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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.

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.

All amniotic fluid 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.

 

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 a direct preparation 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. 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, CV Direct & FISH

Rapid testing for the common aneuploidies in amniotic fluids is normally undertaken by QF-PCR in collaboration with the Department of Molecular Genetics. If this test 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.

Rapid testing on CV samples is carried out by a direct culture. The quality of the chromosomes from the direct preparation will only allow detection of any numerical chromosome abnormalities and may detect large structural chromosome abnormalities. If chromosomes are not obtained from the direct cultures, FISH will be carried out on non-dividing interphase cells to screen for trisomies 13,18, 21 and sex chromosome aneuploidy. If this result is normal and microarray testing is not being carried out, this will be the final result.

FISH testing for prenatal samples is used in certain situations:

  • To confirm aneuploidy detected by the QF-PCR test and to assess trisomy 13 and 18 found on direct preparation
  • To check for aneuploidy on failed CVS direct chromosome preparations
  • To test for a selected group of micro-deletion syndromes, for example DiGeorge syndrome, see constitutional FISH probe list (by clinical request)

 

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 samples) or direct preparation (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 balanced 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 prenatal limitations.

 

Storage and further testing

Material from rapid testing

DNA from the amniotic fluid QF-PCR test is stored for at least 10 years. Fixed material from CV direct preparations with a normal result is stored for 8 months from sample receipt. Fixed material from direct preparations with an abnormal result are 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 02/12/2021. Please note that if printed, this information is only valid on the day of printing.