Fluorescent in Situ Hybridisation (FISH) is a procedure whereby unique DNA fragments are labelled with a flourochrome, hybridised to complementary DNA in nuclei or on chromosomes and visualised under a microscope using ultra-violet light.  The conditions of hybridisation are stringent enough toensure that only complementary sequences are identified and hybridised.

This unique property of DNA complementary hybridisation makes it possible to identify the number of these unique sequences present in an interphase nucleus and thus equate the number of signals to the number of such a chromosome present.  The number of these chromosomes present, is then determined.

For prenatal diagnostic procedures, unique DNA sequences for five (5) chromosomes, namely chromosomes 13, 18, 21, the X and Y are used.  These probes identify almost 95% of the most frequently occurring numeric abnormalities, like trisomy 21.

Full chromosome culture procedures make analysis of both numeric and structural abnormalities of all 46 chromosomes possible after culturing amniotic cells.

Aims of FISH:

To provide a rapid (within 48 hours) and reliable prenatal diagnosis for the most common trisomies.

  1. To reduce the time delay associated with cell culture.
  2. To alleviate anxiety by minimising the waiting time for a result.
  3. To enable the parents and the health team to decide the most appropriate action at the earliest stage of pregnancy.

Advantages of FISH:

The procedure makes prenatal results available within 24 to 48 hours.

FISH is ideal to exclude numeric abnormalities of chromosome 13, 18, 21, the X and Y, after a prenatal screen indicated an increased risk.

FISH can be performed on both early and late pregnancies (13 to 34 weeks).

100% accurate in the detection of major autosomal trisomies.

Concurrent testing for micro-deletion syndromes e.g. Di-George, Williams and Prader Willi can be performed.