How to Interpret Your FISH Test Results

A FISH (fluorescence in situ hybridization) test result tells you whether specific genetic changes are present in your cells, and the report typically boils down to a few key elements: the number and arrangement of colored signals a technician counted under a microscope, whether those signals cross a laboratory-defined threshold for “positive” or “negative,” and sometimes a ratio comparing two signals to each other. The details vary depending on why the test was ordered, whether for breast cancer, a blood cancer, a prenatal screen, or something else entirely. Understanding a few core concepts makes the report much less intimidating.

What the Colored Signals Actually Represent

FISH works by attaching fluorescent tags to short stretches of DNA that match a gene or chromosome region your doctor wants to examine. When those tagged probes find their target inside your cells, they light up under a special microscope. The technician then counts how many bright dots appear per cell, what color they are, and whether they sit together or apart. That count is the backbone of your report.

In a normal cell with two copies of a chromosome, you would expect to see two signals of a given color. Extra signals can mean extra copies of a gene (amplification) or an extra chromosome (trisomy). Missing signals suggest a deletion. When two differently colored signals sit right on top of each other and produce a merged “fusion” signal, that usually means two genes that should be far apart have been joined together by a chromosomal rearrangement.

Not every unusual pattern is straightforward. In a study of over 1,000 bone and soft-tissue tumors tested with break-apart probes, abnormal signal patterns such as deletions, extra copies, and amplifications appeared at rates ranging from about 1% to 32% depending on tumor type, and these ambiguous patterns required confirmation by additional molecular tests in many cases.¹ This is why your report may include a comment that the result is “equivocal” or that further testing was recommended.

HER2 Testing in Breast Cancer

One of the most common reasons people encounter a FISH report is HER2 testing after a breast cancer diagnosis. HER2 is a protein that, when overproduced, drives aggressive tumor growth. If FISH shows too many copies of the HER2 gene, you are a candidate for targeted drugs like trastuzumab. The result hinges on two numbers: the ratio of HER2 gene signals to a reference signal on the same chromosome, and the average number of HER2 copies per cell.

Under current guidelines from the College of American Pathologists and the American Society of Clinical Oncology, a dual-probe FISH assay classifies results into groups. The ratio cutoff for amplification is 2.0 or higher, and an average HER2 copy number of 6.0 or more per cell also qualifies as amplified even if the ratio is below 2.0.² Copies between 4.0 and just under 6.0, with a ratio below 2.0, land in an equivocal zone that requires additional workup.

These thresholds carve up results into five groups. Group 1, where both the ratio is 2.0 or higher and the average copies are 4.0 or more, is straightforwardly positive. Group 5, with a ratio below 2.0 and fewer than 4.0 copies, is clearly negative. Groups 2, 3, and 4 sit between those poles and can cause confusion.³ If your report puts you in one of those in-between groups, your oncologist will likely order a recount on the same tissue, test with an alternative probe, or run immunohistochemistry to look at protein levels directly.

There is also a category pathologists call “non-classical” results: patterns like a high ratio but low total copies per cell (sometimes called monosomy pattern), or low ratio but very high copies (co-amplification). A multi-institutional study grouped these unusual patterns and found they created real diagnostic headaches, sometimes requiring alternate testing strategies to decide whether a patient should receive HER2-targeted therapy.⁴ If your report mentions any of these terms, it is not a sign that something went wrong with the test. It means your tumor’s biology does not fit neatly into the usual boxes, and your care team needs to look more closely.

What Happens When HER2 Results Are Equivocal

An equivocal FISH result is one of the more stressful outcomes to receive, because it means the lab could not definitively call you positive or negative. In practice, many equivocal cases do resolve to amplified when retested with an alternative probe set, potentially making those patients eligible for HER2-targeted treatment. One study found that roughly half of patients retested with an alternate probe moved into the amplified category, though the researchers noted there is not yet clinical trial data confirming that this particular subgroup benefits from targeted therapy to the same degree as classically amplified patients.⁵ If you receive an equivocal result, expect your oncologist to order at least one more round of testing before making treatment decisions.

Blood Cancer Panels

In blood cancers such as multiple myeloma and chronic myeloid leukemia (CML), FISH is not looking for a single gene amplification. Instead, labs run panels of probes targeting several chromosomal regions at once, because different genetic hits carry different prognostic weight.

In multiple myeloma, a standard FISH panel checks for abnormalities like translocation t(4;14), deletion of a region on chromosome 17 (del17p), and gain of an extra copy of a region on chromosome 1 (1q21). Each of these changes shifts your risk category. A study combining gene expression profiling with FISH results found that patients flagged as high-risk by both methods had significantly worse outcomes for progression-free survival and overall survival.⁶ If your myeloma FISH panel comes back with one or more of these hits, your hematologist will use the result alongside other staging tools to decide how aggressively to treat.

