF-PCR (amniotic fluid and chorionic villus samples). Microarray testing will only be undertaken if the results from these tests are normal, or if the results do not explain the scan abnormalities reported.
Fetal Exome Sequencing
This testing is for a fetus with anomalies likely to have a monogenic cause. This test may be arranged via the Nottingham Clinical Genetics Service who will liaise with the testing centre at Great Ormond Street. This test will only be carried out after normal rapid and microarray test results.
Referral categories
The following will be tested by long term culture karyotype:
- Raised screen risk for aneuploidy (only if an abnormal rapid result, normal rapid results will receive no further testing)
The following will be tested by microarray (if the patient opts out of microarray testing, long term culture karyotyping will only be carried out if the rapid test is abnormal):
- One or more structural abnormalities on scan
- Isolated Nuchal translucency (NT) measurement of ≥ 3.5mm
- Known carriers of chromosome rearrangements; for detection of balance v imbalance (carrier status testing not carried out)
- Fetuses with a sex chromosome aneuploidy that is unlikely to explain the USS anomaly
Please note: In order to fully interpret array test results a full description of any scan abnormalities should be included on the referral card.
Reporting target times
Please see our Target Reporting Times.
We aim to report prenatal microarray tests within the 14 day reporting time target however where microarray testing of parental samples (and/or DNA from long term culture) is necessary, the reporting time may exceed 14 days.
Limitations
Please see our limitations of tests page.
Storage and further testing
Material from rapid testing
DNA from the amniotic fluid or CVS QF-PCR test is stored for at least 10 years.
Material from microarray testing
DNA from CV and amniotic fluids used for microarray testing is stored for at least 10 years.
Material from long term cultures
Fixed material from CV and amniotic fluid long term cultures (where karyotype is abnormal) is stored for at least 10 years. Cell pellets from long term cultures and normal fixed material is kept for 8 months from sample receipt. Long term cultures are maintained for 9 weeks after sample receipt.
Page last updated 31/01/2023. Please note that if printed, this information is only valid on the day of printing.