FGFR2 and FGFR3 Genetic Test: Cancer Mutations, Fusions, and Results

FGFR2 and FGFR3 genetic testing looks for changes that can activate fibroblast growth factor receptor signaling in cancer cells. The most clinically important findings include FGFR2 fusions or rearrangements in intrahepatic cholangiocarcinoma and selected FGFR3 mutations or fusions in urothelial cancer, although these alterations also occur in other solid tumors. Testing is performed on tumor tissue, blood-based circulating tumor DNA, or occasionally another specimen selected by the oncology team. A positive result may support an approved targeted therapy, a clinical trial, or a more specific diagnosis, but the meaning depends on the exact variant, cancer type, disease stage, and treatment history. A negative result does not always rule out an FGFR alteration because sample quality, tumor content, and the testing method affect detection. Results should therefore be interpreted with the pathology report, imaging, prior therapies, and the laboratory’s statement about whether the alteration is known to be actionable.

• FGFR2 fusions or rearrangements are most established in unresectable or metastatic intrahepatic cholangiocarcinoma and can guide use of an FGFR inhibitor after appropriate clinical review.
• FGFR3 mutations and fusions are common actionable findings in some urothelial cancers, especially tumors with a luminal-papillary molecular pattern.
• RNA-based testing can improve fusion detection because it evaluates the expressed fusion transcript rather than relying only on DNA breakpoints.
• A positive result is not automatically treatment-eligible; the alteration must be susceptible, the cancer type and treatment setting must fit, and the patient must meet drug-specific criteria.
• A negative blood test may need tissue follow-up when the tumor sheds little DNA or the assay has limited coverage for rearrangements.

Table of Contents

• What the FGFR2 and FGFR3 test detects
• Cancers in which FGFR testing is used
• Testing methods, samples, and timing
• Understanding a positive FGFR result
• Negative, uncertain, and complex results
• Targeted therapy, monitoring, and resistance
• Questions to ask after FGFR testing

What the FGFR2 and FGFR3 test detects

FGFR2 and FGFR3 are genes that encode receptor tyrosine kinases. These receptors sit across the cell membrane and help transmit growth, survival, development, and tissue-repair signals. When a fibroblast growth factor binds to the receptor, the receptor pairs with another receptor and activates signaling pathways inside the cell. Cancer can exploit this system when a gene alteration keeps the receptor active without normal control.

An FGFR test does not look for one single abnormality. It may evaluate several classes of alteration:

• Single-nucleotide variants and small insertions or deletions. These changes alter a small part of the DNA sequence. Recurrent FGFR3 activating mutations include changes at codons such as S249, Y373, R248, and G370 in urothelial tumors. Laboratories usually report the exact protein and DNA notation.
• Fusions and rearrangements. Part of FGFR2 or FGFR3 becomes joined to another gene, creating an abnormal signaling protein or placing FGFR activity under different control. FGFR2 fusions in intrahepatic cholangiocarcinoma can involve many partners, including BICC1, but the partner does not always need to be known for the finding to be clinically meaningful.
• Amplification. Extra gene copies may increase receptor production. Amplification has less consistent predictive value than established activating mutations or fusions and is often interpreted in the context of copy number, tumor type, and clinical evidence.
• Other structural variants. Internal rearrangements, truncations, or unusual breakpoints may be detected by broad sequencing. Their significance can range from clearly activating to uncertain.

The word alteration is broader than mutation. A mutation usually refers to a sequence change, while an alteration may include a fusion, rearrangement, amplification, or another genomic event. Reports sometimes use “FGFR-positive” as shorthand, but the precise alteration is essential because different drugs and trials recognize different findings.

Most cancer FGFR testing is somatic testing, meaning it examines changes acquired in the tumor. A tumor result usually does not indicate that relatives inherited the same change. Rare constitutional or germline FGFR variants cause developmental syndromes, but that is a separate clinical setting. When a tumor report raises a possible inherited finding, the care team may recommend confirmation with a dedicated germline genetic test using blood, saliva, or another non-tumor sample.

The laboratory should classify the result according to available evidence. Common categories include pathogenic, likely pathogenic, variant of uncertain significance, likely benign, and benign. Oncology reports may also assign an evidence tier that reflects whether a finding is linked to an approved therapy, a professional guideline, a clinical trial, or early research. The variant classification and the therapy-evidence tier answer different questions: one addresses whether the alteration affects gene function, and the other addresses whether it currently helps guide cancer care.