For CML, FISH looks for the BCR-ABL1 fusion gene, the hallmark of the disease. Most CML patients have a visible rearrangement between chromosomes 9 and 22, producing what is called the Philadelphia chromosome. But a small subset carry the same fusion gene in a hidden form, created by tiny DNA insertions too small for standard chromosome analysis to see.⁷ In rare cases, even FISH can miss these cryptic rearrangements, which is why molecular testing (RT-PCR) is used alongside FISH for CML monitoring.

The signal patterns on a CML FISH report can get technical. Using a dual-fusion probe system, a typical positive result shows one green signal, one orange signal, and two fusion (yellow) signals. Variations from that standard pattern, such as seeing only one fusion signal instead of two, can indicate that one of the derivative chromosomes lost some material during the rearrangement.⁸ Those details matter to your hematologist because they affect prognosis and treatment response, but as a patient the key question is simply whether the fusion is present or absent.

Why FISH Catches Things That Standard Chromosome Analysis Misses

You may wonder why your doctor ordered FISH on top of a regular chromosome test (karyotyping). The short answer is that FISH is far more sensitive for the specific abnormalities it targets. In a direct comparison among multiple myeloma patients, FISH detected numerical chromosomal abnormalities in about 95% of cases that had them, while karyotyping found only about 33%. When structural abnormalities were present, FISH caught 98% versus karyotyping’s 27%.⁹ Karyotyping still occasionally picks up something FISH does not, particularly complex rearrangements involving chromosomes not included in the FISH panel, so the two tests complement each other rather than being redundant.

Prenatal FISH Testing

If you received FISH results during pregnancy, the test was almost certainly checking for extra or missing copies of chromosomes 13, 18, 21, X, and Y. These are the chromosomes most commonly involved in viable aneuploidies (conditions where a baby has the wrong number of chromosomes), including Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), and Turner syndrome (monosomy X).

Prenatal FISH gives results within one to two days, much faster than the two-week wait for a full karyotype from cultured cells. In a study of 116 high-risk pregnancies, FISH correctly identified all seven abnormal cases, including four with trisomy 21, two with monosomy X, and one with trisomy 13, and every result matched the subsequent full chromosome analysis.¹⁰ A normal prenatal FISH result is reassuring for those five chromosomes, but it does not rule out every possible genetic condition. It only tests what the probes are designed to find.

Microdeletion Testing

FISH is also used to detect very small missing pieces of chromosomes that standard karyotyping cannot see. The most well-known example is the 22q11.2 deletion, which causes DiGeorge syndrome and related conditions involving immune problems, heart defects, and characteristic facial features. High-resolution chromosome banding, an enhanced version of karyotyping, failed to detect the deletion in cases where FISH clearly found it.¹¹

If your child is being evaluated for immune deficiency or certain heart abnormalities, your doctor may have ordered a FISH test targeting 22q11.2 specifically. In one series of 43 patients with suspected DiGeorge syndrome or a related condition, FISH confirmed the deletion in five, a detection rate that underscores how the clinical suspicion must be fairly strong before the test is ordered.¹² A negative FISH result in that context does not necessarily mean the child’s symptoms have no genetic basis; it means this particular deletion was not found, and other genetic tests may follow.

Cutoff Values and What “Positive” Really Means

One of the least intuitive parts of a FISH report is the cutoff threshold. Because no test is perfect, a small percentage of normal cells will show an apparently abnormal signal pattern just by chance, perhaps because two signals overlap, a probe did not bind cleanly, or a cell was sliced at an awkward angle during sample preparation. To distinguish real abnormalities from this background noise, each lab establishes a cutoff: a percentage below which an abnormal signal pattern is considered a false positive.

Labs generate these cutoffs by running the same probe on a pool of at least 20 to 25 samples that are known to be normal for the abnormality in question and then calculating the upper limit of the false-positive rate using statistical methods.¹³ The resulting number varies by probe. For some probes used in myeloma, a cutoff of 5% has been validated as reliable, meaning that if more than 5% of the cells counted show the abnormal signal, the result is called positive.¹⁴

This matters practically if your report shows a percentage close to the cutoff. A result of 4% abnormal cells when the cutoff is 5% would be called negative, even though a few abnormal-looking cells were spotted. Your doctor might recommend repeat testing in a few months to see if the number is rising. Conversely, a result well above the cutoff is unambiguous.

In mosaicism testing, where the question is whether someone carries two genetically different cell lines, the threshold matters even more. For Turner syndrome, one study considered a case to be a true mosaic only if more than 1% of cells showed a second sex-chromosome signal, after scoring at least 200 to 500 cells depending on the pattern.¹⁵ If your report mentions mosaicism, it means the lab found two different cell populations and the percentage of each is clinically meaningful.

