Fluorescence in situ hybridization, or FISH, is a test that detects chromosomal abnormalities (aneuploidy). It is used in cases with a suspicion for chromosomal abnormalities that require a quick answer.
Unlike karyotype tests where results are given after 2-3 weeks, FISH test results are usually given within 48 hours. It is considered effective in cases of a high suspicion for chromosome abnormalities, or in advanced stages of the pregnancy.
FISH is a biological test performed on an amniotic fluid sample. It assesses for different chromosome abnormalities. In addition, biological markers can allow the identification of specific chromosomal abnormalities, but this is usually performed in special cases where an in-depth assessment is required.
When is the test needed?
FISH tests are performed when there is a specific suspicion for a chromosome abnormality, usually after the detection of abnormalities by ultrasound examination or alpha feto-protein examinations.
In most pregnancies with a high risk of chromosome abnormalities, parents are not sent directly to a FISH test, but instead are offered a karyotype test first. A karyotype test analyzes all the chromosomes, and its answers are available after several weeks.
In special cases, when an answer is needed quickly, doctors prefer offering FISH testing, where results are available within 48 hours. Such cases include:
Advanced pregnancy – when, for various reasons, the amniocentesis is performed in advanced stages of the pregnancy (such as after a late anatomy scan or in order to wait for alpha fetoprotein test results) and there is a risk of chromosomal abnormalities, there is a need for quick results. In these cases, parents who still want to be able to decide whether to continue the pregnancy do not have the time to wait three weeks until the arrival of test results and need an immediate answer, which can be given via a FISH test.
Suspicion of chromosome abnormalities – The different prenatal tests are meant to screen for possible defects and ensure the fetus’ wellbeing. These tests include nuchal translucency, alpha fetoprotein, anomaly scans and maternal blood screening in the 1st trimester. When the test results are irregular, a FISH test is recommended for a fast result.
Unlike other cases that require an amniocentesis, such as an advanced maternal age or irregular nuchal translucency test results, cases of a high risk of chromosomal abnormality require immediate answers, parents are sent to FISH testing since there is no time to wait.
Parents’ reassurance – Some doctors recommend a FISH test even in cases of a low risk of chromosome abnormalities, to reassure the parents. For instance, when screening test results are unclear, or if an unclear finding is shown in an ultrasound test, doctors prefer recommending a FISH test to reassure the worried parents until the arrival of the karyotype test, which may take three weeks.
It is important to point out that FISH testing does not replace karyotype testing but is usually complementary. FISH tests have a success rate of around 85%, and it does not give information on all chromosomes, so parents are almost always offered karyotyping in addition to the FISH testing.
How does the test work?
After receiving genetic material samples from an amniocentesis or CVS, the fish test is performed by adding fluorescent markers to the sample. These markers attach to the chromosomes which allows identifying the pairs of markers (representing the pairs of chromosomes) through a microscope.
In irregular test results, instead of a pair of markers the test will show three markers in the relevant chromosome instead of two, indicating Down’s syndrome (Down’s syndrome is caused by three copies of chromosome 21 instead of two).
Most FISH test kits include markers for the main chromosomes relevant to trisomies 21, 18 and 13, and to the sex chromosomes (X and Y). in addition, extra dyes can be added to attach to specific DNA fragments with a risk of a defect. These cases are more complex and are usually nor performed unless specifically required.
What conditions can the test detect?
FISH testing can detect diseases related to the sex chromosomes. It can be used to eliminate most chromosomal abnormalities – Down’s syndrome, Edwards syndrome, Patau syndrome, Turner syndrome and Klinefelter syndrome.
It is important to stress that FISH tests can identify around 85% of chromosomal abnormalities, and in any case, parents should wait for the karyotype test results in order to be 100% sure.