Cancers in which FGFR testing is used

FGFR alterations appear across many tumor types, but the strongest routine uses are concentrated in several cancers. The diagnosis on the pathology report remains central; the same alteration can have different treatment implications in different organs.

Intrahepatic cholangiocarcinoma

FGFR2 fusions or rearrangements occur in a clinically important subset of intrahepatic cholangiocarcinoma, a bile duct cancer arising within the liver. Published series commonly place the frequency at roughly 10% to 15%, although estimates vary with population, tumor selection, and assay design. These alterations are much less characteristic of extrahepatic bile duct and gallbladder cancers.

For advanced intrahepatic cholangiocarcinoma, broad molecular profiling is often considered because several genomic findings can influence therapy. Identifying an FGFR2 fusion or rearrangement may make a patient eligible for an FGFR inhibitor in a later treatment line, depending on jurisdiction, prior treatment, drug labeling, and current guidelines. Testing should ideally occur early enough that the result is available before a change in systemic therapy is needed.

Urothelial carcinoma

FGFR3 activating mutations and FGFR2 or FGFR3 fusions occur in urothelial carcinoma of the bladder and upper urinary tract. FGFR3 alterations are particularly frequent in low-grade, non-muscle-invasive bladder tumors, but actionable testing has been most consequential in locally advanced or metastatic disease. A susceptible FGFR alteration can support use of an FGFR-directed drug in an appropriate treatment setting.

The prevalence depends strongly on stage, grade, and molecular subtype. A mutation that helped explain the biology of an earlier bladder tumor may still be relevant later, but retesting can be useful because tumors evolve and a small biopsy may not represent every disease site.

Other solid tumors

FGFR2 or FGFR3 alterations also occur in endometrial, gastric, breast, lung, brain, cervical, salivary gland, and other cancers. However, many findings outside established indications are investigational. A broad solid-tumor next-generation sequencing panel may identify them alongside other drivers, which can support a basket trial that enrolls patients by molecular alteration rather than tumor origin.

An FGFR finding can also contribute to diagnosis. Certain fusions or mutation patterns may support a tumor classification when reviewed with histology and immunohistochemistry. Molecular data should not replace the pathologist’s assessment; it adds another layer of evidence.

Testing may be ordered to identify an approved therapy or trial, clarify an unusual diagnosis, examine resistance, or avoid using limited tissue for several separate tests.

Because approved indications and guideline recommendations change, a result that was not actionable when first reported may become useful later. Some laboratories offer report updates or knowledge-base reinterpretation, but patients should not assume that this happens automatically.

Testing methods, samples, and timing

The best method depends on which alteration is being sought. Small mutations, fusions, and copy-number changes require different analytical strengths, so an assay’s gene list alone does not prove that it detects every clinically important FGFR event.

Tumor tissue testing

Formalin-fixed, paraffin-embedded tissue from a biopsy or surgery is the most common specimen. A pathologist marks tumor-rich areas and estimates tumor percentage. The laboratory extracts DNA, RNA, or both. Older tissue may still work, but prolonged fixation, decalcification, small cell numbers, and degraded nucleic acid can reduce success.

DNA next-generation sequencing can detect point mutations, small insertions and deletions, copy-number changes, and some rearrangements. Fusion detection is harder because breakpoints may occur in large introns or repetitive regions that a DNA panel does not fully cover. A report should state whether FGFR2 and FGFR3 rearrangements were assessed and what limitations apply.

RNA sequencing detects fusion transcripts produced by tumor cells. It can confirm that a rearrangement is expressed and may discover partners not predefined by the assay. For tumors in which fusions are a central biomarker, a combined DNA-and-RNA approach can reduce missed findings. The related RNA sequencing test requires sufficiently preserved RNA, which can be challenging in small or old specimens.

Fluorescence in situ hybridization, or FISH, uses probes to detect separation or abnormal arrangement of a gene region. It can work with limited tissue and may detect a rearrangement without identifying the fusion partner. Reverse-transcription PCR is highly sensitive for specific known transcripts but may miss unexpected partners. Immunohistochemistry is not generally a universal stand-alone substitute for genomic confirmation of actionable FGFR2 or FGFR3 alterations.

