FISH does not have false positive results. If the fetal cells contain certain chromosomal abnormalities, this will be apparent with the FISH test. The only exception might be if the parent has a chromosomal abnormality and the fetal cells were contaminated with parental cells.
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 or , body wall defects such as , or .
The FISH test involves fixing, permeablizing, and exposing the cells to a fragment of DNA bound to a florescent 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.
FISH, or Fluorescence In Situ Hybridization, is a diagnostic prenatal test which looks for a few common chromosomal abnormalities. 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.