Prenatal Cytogenetics

Prenatal detection of cytogenetic abnormalities is either by amniocentesis or by chorionic villus sampling. Samples are received from Obstetrics clinics in Ninewells.

Amniocentesis:

The test is routinely performed at about 16-weeks gestation, mostly as a consequence of maternal serum screening for Down syndrome. A sample of amniotic fluid (about 15ml) is drawn off transabdominally under ultrasound guidance. Cells from the fluid are cultured and karyotyped. As well as detecting Down syndrome, this technique may also be used for prenatal diagnosis in families carrying known chromosomal rearrangements, or in pregnancies with an abnormal ultrasound scan.

Results are routinely available in 12-14 days.

cytostube

Additionally, a QF-PCR rapid aneuploidy screening is carried out on all amniotic fluid samples, to detect trisomies responsible for Down, Edwards and Patau syndromes. Preliminary results are routinely available within 24-72 hours of the procedure, but are always backed up by full karyotyping, as not all chromosome anomalies are detected.

Sample requirements:

The amniotic fluid sample should be split between two sterile universal containers, labelled with patient details: 2ml to one tube, and the remainder to the other. A 5ml maternal blood sample should also be taken into an EDTA tube (RED or PURPLE top). The samples should be accompanied by an appropriate request form, and promptly dispatched to the laboratory. Gestational age and any relevant obstetric details or scan findings should be noted.

For NHS Highland amniocentesis the sample is split into three in clinic as previously discussed with local clinicians.

Samples with low cellularity, or significant maternal contamination may be unsuitable for QF-PCR. Follow up on long term culture takes up to 14 days.

Chorionic Villus Sampling (CVS):

A small sample of placenta is taken, transabdominally, under ultrasound guidance. CVS can be taken earlier in pregnancy than amniocentesis. However, the result may be slightly less reliable than amniocentesis, due to the possibility of confined placental mosaicism, and patients should be counselled accordingly.

cytopetri

A QF-PCR rapid aneuploidy screening is carried out on all routine CVS samples (as above). Preliminary results are routinely available within 24-72 hours of the procedure. Occasionally, a preliminary analysis on “direct” preparations may be performed instead of QF-PCR, for instance if there is a known familial rearrangement. Result from this is usually available the following day. Follow up from long term culture takes up to 14 days.

CVS sampling is particularly useful in cases where abnormality is detected on scan, but the pregnancy is not far enough advanced for amniocentesis sampling, or in cases where there is a previously known family history of a chromosomal or other genetic condition.

Solid Tissue Samples:

Solid tissues are referred following: recurrent pregnancy loss (3 or more); fetal abnormality detected prenatally or at post mortem. Samples may also be sent for mosaicism studies; or for cell culture for molecular genetic or biochemical studies.

Results are usually available within 28 days.

Sample requirements:

Fresh samples only should be sent, formalin fixed specimens are unsuitable. Sterile tissue transport medium is available from the laboratory on request (x 33107). Cord blood is also acceptable for late loss or termination. Fresh skin biopsy from live patients MUST be by prior arrangement.

Consult User Manual or phone the department for more details on sample suitability.

For specific queries about the prenatal cytogenetics service, contact Miss. C. Maliszewska at the department on 01382-496735 or E-mail: cmaliszewska@nhs.net

Patient queries should be directed to the Down screening service, Ninewells Hospital (Tel. Ninewells hospital, extension 35621; Direct Dial: 01382-425621), and to the individual consultants at Perth.