Blood-based testing

A liquid biopsy analyzes circulating tumor DNA released into blood. It can be useful when tissue is unavailable, unsafe to obtain, or too slow to acquire. It can also sample DNA from multiple metastatic sites and reveal resistance mutations. However, a negative plasma result is not definitive when the cancer sheds little DNA, the disease burden is low, or the assay is less sensitive for structural variants.

Fusion detection in plasma varies among platforms. Some DNA-based assays infer rearrangements from captured breakpoints, while others have limited coverage. A “no alteration detected” result should be read with the assay’s tumor-fraction estimate and technical notes. When clinical suspicion remains high, tissue-based testing with RNA analysis may be appropriate.

When to order the test

Testing at diagnosis of advanced disease often prevents a later delay. In intrahepatic cholangiocarcinoma, molecular profiling is commonly requested when unresectable or metastatic disease is recognized. In urothelial cancer, timing may depend on stage and expected treatment sequence. Repeat testing can be reasonable after progression, especially when the original test was narrow, the sample was poor, or resistance to targeted therapy is being investigated.

No fasting or special preparation is usually needed for a blood draw. A tissue test may use an existing specimen, so no new procedure is required unless the old sample is inadequate. A fresh biopsy has procedure-specific risks that should be discussed separately.

Before testing, confirm that the panel includes the alteration types relevant to the cancer. “FGFR2 included” may mean point mutations only, even when the clinical need is fusion detection. The requisition and report should identify specimen type, tumor percentage, sequencing method, genes and regions covered, detection limits, quality metrics, and whether orthogonal confirmation was performed.

Understanding a positive FGFR result

Interpret a positive report at four levels: the exact alteration, evidence that it activates FGFR signaling, relevance to the tumor type, and eligibility for a specific therapy or trial. The gene name alone is insufficient.

For example, “FGFR2 rearrangement detected” in intrahepatic cholangiocarcinoma may be highly relevant when the assay demonstrates an in-frame fusion or another qualifying rearrangement. “FGFR2 amplification” is a different biological event and may not meet the same treatment criteria. Likewise, a recurrent activating FGFR3 mutation in metastatic urothelial carcinoma carries stronger evidence than an FGFR3 variant of uncertain significance.

A report may include:

• gene and transcript used for naming;
• DNA-level notation, such as a coding change;
• protein-level notation, such as p.Ser249Cys;
• fusion partner and exons, when known;
• variant allele fraction, or VAF;
• copy-number estimate for amplification;
• classification and evidence tier;
• associated drugs, tumor types, and trials; and
• assay limitations.

The VAF is the fraction of sequence reads containing the variant. It is influenced by tumor purity, copy number, normal-cell contamination, and tumor heterogeneity. A VAF of 20% does not mean that 20% of the body carries the mutation, and it does not directly measure how aggressive the cancer is. In plasma, VAF may rise or fall with tumor DNA shedding, but cross-time comparisons are most reliable when the same validated assay and similar clinical conditions are used.

A fusion partner can help confirm an oncogenic event, but the absence of a named partner does not always invalidate a rearrangement. Some assays report a breakpoint near FGFR2 without fully resolving the partner. The molecular tumor board or laboratory may review whether the structure preserves the FGFR kinase domain and is likely to produce an active protein.

Positive results may be described as actionable, potentially actionable, or investigational. Actionable generally means the finding could influence management now, but it does not guarantee that treatment is appropriate. Eligibility may depend on:

• exact diagnosis and site of origin;
• locally advanced, unresectable, or metastatic status;
• prior systemic therapy;
• measurable disease and performance status;
• organ function and comorbidities;
• previous exposure to an FGFR inhibitor;
• regulatory approval in the patient’s country; and
• trial-specific inclusion and exclusion criteria.

A positive FGFR alteration can coexist with other mutations. The oncology team weighs which finding is the main driver and which treatment has the strongest evidence. Broader tumor molecular profiling helps place the FGFR result in that wider context.

Negative, uncertain, and complex results

A negative result means the assay did not detect a reportable alteration within its tested regions and technical limits. It does not prove that FGFR signaling is normal or that no alteration exists anywhere in the tumor.

