What is Fluorescence In Situ Hybridization (FISH) test?

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Last updated: March 6, 2026 • View editorial policy

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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

References

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.