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Your molecular pathology report
Variant Allele Frequency (VAF): — What the Number in Your Report Means
VAF — variant allele frequency — is the percentage attached to each mutation found on your cancer gene panel. It tells your oncologist how many of the cancer cells in your sample carry that specific change, and it shapes decisions about targeted therapy eligibility and treatment response.
Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed August 2026
- It is not a cancer severity score — VAF measures how common a mutation is within the sample, not how advanced the cancer is.
- It guides targeted therapy decisions — A mutation with a high VAF means most cancer cells carry the change a matched drug is designed to act on.
- It changes over time — Tracking VAF across tests tells your team whether treatment is working before a scan shows a size change.
- Sample type matters — Blood-based tests and tissue biopsies give different VAF ranges for the same tumour.
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VAF — variant allele frequency — is the percentage of DNA copies in your sample that carry a specific mutation. A higher VAF means the mutation is present in more of your cancer cells. Your oncologist uses this number alongside the mutation type to assess targeted therapy eligibility and to track how the cancer responds to treatment.
How does your oncologist read a VAF result?
Identify the mutation
The report names the gene and the exact change — for example, EGFR exon 19 deletion, KRAS G12C, PIK3CA H1047R, or BRAF V600E. VAF is the number attached to that specific finding, not to the report as a whole.
Read VAF as a proportion of the sample
The laboratory sequences thousands of short DNA fragments from your sample. VAF is the share of those reads in which the mutation appears. A higher share means the mutation is more prevalent among the cells sampled.
Interpret it against the sample type
Tissue biopsies and liquid biopsies give different VAF ranges for the same tumour. Your oncologist reads the number against how the sample was collected and how much tumour material it contained — not against a single universal scale.
Compare across time points
A single VAF reading is informative. Serial readings — before treatment, during, and after — are more powerful. A falling VAF suggests mutation-carrying cells are decreasing. A rising VAF can be an early warning of resistance.
Questions to ask your oncologist about your VAF result
- Which mutation does this VAF apply to, and what are the treatment options linked to that mutation?
- Is this VAF considered high or low for my cancer type and sample type?
- Does this VAF affect whether I qualify for a targeted therapy?
- Will you track this number on my next test, and what change would concern you?
- Was the sample quality good enough for this result to be reliable?
- If the VAF is low, should we retest with a larger sample or a different method?
What does a high VAF mean for your treatment options?
A high VAF means the mutation is present across a large proportion of the cancer cells in your sample. Oncologists call this a clonal mutation — it was likely present in the original cancer cell and inherited by all the cells that grew from it.
When a mutation such as EGFR exon 19 deletion or KRAS G12C carries a high VAF, your oncologist can be more confident that most cancer cells carry the change a matched targeted therapy is designed to act on. That confidence matters when deciding whether to use a drug directed at that specific mutation.
A high VAF is not a sign the cancer is more aggressive or more advanced. It is information about how widespread the mutation is within the tumour — useful for treatment planning, not a measure of severity.
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What does a low VAF mean, and does it affect your options?
A low VAF means the mutation was found in fewer of the DNA copies in your sample, but the reason behind that matters as much as the number itself.
The mutation may be subclonal — present in only a smaller group of cancer cells rather than the whole tumour. This happens when a second mutation arises later in the cancer's growth, producing a minority population with a distinct molecular profile.
A low VAF can also result from a sample that contained few cancer cells relative to normal tissue, or from a liquid biopsy where very small amounts of tumour DNA circulate in the blood. In those cases the mutation may be more prevalent in the tumour than the VAF suggests.
Whether a low-VAF mutation still guides treatment depends on the mutation, the cancer type, and the current evidence. Ask your oncologist directly whether this result changes your options, rather than assuming a low number means the finding is unimportant.
Why does VAF change between tests, and what does that tell your team?
VAF is not fixed. It reflects the proportion of mutation-carrying cells at the time of sampling, and that proportion shifts as the cancer responds to treatment, grows, or acquires new mutations.
A falling VAF on a repeat test after starting treatment usually means fewer cancer cells carry that mutation — consistent with the treatment working. A rising VAF can mean those cells are multiplying again, sometimes because a resistance mutation has emerged in a subpopulation.
Serial VAF tracking is especially useful in liquid biopsy, where blood can be retested without a repeat tissue procedure. It gives your oncologist an early signal about response — sometimes weeks before a scan shows a change in tumour size.
Did you know?
In liquid biopsy, VAF values are often much lower than what would be seen in a tissue sample from the same tumour. Only a small fraction of the tumour's DNA escapes into the bloodstream — the rest of the DNA in the blood sample comes from healthy cells.
A low VAF on a blood test does not automatically mean a low VAF in the tumour itself. This is why your oncologist interprets liquid biopsy results differently from tissue results, and why the two are sometimes used together.
Source: ESMO Recommendations on the Use of Next-Generation Sequencing for Patients with Advanced Cancers
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Common questions
Frequently asked questions
Why is VAF shown as a percentage in my report?
VAF is a proportion — the share of DNA copies in your sample that carry a specific mutation. The laboratory sequences thousands of short DNA fragments from your sample. If the EGFR exon 19 deletion appears in a certain share of those reads, that share is reported as the VAF. It is a measure of how common the mutation is within the sampled material, not an absolute count of cancer cells in your body.
Does a higher VAF mean my cancer is more serious?
No. VAF measures how prevalent the mutation is within the sample, not how advanced the cancer is or how it will behave. A high VAF means the mutation is well established across the tumour — which is useful information when a targeted therapy exists for that mutation, because it suggests most cancer cells carry the change the drug is designed to act on. Stage, grade, and spread are what determine severity, and those are reported separately from VAF.
What is a normal or expected VAF for a cancer mutation?
There is no single expected VAF that applies across all cancers, sample types, or mutations. Tissue samples, liquid biopsies, and bone marrow samples each produce different typical ranges for the same mutation. What matters is whether the VAF is consistent with the sample quality, whether it changes meaningfully over time, and whether it is above the threshold your laboratory considers reliable for that assay. Your oncologist will interpret your specific result in that context rather than against a universal number.
Can a mutation with a low VAF still affect my treatment decision?
Yes, in some cases. Whether a low-VAF mutation is actionable depends on the mutation type, the cancer, and the available evidence. For some mutations, even a smaller proportion of mutation-carrying cells is considered sufficient to guide treatment. For others, the evidence supports treatment only when the mutation is present at a higher level. Ask your oncologist whether this specific mutation at this VAF is considered relevant for your plan, rather than assuming low means unimportant.
Why is the VAF different between my tissue biopsy and my blood test?
Tissue biopsy and liquid biopsy measure the same mutations very differently. Tissue gives a direct read from the tumour, so VAF can be relatively high when the sample is rich in cancer cells. In blood, only a small fraction of the DNA comes from tumour cells — the rest comes from healthy cells — so even a prevalent mutation in the tumour shows a much lower VAF in blood. The two tests are complementary rather than directly comparable, and your oncologist will interpret each result in the context of how it was obtained.
What happens if my VAF rises during treatment?
A rising VAF during treatment is a signal your oncologist will take seriously. It can mean that cells carrying the original mutation are increasing in number again, which may indicate the treatment is becoming less effective. It can also mean a new population of cells with a resistance mutation is emerging. In either case, a rising VAF typically prompts reassessment — which may include a repeat biopsy, a scan, or a discussion about adjusting treatment. It is an early warning, and earlier action usually means more options remain available.