What is Fluorescence In Situ Hybridization (FISH) test?

What is the FISH Test?

Fluorescence In Situ Hybridization (FISH) is a molecular cytogenetic technique that uses fluorescent DNA probes to detect and localize specific genetic sequences on chromosomes in both dividing (metaphase) and non-dividing (interphase) cells. 1

Core Principle

FISH is based on the complementary nature of DNA strands, where fluorophore-labeled DNA probes hybridize to their complementary sequences in tested cells and tissues, which are then visualized through fluorescence microscopy. 2

Two Main Testing Approaches

Metaphase FISH

• Performed on dividing cells to gain information about chromosome structure not readily visible by conventional banding techniques 1
• Functions as an adjunct to conventional chromosome analysis 1
• Common applications include:
  • Detection of microdeletions 1
  • Detection of cryptic rearrangements involving subtelomeric regions 1
  • Characterization of structural chromosomal abnormalities 1

Interphase FISH

• Performed on non-dividing cells to detect and quantify specific genomic targets 1
• Key advantage: Does not require mitotic cells, making it practical to examine large numbers of cells and samples with low or no mitotic index 1
• High sensitivity: Often eliminates the need for repeated chromosome analysis in disease monitoring 1
• Common applications include:
  • Detection of aneuploidy in uncultured amniocytes 1
  • Detection and quantification of abnormalities in hematological malignancies 1
  • Analysis of solid tumor specimens 1

Clinical Diagnostic Applications

FISH has become one of the fastest-growing areas in cancer diagnosis, using panels of gene-specific probes to detect somatic recurrent losses, gains, and translocations in both hematologic and solid tumors. 2

Additional diagnostic uses include:

• Constitutional genetic disorders: Common aneuploidies, microdeletion/microduplication syndromes, and subtelomeric rearrangements 2
• Infectious disease detection: Identification of microbes and parasites like malaria in human blood cells 2
• Gene mapping and identification of novel oncogenes 3

Important Limitation

FISH detects only its intended targets and provides no information about additional abnormalities that may signal disease progression or secondary disease. 1 This target-specific nature means clinicians must select appropriate probes based on the suspected diagnosis.

Regulatory Considerations

FISH probes fall into four FDA regulatory categories: 1

• FDA-approved probes/kits for in vitro diagnostics
• Analyte-specific reagents (ASRs) manufactured under good manufacturing practices
• Research use only (RUO) or investigational use only (IUO) probes requiring disclosure of FDA status when reporting results
• In-house developed probes not actively regulated by FDA but requiring performance characteristic validation