FISH, or Fluorescence In Situ Hybridization, is a diagnostic prenatal test which looks for a few common chromosomal abnormalities. A fluorescent dye is used to visualize and map genetic material in cells. FISH is usually performed with the same genetic material gathered for testing during CVS or amniocentesis. The advantage of FISH testing is that results are often available in as little as 24-48 hours.
FISH has a low rate false positive results. If the fetal cells contain certain chromosomal abnormalities, this will be apparent with the FISH test. An exception might be if the parent has a chromosomal abnormality and the fetal cells were contaminated with parental cells.
A negative FISH result does not automatically mean the fetus is healthy. It only means the common chromosomal abnormalities FISH was used to test for are not present. Typically these are limited to Trisomy 13 (Patau syndrome), Trisomy 18 (Edward’s syndrome) and Trisomy 21 (Down syndrome) although this can vary by care provider or medical lab.
The FISH test does not reveal other, less common chromosomal anomalies unless the lab has set the test up to look for those specifically (which would be unusual). It also doesn’t reveal neural tube defects such as spina bifida or anencephaly, body wall defects such as omphalocele, brain defects or skeletal dysplasias.
There are a host of other genetic problems that could be detected by microarray or karyotyping. But there again, the results are only as good as the test. Karyotype will identify only large chromosomal abnormalities (deletions, translocations, extra copies) but not small defects in DNA. Microarray includes probes for many genetic markers, but not all of them.
The FISH test involves fixing, permeabilizing, and exposing the cells to a fragment of DNA bound to a fluorescent probe. This fragment of DNA is specific to a gene. If the gene is present, or if there is an increased number of gene copies, the cells will fluoresce and they can be seen with a microscope.
In the second test with cells gathered by amnio or CVS, cells are grown in a culture and then treated so they will all be at the same, optimal stage of cell division. This can take a week or two. Then cells are evaluated by karyotyping: images of each individual chromosome are collected and studied to determine exactly how many copies of each chromosome are present in each cell.
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