Prenatal Fluorescence In Situ Hybridisation (FISH)

Fluorescence in situ hybridisation (FISH) is a technique that uses molecular DNA probes to detect complementary DNA sequences along a chromosome. The single-stranded probe DNA is allowed to hybridise to the single-stranded target DNA. The probe is labelled with a fluorochrome, which fluoresces when it is excited by a particular wavelength of light, and can be visualised using a fluorescence microscope.

FISH is a very rapid and specific test, which can be used to analyse numerical and structural aberrations.

Prenatal Interphase FISH

Aneuploidies of chromosomes 13,18,21, X and Y account for 95% of all chromosome abnormalities detected at prenatal diagnosis. Prenatal FISH involves the application of chromosome specific probes ( 13,18,21,X and Y) to the undividing cells within the amniotic fluid. The results give an indication as to the number of each of these chromosomes present. Three copies of chromosome 21 for example, would indicate that the baby is likely to have Down syndrome. Prenatal FISH with the chromosome 22 probe TUPLE1 can be used to look for deletions associated with chromosome 22q11.2 deletion syndromes. This test can be requested in conjunction with anueploidy FISH.

Requirements for Prenatal FISH

5-10 mls of amniotic fluid is required for FISH. This is additional to the normal volume required for cell culture.

Prenatal FISH may be requested for any sample. The following guidelines apply:

• Where there is a strong suspicion of chromosome aneuploidy based on ultrasound findings there will be no charge to the patient.
• Pregnancies at a gestation of 20 weeks or more will not be charged.
• Other referrals will be charged a fee ( non-refundable through Medicare)

Results are ready within 24-48 hours.

Limitations of Prenatal FISH

Prenatal FISH cannot exclude structural changes of any chromosome or a numerical abnormality of any of the remaining untested chromosomes.

Occasionally the test will fail and a report will not be able to be issued. There will be no cost to the patient if the test fails. This can be due to any one of the following reasons:

• Poor hybridisation of the probes to interphase cells resulting in an ambiguous result.
• Insufficient cells in fluid ( amniotic fluids taken at less than 14 weeks can be problematic)
• Blood stained fluids may lead to equivocal result due to maternal cell contamination.

Back to Cytogenetics Laboratory

^TOP	FEEDBACK | SITE INDEX | DISCLAIMER | CONTACT | COPYRIGHT | PRIVACY Printer-friendly version Last Modified: 27-07-2004 14:21:30