How to Interpret Your NGS Report? 9 Critical Facts That Change Clinical Decisions

An NGS report shows the genomic alterations detected in the tumor or blood; however, it does not, by itself, determine which treatment will definitely work. Results must be evaluated together with specimen quality, tumor type, disease stage, prior treatments, the variant's level of evidence, and the possibility of germline or CHIP origin.

An “actionable” alteration does not guarantee that treatment will work

The presence of a drug name alongside a variant in an NGS report does not indicate that the tumor is dependent on that drug or that the patient will definitely benefit from it.

In the initial molecular screening results of the NCI-MATCH trial, 93% of samples were successfully profiled, and an alteration matching one of the study treatments was detected in 37.6% of patients. However, in the subsequent overall assessment, the objective response rate across treatment arms was approximately 10.3%.

This discrepancy may have several explanations:

• The detected alteration may be a biologically secondary event rather than the tumor's core driver.
• Co-mutations present in the same tumor may diminish the efficacy of targeted therapy.
• The same variant may not produce the same biological outcome across different tumor types.
• The alteration may be present in only a small subclone of tumor cells.
• The drug match may be based on early-phase or preclinical evidence rather than an approved indication.

Detectable alteration ≠ actionable alteration ≠ validated predictive biomarker ≠ clinical benefit

To determine whether a variant translates into a treatment option, biomarker evidence levels and tumor type-specific guidelines must be reviewed together.

A negative NGS result is not always a true negative

A variant being “not detected” does not necessarily mean it is absent from the tumor. The result can be influenced by specimen quality, tumor cell fraction, DNA/RNA quantity, read depth, panel design, variant allele frequency, and the bioinformatic thresholds applied.

A negative result on plasma-based liquid biopsy in particular must be interpreted with caution. The FDA's technical review of Guardant360 CDx explicitly states that a negative plasma result does not guarantee the tumor is negative for the genomic alteration in question, and that reflex testing on tissue should be pursued when appropriate.

Key clinical message: A negative liquid biopsy can miss a tumor alteration if ctDNA shedding is insufficient. Therefore, if clinical suspicion persists, confirmation with an appropriate tissue specimen may be warranted.

The right question is not simply “Was a mutation found?” The following should also be asked:

• Can the test reliably capture the relevant variant type?
• Is there sufficient tumor DNA in the specimen?
• Is RNA analysis needed for a fusion or exon-skipping event?
• Could confirming the negative result with an orthogonal method change the clinical decision?

Tumor-only testing cannot definitively distinguish somatic from germline origin

Some alterations detected in tumor tissue in BRCA1, BRCA2, PALB2, ATM, TP53 or mismatch-repair genes may be germline in origin. However, a test that analyzes the tumor alone cannot always definitively establish whether the alteration is a hereditary variant present in all of the patient's cells or a somatic event that arose only within the tumor.

Although variant allele frequency is an important clue, it does not by itself prove germline origin. ESMO recommends the use of matched tumor-normal analysis when appropriate consent and clinical conditions allow, and that confirmatory germline testing and genetic counseling be considered when a possible germline finding is identified in the tumor.

This distinction matters not only for the patient's treatment but also for the cancer risk of family members.

Not every mutation detected on liquid biopsy originates from the tumor

Clonal hematopoiesis, associated with aging, smoking, and prior cytotoxic therapy, can lead to the development of somatic clones in blood cells. Alterations arising from these clones can appear in cell-free DNA analysis and be mistakenly interpreted as tumor mutations.

Genes such as DNMT3A, TET2, ASXL1, TP53, PPM1D and JAK2 warrant particular attention in this regard. Matched leukocyte/buffy-coat analysis can help determine whether a plasma alteration is of hematopoietic origin.

There is also another side to the problem: some tumors shed very little ctDNA into circulation. In low disease volume, disease confined to the central nervous system, or certain patterns of peritoneal disease, plasma testing can miss a true alteration. For this reason, liquid biopsy must be evaluated within its clinical context, given the risks of both false-positive and false-negative interpretation.

A single biopsy may not represent the entire tumor

Cancer is not a static structure composed of a uniform cell population. Different regions of the same tumor can harbor different subclones, and the proportions of these clones can shift over time under treatment pressure.

The TRACERx study examined 1,644 tumor regions taken from 421 patients with non-small cell lung cancer. The study demonstrated that intratumor heterogeneity and subclonal evolution can be associated with clinical outcomes.

As a result, analysis of archival material taken from the primary tumor years earlier may incompletely reflect the current molecular makeup of metastatic disease that has developed after multiple lines of systemic therapy. Specimen age, the region sampled, and the treatments given in the interim are integral parts of NGS interpretation.

An NGS report is not the tumor's fixed identity; it is a molecular snapshot obtained at a particular place and time.

A DNA alteration is not the same thing as active tumor biology

The presence of a copy number gain or mutation in a gene does not, by itself, indicate that the gene is producing high levels of RNA, that the protein is present on the cell surface, or that the corresponding signaling pathway is actually active.

The reverse is also possible: when a DNA panel's intronic coverage is insufficient, some clinically important fusions can be missed. By directly demonstrating the resulting fusion transcript, RNA-NGS can complement DNA analysis, particularly in detecting and confirming kinase fusions.

In situations that could change the clinical decision, multi-omic and orthogonal confirmation should be considered:

• FISH/ISH for a copy number alteration.
• Protein/IHC for RNA expression.
• RNA-NGS for a suspected fusion or exon-skipping event.
• IHC, PCR, or another validated method for MSI discordance.
• Independent molecular confirmation for a critical variant with low VAF.

