Cytogenetics - prenatal and fetal

Specimen:

Consult pathologist prior to specimen collection. All specimens must be delivered immediately to the laboratory.

Chorionic villus sampling (CVS): 5-25 mg of tissue in a sterile container with tissue culture medium (for conventional karyotype or microarray testing).

Amniotic fluid: 20 mL of amniotic fluid collected into 2 x 10 mL sterile containers.

Fetal blood: 1-2 mL in sterile heparin tube or EDTA if microarray testing is required.

If fetal death in utero has occurred: fetal tissue (especially cartilage or cord) collected aseptically and placed in sterile saline or tissue culture medium.

Method:

Direct, short term and/or long term cultures are used depending on clinical requirements.

Results may take from 2 days to 3 weeks for completion.

The type and duration of culture vary for the indication and specimen type.

Cytogenetic harvesting, slidemaking, banding and karyotyping are performed on all specimens. FISH or rapid molecular methods such as Quantitative Fluorescent PCR are often used to rapidly screen samples for common aneuploidies before definitive (karyotype) results are available.

Fluorescence in situ hybridisation (FISH) on metaphase spreads may increase the accuracy of cytogenetic diagnoses in some situations.

See also Fluorescence in situ hybridisation.

Microarray methods are now available in some laboratories for prenatal testing.

Microarray testing has a higher resolution for gains/losses of genetic material than conventional karyotype, but will not detect balanced chromosomal rearrangements, Microarray testing may not detect mosaic gains or losses of genetic material.

Application:

Pregnancies at risk include:

women of advanced maternal age;

those with a previous history of children with chromosome abnormalities;

parent with a known chromosome abnormality; and

those with a high risk identified by maternal serum screening.

Investigation is also appropriate if abnormalities are detected on ultrasound or identified after delivery and following unexplained stillbirth or neonatal death.

Interpretation:

Findings are reported in terms of numerical and/or structural chromosome abnormalities (eg, deletions, translocations) - consult pathologist.

Reference:

Rooney DE ed. Human Cytogenetics, Constitutional Analysis, a Practical Approach. 3rd ed. Oxford University Press 2001. 

Gardner RJM and Sutherland GR. Chromosome Abnormalities and Genetic Counselling. 4th ed. Oxford University Press 2011.