1. SECTIONS:
2. preface & acknowledgements
3. genetics at a glance
4. contacts, support & testing
5. testing & pregnancy
6. newborn screening
7. cancer in the family
8. cardiovascular conditions
9. chromosomal conditions
10. clotting & bleeding conditions
11. cystic fibrosis
12. diabetes
13. fragile X syndrome and other causes of developmental delay
14. haemoglobinopathies
15. hereditary haemochromatosis
16. neurofibromatosis
17. neurological conditions
18. psychiatric conditions
19. genetics in practice
20. emerging genetic technologies
21. glossary

Genetic testing

 

Chromosome analysis

1. Prenatal diagnostic testing involves fetal or placental cells being examined to look at the number and
   structure of each chromosome. A full chromosome analysis, called a karyotype, allows the diagnosis
   of chromosomal abnormalities. A karyotype takes 7 to 14 days and the result will be sent to the referring clinician.
2. Where there are strong indications of a fetal anomaly (eg a markedly increased risk screening result), or where a preliminary result is required quickly, FISH (fluorescent in situ hybridisation) analysis may also be performed on samples obtained using CVS or amniocentesis. FISH results may be available in 1 to 2 days.

 

Indications for FISH:

1. Fetal anomaly detected on routine second trimester ultrasound scan.
2. Markedly increased risk result on a screening test.
3. Late gestation.
4. Parental anxiety.

 

Benefits of FISH:

1. Provides a quick preliminary result for the presence of Down syndrome and certain other chromosome trisomies within 24 to 72 hours.
2. FISH will detect chromosome abnormalities. The number of probes used will determine the type of chromosome abnormalities detected.
3. ‘Three-probe FISH’ will detect Down syndrome and sex chromosome abnormalities
4. ‘Five-probe FISH’ will detect Down syndrome, trisomy 18, trisomy 13 and sex chromosome abnormalities

 

The limitations of FISH

1. FISH can give false positive results, and thus an abnormal result needs to be interpreted cautiously if other indications of trisomy are not present.
2. FISH can give false negative results. A normal FISH result only excludes full trisomies of the chromosomes tested (usually 13, 18, 21, X, Y). FISH usually cannot rule out structural abnormalities of these chromosomes, nor trisomies of other chromosomes.
3. FISH does not replace a complete chromosome analysis (karyotype) and this should still be completed.
4. FISH results may be inconclusive if both normal and abnormal cells are present (mosaicism).

 

Arranging a FISH test

1. FISH is requested by the obstetrician or the ultrasonographer.
2. There is no Medicare rebate for FISH.
3. The charge for FISH depends on the laboratory doing the testing and the number of probes used.
4. Costs for three-probe FISH may be less than for five-probe FISH.
5. Contact the pathology or ultrasound service for details of costs.
6. Results are usually available in 24 to 72 hours.

Copyright | Disclaimer | Privacy