Overview
FISH hybridizes fluorescent DNA probes to complementary chromosomal targets, visualized by fluorescence microscopy to detect copy-number and structural abnormalities.
Clinical Importance
Detect oncogenic rearrangements and amplifications (e.g., HER2, ALK, BCR-ABL confirmation contexts)
Identify microdeletion/duplication syndromes when ordered
Specimen Requirements
- Specimen: formalin-fixed paraffin-embedded (FFPE) tissue, fresh/frozen tissue, or cytology material as validated
- Tumor content: pathologist assessment of neoplastic cellularity often required before testing
- Handling: minimize fixation delays and harsh acid decalcification that damage nucleic acids
- Input: provide adequate tissue; microdissection may be used to enrich tumor
- Hematologic FISH often uses bone marrow or peripheral blood in appropriate anticoagulant
Analytical Method
Cells/tissue are fixed, denatured, and hybridized with locus-specific, centromeric, or break-apart probes. Signal patterns (fusion, split, amplification) are scored in a defined number of nuclei per SOP.
Instruments Used
Automated analyzers
- Fluorescence microscopes with FISH imaging systems
- Automated FISH spot counters (selected laboratories)
Manual methods
- Manual probe hybridization and microscopic signal scoring
Reference Values
Molecular cutoffs, units (IU/mL, copies/mL, Ct), reportable ranges, and variant classifications are assay- and laboratory-specific. Always interpret against the performing laboratory report, manufacturer IFU, and current CLSI / CAP / AMP / ACMG guidance as adopted locally. Limit of detection and inhibition controls are essential for negative results.
Cutoff percentages for abnormal signal patterns are probe- and laboratory-validated. Normal = pattern within cutoff; abnormal = exceeds validated threshold.
Interpretation
Common Causes of Increased Results
- Abnormal signal pattern above cutoff; rearrangement/amplification/deletion detected as defined by the probe set
Common Causes of Decreased Results
- Normal pattern; does not exclude abnormalities outside probe coverage
Clinical Significance
- Solid tumor and hematologic molecular cytogenetics
- Companion diagnostics (e.g., HER2) when indicated
Factors Affecting Results
Pre-analytical
- Poor fixation or over-fixation
- Decalcification damaging probes targets
Analytical
- Scoring bias; inadequate nuclei counted
- Probe failure / QC failures
Post-analytical
- Reporting Ct or raw copies without assay-specific interpretive rules
- Misapplying another laboratory’s LOD or units
- Variant classification without current ACMG/AMP framework when required
Advantages and Limitations
Advantages
- Morphology retained
- Detects large structural changes PCR may miss
Limitations
- Targeted to chosen probes only
- Labor-intensive scoring
Clinical Pearls
FISH only sees the probe you paid for; a normal HER2 FISH says nothing about EGFR.
References
Authoritative textbooks, guidelines, and reviews supporting this laboratory test reference. Verify reference intervals, critical limits, and method details against institutional protocols and current guideline editions.
Textbooks
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2021. (Molecular diagnostics chapters, verify current edition.)
- Buckingham L. Molecular Diagnostics: Fundamentals, Methods, and Clinical Applications. 3rd ed. F.A. Davis; 2019. (Verify current edition and institutional adoption.)
CLSI and Professional Guidelines
- Clinical and Laboratory Standards Institute (CLSI). Consult current CLSI molecular method documents available through your laboratory, document numbers and editions change; do not treat any single CLSI title as universal.
- Institutional molecular laboratory standard operating procedures (SOPs), assay manufacturer instructions for use (IFU), and locally adopted accreditation requirements. Always verify current local practice.
- Clinical and Laboratory Standards Institute (CLSI). Molecular method evaluation and related documents (verify current CLSI editions applicable to the assay).
- College of American Pathologists. Molecular pathology accreditation and proficiency testing expectations (verify current CAP checklist requirements).
- Association for Molecular Pathology (AMP). Practice guidelines and position statements for molecular diagnostics (verify current AMP documents for the assay).
- American College of Medical Genetics and Genomics (ACMG). Standards and guidelines for interpretation of sequence variants and related genetic testing practice (verify current ACMG documents).
WHO Publications
- World Health Organization. Laboratory and diagnostic guidance relevant to molecular testing (verify current WHO publication for the assay/pathogen).
Frequently Asked Questions
Common questions about using this FISH laboratory test Medical Laboratory Library reference in Medical Laboratory Science education.
What is the FISH laboratory test reference used for?
This Medical Laboratory Library page summarizes the clinical importance of FISH. Review the Clinical Importance section for why the test is ordered in laboratory medicine practice.
Where can I find FISH specimen requirements?
See the Specimen Requirements section on this page for specimen information related to FISH. Confirm collection details against your laboratory protocol.
Where are FISH reference values listed?
See the Reference Values section on this Laboratory Library page. Reference ranges can vary by method and population, so verify intervals with your laboratory.
How do I interpret FISH results?
Use the Interpretation section for laboratory interpretation framing, including increased and decreased result patterns when provided. Do not treat educational text as a patient-specific diagnosis.
What analytical method is used for FISH?
See the Analytical Method section for principle and method notes. Method details may differ by instrument and institutional SOP.
Are related laboratory tests linked from FISH?
Yes. The Related Laboratory Tests section lists connected references for continued lookup in the Medical Laboratory Library.
How can I continue learning after reading FISH?
Use Continue learning links on this page for related Academy lessons, medical laboratory quizzes, or laboratory case studies when available for this Molecular Diagnostics topic.
What are the clinical pearls for FISH?
See the Clinical Pearls section for concise laboratory practice notes associated with FISH. Apply pearls alongside institutional protocols.
Reading progress
Awards XP once after you have meaningfully reviewed this article.