A FISH test in pregnancy, specifically Fluorescence In Situ Hybridization, is a rapid prenatal test used to detect common chromosome abnormalities in a developing fetus. It uses a sample of amniotic fluid.
Understanding FISH
Fluorescence In Situ Hybridization (FISH) is a molecular cytogenetic technique that uses fluorescent probes that bind to specific DNA sequences on chromosomes. This allows for the identification and enumeration of specific chromosomes or chromosome regions. In a prenatal setting, FISH is primarily used to rapidly screen for common aneuploidies (abnormal number of chromosomes).
How FISH is Performed Using Amniotic Fluid
The FISH test, when performed on amniotic fluid, involves the following steps:
- Amniocentesis: A small sample of amniotic fluid is obtained through amniocentesis, a procedure where a needle is inserted into the amniotic sac under ultrasound guidance.
- Amniocytes Preparation: The amniotic fluid contains fetal cells called amniocytes. These cells are processed for FISH analysis.
- Hybridization: Fluorescent probes specific to chromosomes 13, 18, 21, X, and Y are applied to the prepared amniocytes. These probes bind (hybridize) to their corresponding DNA sequences on the chromosomes.
- Visualization: The hybridized cells are then visualized under a fluorescence microscope. The number of fluorescent signals for each chromosome is counted.
- Interpretation: An abnormal number of signals indicates a potential chromosome abnormality. For example, three signals for chromosome 21 would suggest Trisomy 21 (Down syndrome).
Advantages of FISH in Prenatal Diagnosis
- Rapid Results: FISH provides results much faster (within 24-48 hours) than traditional karyotyping, which can take 1-2 weeks. This allows for quicker decision-making regarding pregnancy management.
- Detection of Common Aneuploidies: FISH is highly effective in detecting the most common aneuploidies, such as Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), Trisomy 13 (Patau syndrome), and sex chromosome aneuploidies (e.g., Turner syndrome, Klinefelter syndrome). The reference confirms that FISH is a "rapid and reliable assay for detecting chromosome abnormalities in a prenatal setting."
- Reduced Anxiety: The faster turnaround time can help reduce parental anxiety associated with waiting for test results.
Limitations of FISH
- Limited Scope: FISH only screens for a specific set of chromosome abnormalities. It does not detect all possible genetic conditions.
- Confirmation Required: FISH results are considered preliminary and should be confirmed by traditional karyotyping or chromosomal microarray analysis (CMA).
Example Scenario:
A pregnant woman is at high risk for Down syndrome based on first-trimester screening. She undergoes amniocentesis and FISH analysis is performed on the amniotic fluid. The FISH results show three signals for chromosome 21. This indicates a high probability of Trisomy 21 (Down syndrome) in the fetus. Karyotyping is then performed to confirm the FISH results.
