Cytogenetics - constitutional

Specimen:

Blood is collected aseptically into sterile heparinised containers for conventional karyotyping. For microarray testing, an EDTA container is required. All specimens must be delivered immediately to the laboratory.

Infant: 1-2 mL of cord or peripheral blood

Child: 5 mL blood

Adult: 5-10 mL blood

Skin biopsies (for fibroblast culture) should be full thickness (2-3 mm in depth) obtained aseptically and placed into sterile saline or tissue culture medium.

Method:

Cultures of blood cells or fibroblasts are used for conventional karyotyping, depending on clinical requirements.

Whilst results of blood cell cultures may be available within 7 days, fibroblast cultures may take up to 3 weeks for completion.

Microarray test results may be available within 4-6 weeks.

Cytogenetic harvesting, slidemaking, banding and karyotyping are performed where ‘karyotype’ or ‘cytogenetics’ is requested. DNA extraction and microarray testing are performed.

Application:

Conventional karyotyping +/- FISH is used for:

Investigation of congenital anomalies;

psychomotor delay;

growth retardation;

recognisable syndromes;

delayed puberty; infertility;

recurrent spontaneous abortion (test both parents); and

microdeletion syndromes,see also Fluorescence in situ hybridisation.

Microarray testing is used to investigate patients with developmental delay/intellectual disability and/or multiple congenital anomalies.

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.

Microarray testing is rebated by Medicare only for investigation of developmental delay/intellectual disability OR autistic spectrum disorder OR patients with two or more congenital abnormalities. Note that karyotype and microarray testing will not be rebated by Medicare if requested on the same test episode.

Karyotype or microarray studies should be performed on the affected individual and sometimes family members may also require investigation.

Molecular genetics has now replaced cytogenetics for the diagnosis of Fragile X syndrome.

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. Oxford University Press 3rd ed 2001. 

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