Prenatal tests

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 array testing (see below) minimum amount 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 2pm 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 will be accepted and attempted however if less than 1ml QF-PCR is not available. 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.

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.

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
  • To check for sex chromosome aneuploidy as part of rapid screen (by clinician request, e.g. indication of Turner syndrome)
  • 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 tests(by clinical request)
  • To investigate structural chromosome abnormalities

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.

 

Long term culture karyotyping

After these preliminary rapid results the final karyotype will be reported from the long term cell cultures usually by 14 days. If array testing shows a normal result, karyotyping of the long term cultures will not be undertaken.

 

Array 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.  Informed consent should only be taken by staff members who have been trained.

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 to aid the interpretation of prenatal array results.

Samples referred for microarray testing will have preliminary rapid tests, QF-PCR (amniotic fluid samples) or direct culture (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.

 

Referral categories

The following will be tested by karyotype:

  • Raised screen risk for aneuploidy.
  • Prenatal testing for known carriers of chromosome rearrangements (detectable by karyotype/FISH)
  • Family history of numerical chromosome abnormalities
  • Cytogenetic testing of prenatal samples referred for monogenic or biochemical studies (samples for onward referral  for  monogenic or biochemical studies will require accompanying information regarding the tests required, the identity of the laboratory to carry out these tests and who will be responsible for the costs of testing)
  • Prenatal testing because of maternal anxiety (at the clinician’s request).

 

The following will be tested by microarray (unless otherwise requested, if microarray testing is not required please state clearly on the referral form):

  • One or more structural abnormalities on scan
  • Isolated Nuchal translucency (NT) measurement of ≥ 3.5mm
  • Known carriers of chromosome rearrangements (not detectable by conventional cytogenetic techniques)
  • 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 reporting timetable.

We aim to report prenatal array tests within the 14 day reporting time target however where array testing of parental samples (and/or DNA from long term culture) is necessary there may be a delay.

 

Limitations

Please see our prenatal limitations of testing.

 

Storage and further testing

Fixed material from reported cases is stored for 8 months from receipt of the sample. At this point cases reported as normal karyotype are discarded; for cases reported with an abnormal karyotype the fixed material is stored for at least 10 years. Cultured cells are maintained for up to 6 weeks from the date of report at which time they are discarded. Requests for any further testing/onward referral from these cultures should be made before that date. DNA is stored indefinitely.

Page last updated 03/05/2018. Please note that if printed, this information is only valid on the day of printing.