Several factors can produce a negative result despite an underlying alteration:

• too few tumor cells in the specimen;
• degraded DNA or RNA;
• a breakpoint outside the covered region;
• an uncommon fusion partner not detected by a targeted method;
• low circulating tumor DNA in plasma;
• tumor heterogeneity, with the alteration present in an unsampled site;
• prior treatment changing the tumor population; or
• a reporting filter that excludes low-level or uncertain events.

The next step depends on why the test was ordered. If a DNA-only panel is negative in intrahepatic cholangiocarcinoma and fusion testing was incomplete, RNA-based analysis may add value. If a plasma test is negative and tissue is available, tissue testing can be more informative. If both are adequate and comprehensive, the team usually focuses on other biomarkers and standard treatments rather than repeatedly testing the same low-probability question.

A variant of uncertain significance, or VUS, is a change whose effect is not established. It should not be treated as an actionable FGFR mutation simply because it alters the gene. Evidence may later change, but treatment should be based on current classification, laboratory review, and clinical data. A useful variant classification explanation can help distinguish pathogenic findings from uncertain or benign changes.

Some reports identify an FGFR alteration at a very low VAF. In tissue, this may represent a small tumor subclone, sequencing artifact, or low tumor purity. In plasma, it could reflect a true tumor clone, but interpretation also depends on assay quality and whether the alteration is biologically plausible. Unlike certain blood-cell mutations, recurrent FGFR2 and FGFR3 driver events are less commonly attributed to clonal hematopoiesis, yet every unexpected plasma result still deserves correlation with tissue and diagnosis.

An amplification result needs special caution. Copy-number calls can be affected by tumor purity and overall chromosome changes. A modest gain is not equivalent to high-level focal amplification, and neither necessarily predicts response as reliably as a well-established activating mutation or fusion. The report’s copy-number estimate and evidence statement are more useful than the word “positive” alone.

Discordant tissue and plasma findings are possible. The tissue may be old, while plasma reflects current metastatic disease; conversely, plasma may have too little tumor DNA. Neither sample is automatically correct in every case. Clinicians compare dates, disease sites, treatment exposure, assay coverage, and quality metrics before deciding whether confirmation is needed.

Targeted therapy, monitoring, and resistance

FGFR inhibitors block signaling from abnormal fibroblast growth factor receptors. Several drugs have established roles in selected FGFR-altered cancers, while others are being studied. Treatment choice must follow the current drug label, guideline, and patient-specific clinical situation.

In previously treated, unresectable locally advanced or metastatic intrahepatic cholangiocarcinoma with qualifying FGFR2 fusions or rearrangements, approved options in some regions include pemigatinib and futibatinib. In advanced urothelial carcinoma with susceptible FGFR3 or FGFR2 alterations, erdafitinib is an established option in a defined treatment setting. Regulatory wording and required prior therapy can change, so the oncology team should verify the current indication rather than relying on an older molecular report.

FGFR inhibitors have distinctive adverse effects because normal FGFR signaling helps regulate phosphate, skin, nails, eyes, and mucosal tissues. Common or clinically important issues can include:

• increased serum phosphate;
• mouth sores, dry mouth, or taste changes;
• diarrhea or constipation;
• fatigue;
• dry skin, hand-foot symptoms, and hair changes;
• nail lifting, brittleness, or infection;
• eye disorders, including central serous retinopathy or retinal pigment epithelial detachment; and
• changes in liver or kidney laboratory values.

Monitoring often includes serum phosphate, kidney and liver function, medication review, symptom checks, and scheduled eye examinations. Hyperphosphatemia can be an expected pharmacodynamic effect, but it still requires management. The clinician may recommend diet changes, phosphate binders, dose interruption, reduction, or discontinuation according to severity and the specific drug protocol. New blurred vision, visual distortion, dark spots, or sudden visual change should be reported promptly.

Drug interactions matter. Acid-reducing medicines, strong enzyme inhibitors or inducers, and drugs that affect cardiac rhythm may alter exposure or risk for some FGFR inhibitors. Patients should provide a full list of prescriptions, over-the-counter medicines, vitamins, and supplements. Pregnancy prevention requirements and fertility questions should be discussed before treatment.