When FISH Results Can Be Wrong

False positives and false negatives both occur, and knowing the common causes helps you ask the right questions. One well-documented source of false positives involves cells that have extra sets of chromosomes (polyploidy) rather than a true gene rearrangement. In lung cancer testing for ALK gene rearrangements, tumor cells with higher ploidy levels and larger nuclei have an increased chance of producing single signals that mimic a break-apart pattern, even though no rearrangement has occurred. Researchers confirmed this by showing that follow-up testing with a different method found no actual rearrangement in cases with borderline ALK FISH results.¹⁶

False negatives happen too. In CML, the BCR-ABL1 fusion gene can be generated by DNA insertions so small that FISH probes do not span them, producing a negative FISH result despite the disease being present.⁷ Tissue quality also plays a role: if the sample is old, poorly fixed, or too small, probes may not bind well and the signals can be hard to read. This is why some FISH reports include a comment about specimen adequacy.

Reading the Nomenclature on Your Report

FISH reports often include a string of abbreviations following the International System for Human Cytogenomic Nomenclature (ISCN). Something like “nuc ish(D7S486,CEP7)x2” means that on an interphase nucleus, two signals were seen for the D7S486 locus and two for the chromosome 7 centromere probe, a normal result. If you see “x3” for a probe, that means three copies were detected. Multiplication signs, probe names, and chromosome locations are all standardized, though the notation has been revised multiple times, most recently in 2024, to improve consistency across laboratories.¹⁷

In practice, though, the nomenclature can be inconsistent between labs. A survey by the College of American Pathologists found that the same biological condition was described in a multitude of different ways by different laboratories, making the notation impractical for direct comparison between reports without a pathologist to interpret it.¹⁸ The plain-language interpretation section of your report, where the pathologist writes whether the result is positive, negative, or equivocal, is the part designed for clinical decision-making. The coded nomenclature is more like the raw data.

Lung Cancer and Other Solid Tumors Beyond Breast

FISH is widely used in lung cancer to look for gene rearrangements that can be targeted with specific drugs. The two most common targets are ALK and ROS1 fusions, which are found in a small but important fraction of non-small cell lung cancers. Testing for these fusions on cytology specimens, such as fluid or fine-needle aspiration samples, is routine.¹⁹ If your lung cancer FISH report shows a positive ALK or ROS1 rearrangement, you are likely eligible for a class of drugs called tyrosine kinase inhibitors, which can produce strong responses in fusion-positive tumors.

FISH testing is also used for FGFR2 rearrangements in bile duct cancers (cholangiocarcinoma), where a positive result opens the door to targeted FGFR inhibitors. In a study of 226 cases, FISH detected FGFR2 fusions or rearrangements in about 10% of tumors, with a sensitivity of roughly 95% when compared against RNA-based sequencing as the reference standard.²⁰

How FISH Compares to Next-Generation Sequencing

You may hear that sequencing-based tests are replacing FISH, and there is truth to that trend, but FISH is not obsolete. RNA-based next-generation sequencing (NGS) can scan for dozens of gene fusions simultaneously and often identifies the exact partner genes involved, while FISH tests only the specific gene the probe was designed for. A comparison in lung cancer samples found that overall concordance between FISH and NGS for ALK, RET, and ROS1 fusions was above 97%, though NGS caught a few fusions that FISH missed and FISH remained useful when tissue was too scarce for sequencing.²¹ A broader comparison across methods confirmed concordance above 90% and noted that targeted RNA NGS was the most efficient single technique, but FISH picked up fusions in cases where molecular testing failed due to tissue limitations.²²

The practical takeaway: if your report was generated by FISH alone, the result is highly reliable for the specific targets tested but does not cover genes outside the probe panel. If your report includes both FISH and NGS, the two methods serve as cross-checks, and a discordance between them is something your oncologist will investigate rather than ignore.

FISH and Tumor Heterogeneity

One advantage FISH has over bulk sequencing methods is that it works at the single-cell level. A technician literally looks at individual cells and counts their signals one at a time. This means FISH can reveal that a tumor is not genetically uniform, a phenomenon called tumor heterogeneity. Some cells might show amplification while their neighbors do not. Research integrating FISH data with bulk sequencing data in glioblastoma cases demonstrated that FISH substantially improved the ability to reconstruct how different clonal populations within a tumor had evolved over time.²³

For you as a patient, heterogeneity on a FISH report can be reported as a percentage: “60% of cells showed amplification, 40% did not.” This kind of mixed result does not mean the test failed. It means your tumor has more than one genetic population, and your treatment team may weigh that information when choosing therapies, since a drug targeting the amplified population might leave the non-amplified cells unaffected.