TMB is not a universal, laboratory-independent number

Tumor mutational burden (TMB) can vary depending on panel size, the genomic regions covered, sequencing depth, the germline filtering method used, and which variants are counted.

The Friends of Cancer Research TMB Harmonization Project demonstrated variability among TMB results from different NGS panels and showed that measurements need to be calibrated against common reference standards.

For this reason, an identical numeric TMB value reported by two different laboratories is not always exactly equivalent. TMB should be interpreted together with tumor type, MSI status, POLE/POLD1 biology, specimen quality, and the available clinical evidence.

A VUS is not a treatment target, and its classification can change

Variants of uncertain significance (VUS) are findings for which the available evidence is insufficient to establish whether the alteration is pathogenic or benign. A VUS should not become the rationale for targeted therapy simply because it appears in the report.

In a study evaluating germline test results from approximately 1.45 million individuals, 7.7% of unique VUS were reclassified over time. Of those reclassified, 91.2% shifted toward benign/likely benign and 8.7% toward pathogenic/likely pathogenic.

These data point to two important conclusions:

1. Treatment or family-screening decisions should not be based on a VUS.
2. Molecular reports should be regarded not as static PDFs, but as living documents that can be re-evaluated as new evidence emerges.

The real bottleneck in NGS is not sequencing, but interpretation

Sequencing hundreds of genes is becoming increasingly accessible today. The real challenge is determining the clinical significance of the alterations found in a reliable and reproducible manner.

AMP/ASCO/CAP has published a standard framework for tiering somatic variants according to clinical significance. However, correct clinical interpretation requires considering far more data than the variant alone:

1. Histopathologic diagnosis and disease stage.
2. Specimen type, age, and analytical quality.
3. Prior treatments and treatment sequence.
4. Clonality and variant allele frequency.
5. Co-mutations and resistance biology.
6. DNA, RNA, protein, and, when relevant, methylation data.
7. Tumor type-specific level of clinical evidence.
8. Regulatory approval, guideline, clinical trial, and access status.
9. The patient's performance status, organ function, and treatment goals.

From NGS report to clinical decision: the MENTHIS approach

Developed within NgsAdvisory, MENTHIS was not designed merely to re-summarize an NGS report. MENTHIS is a vendor-independent, multi-omic clinical reasoning approach aimed at placing molecular data from different providers into clinical context, making the level of evidence visible, and reducing the risk of false certainty.

This approach places particular emphasis on the following questions:

• Is the negative result analytically reliable?
• Is there a possibility of germline or CHIP origin in the finding?
• Could co-mutations alter sensitivity to targeted therapy?
• Are the DNA, RNA, and protein results concordant with one another?
• Could orthogonal confirmation change the treatment classification?
• Is the level of evidence for the proposed treatment appropriate for the tumor type and line of therapy?
• Are the findings clearly distinguished as standard-of-care, clinical trial, or investigational options?

Learn about the MENTHIS clinical reasoning approach →

NGS report interpretation checklist

Before an NGS report is translated into a clinical decision, the following questions should be answered:

• Does the specimen represent the current site of disease?
• Are tumor cell fraction and DNA/RNA quality adequate?
• Is the test designed to capture the variant type being sought?
• Does the negative result genuinely reduce clinical suspicion?
• Could the variant be somatic, germline, or CHIP-derived?
• Is the variant's level of evidence appropriate for the tumor type and line of therapy?
• Could co-mutations or tumor heterogeneity affect treatment response?
• Has RNA, IHC, FISH, or another confirmatory test been performed when needed?
• Is the recommendation clearly classified as an approved therapy, off-label use, clinical trial, or investigational option?

Conclusion: The NGS report is the starting point of the treatment decision

NGS is one of the most powerful tools in modern oncology, but it is not, by itself, an automatic treatment selector. Clinical value comes not from the number of variants but from correct specimen selection, awareness of analytical limitations, multi-omic confirmation, evidence calibration, and patient-specific clinical interpretation.

How should an NGS report be interpreted? First, whether the finding is technically reliable is assessed; then its biological meaning, level of evidence, and true relevance within the patient's clinical context. A variant's presence in the report does not necessarily mean it is a treatment target.

The most critical message is this:

NGS can find numerous alterations. The correct clinical decision requires distinguishing which of these alterations matter, which need confirmation, and which can genuinely change treatment.

When evaluating the clinical significance of the findings in your NGS report, topics such as molecular tumor board review, the interpretation of liquid biopsy results, and the need for germline testing should be considered together.

This content is intended for general scientific information purposes and does not substitute for individualized medical evaluation, diagnosis, or treatment recommendations. Molecular findings should be interpreted together with the patient's clinical characteristics and assessment by the relevant specialties.

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• O’Dwyer PJ, Gray RJ, Flaherty KT, et al. The NCI-MATCH trial: lessons for precision oncology. Nature Medicine. 2023;29:1349–1357.
• U.S. Food and Drug Administration. Guardant360 CDx: Summary of Safety and Effectiveness Data. FDA. 2026.
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• Mersch J, Brown N, Pirzadeh-Miller S, et al. Prevalence of variant reclassification following hereditary cancer genetic testing. JAMA. 2018;320(12):1266–1274.
• Li MM, Datto M, Duncavage EJ, et al. Standards and guidelines for the interpretation and reporting of sequence variants in cancer. Journal of Molecular Diagnostics. 2017;19(1):4–23.

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