Cancer can develop resistance even after an initial response. In FGFR2-rearranged cholangiocarcinoma, secondary mutations in the FGFR2 kinase domain can reduce drug binding. Multiple resistance mutations may arise in different tumor clones at the same time. Other tumors escape through alternative signaling pathways or changes outside FGFR.

Repeat tissue testing or a circulating tumor DNA liquid biopsy at progression may identify resistance mechanisms. The result can support a trial, next-generation inhibitor, or non-FGFR therapy, although resistance testing may not reveal a usable target.

Treatment response is assessed primarily with imaging, symptoms, physical examination, and routine laboratory tests. Molecular changes in plasma can add information but do not replace standard response criteria unless used within a validated protocol. A falling variant level may be encouraging, yet it should not prompt treatment changes by itself when scans and clinical findings disagree.

Clinical trials remain important because FGFR biology is diverse. Trials may study more selective FGFR2 or FGFR3 inhibitors, agents active against resistance mutations, antibody-drug conjugates, combinations, or tumor-agnostic approaches. A trial search should use the exact alteration, cancer type, prior treatments, location, and current disease status.

Questions to ask after FGFR testing

A structured review of the report helps turn a technical finding into a usable care plan. Bring the complete molecular report, not only a portal message that says “positive” or “negative.”

Ask the oncology team or molecular specialist:

1. What exact FGFR2 or FGFR3 alteration was found? Request the gene, variant or fusion, classification, and evidence tier.
2. Was the test designed to detect both mutations and fusions? Confirm whether DNA, RNA, FISH, or another method was used.
3. Is this alteration considered susceptible for an approved drug in my cancer type and treatment setting? A laboratory therapy list may include options that are approved only for another tumor.
4. Does the specimen have enough tumor and adequate quality? Review tumor percentage, sequencing quality, and any “limited” or “quantity not sufficient” statements.
5. Would another specimen change confidence? Ask whether tissue confirmation, RNA testing, or plasma testing would fill a specific gap.
6. What treatment options have stronger or equal evidence? Consider standard chemotherapy, immunotherapy, surgery, radiation, other biomarkers, and clinical trials.
7. What monitoring is required with an FGFR inhibitor? Clarify phosphate testing, eye examinations, medication interactions, and symptoms that need prompt reporting.
8. Could repeat molecular testing help at progression? Discuss when it would be useful and whether tissue or blood would be preferred.

Patients can also ask whether the case should be reviewed by a molecular tumor board. These teams combine oncology, pathology, genetics, pharmacy, and laboratory expertise, which is especially helpful for rare fusions, unusual tumor types, or conflicting reports.

Keep a dated copy of every pathology and molecular report. Record the specimen site, collection date, assay name, and treatments given before and after testing. That timeline helps clinicians determine whether an older result still represents the current cancer.

Confirm insurance authorization, laboratory coverage, and possible financial assistance before testing or treatment.

An FGFR result is most useful when it answers a defined clinical question. A activating alteration detected by an appropriate method can open a targeted option. A negative or uncertain result can still be valuable by preventing an unsupported therapy and directing attention to other biomarkers. The report should lead to a plan: treat now, seek a trial, confirm with another method, monitor for future relevance, or proceed with non-FGFR care.

References

• Futibatinib for FGFR2-Rearranged Intrahepatic Cholangiocarcinoma 2023 (Clinical Trial)
• Erdafitinib or Chemotherapy in Advanced or Metastatic Urothelial Carcinoma 2023 (RCT)
• Biliary tract cancer: ESMO Clinical Practice Guideline for diagnosis, treatment and follow-up 2023 (Guideline)
• Expert Consensus on the Diagnosis and Treatment of FGFR Gene-Mutated Solid Tumors 2024 (Consensus Statement)
• Precision oncology targeting FGFRs: A systematic review and meta-analysis 2024 (Systematic Review)
• Erdafitinib in patients with advanced solid tumours with FGFR alterations (RAGNAR): a phase 2, multicentre, open-label, single-arm study 2023 (Clinical Trial)

Disclaimer

This information is educational and does not replace individualized advice from an oncologist, pathologist, genetic counselor, or pharmacist. FGFR treatment eligibility depends on the exact alteration, cancer type, prior therapy, current approvals, and overall health. Do not start, stop, or change cancer treatment based only on a molecular report or this article.