Tempus xF is a blood-based next-generation sequencing test that analyzes circulating tumor DNA (ctDNA) to look for genomic changes in solid cancers. Its main purpose is molecular profiling for treatment selection and resistance, especially when tumor tissue is difficult to obtain, when a faster blood-based result is useful, or when clinicians want a current view of mutations shed by cancer sites throughout the body. The current xF assay covers 105 genes and can report single-nucleotide variants, insertions and deletions, copy-number gains, selected gene rearrangements, microsatellite instability-high (MSI-H) status, and blood tumor mutational burden (bTMB). Tempus also offers xF+, a broader 523-gene liquid-biopsy panel. A mutation detected on xF can be clinically important, but a negative test does not prove the tumor lacks that alteration because low ctDNA shedding can make plasma genotyping falsely negative. Results therefore need to be interpreted with tumor tissue testing, cancer type, stage, treatment history, and the amount of ctDNA available in the sample.
- What Tempus xF tests: A 105-gene ctDNA panel for cancer-associated mutations, copy-number gains, selected rearrangements, MSI-H, and bTMB.
- What xF+ adds: Broader 523-gene coverage plus expanded genomic profiling and clonal-hematopoiesis-related annotation.
- Positive mutation result: A cancer-associated alteration was detected in plasma and may affect treatment, prognosis, or resistance interpretation.
- Negative result: No reportable alteration was found in blood; low tumor shedding can make tissue testing necessary.
- Typical turnaround: Tempus currently states xF and xF+ results are generally expected about 6 days after specimen receipt.
Table of Contents
- What Tempus xF is and what it measures
- Tempus xF versus xF+
- When a Tempus xF liquid biopsy may be useful
- How to read Tempus xF mutation results
- What a negative result and low ctDNA can mean
- Performance, clonal hematopoiesis, and limitations
- How xF compares with tissue NGS and MRD tests
What Tempus xF is and what it measures
Tempus xF is a liquid-biopsy genomic profiling assay. It isolates cell-free DNA from plasma and uses hybrid-capture next-generation sequencing to identify cancer-associated genomic alterations that are circulating in the blood.
The key word is profiling. xF is designed mainly to characterize the tumor genome from blood, not simply to say whether cancer is present. This makes it similar in purpose to other broad ctDNA mutation panels used in precision oncology.
The current 105-gene xF panel can detect several classes of alterations:
- single-nucleotide variants (SNVs), such as a one-letter DNA change;
- insertions and deletions (indels);
- copy-number gains, in which a gene is present in extra copies;
- selected gene rearrangements or fusions;
- MSI-H status;
- blood tumor mutational burden, or bTMB.
These findings may help identify an oncogenic driver, a resistance mechanism, or a biomarker relevant to a targeted therapy or immunotherapy decision. The clinical meaning depends on the cancer. The same gene alteration may be actionable in one tumor type, investigational in another, and irrelevant to treatment in a third.
xF is often most useful in advanced solid tumors, where cancer deposits may shed enough DNA for plasma genotyping and where treatment decisions can depend on the current molecular profile. A liquid-biopsy resistance mutation test can also capture changes that appeared under drug pressure and were not present in an older tissue specimen.
Tempus reports that xF results are typically expected in about six days after the specimen reaches the laboratory. This can be helpful when waiting for a new tissue biopsy would delay a treatment decision.
Tempus xF versus xF+
Tempus currently offers two related plasma genomic panels: xF and xF+. They use the same general liquid-biopsy concept but differ in breadth and some reported features.
| Feature | Tempus xF | Tempus xF+ |
|---|---|---|
| Panel size | 105 genes | 523 genes |
| Primary use | Focused liquid-biopsy genomic profiling | Broader comprehensive liquid-biopsy profiling |
| Variant types | SNVs, indels, copy-number gains, selected rearrangements | SNVs, indels, copy-number gains, selected rearrangements across broader coverage |
| MSI-H | Reported when supported | Reported when supported |
| bTMB | Reported under applicable laboratory conditions | Reported |
| Clonal hematopoiesis | Uses filtering methods intended to reduce non-tumor calls | Can identify variants potentially associated with clonal hematopoiesis |
| Turnaround | Typically about 6 days after specimen receipt | Typically about 6 days after specimen receipt |
The broader panel is not automatically “better” for every patient. A focused 105-gene panel can cover many established drivers and resistance genes with less genomic territory. A 523-gene panel can detect a wider range of alterations and may be helpful when the cancer lacks an obvious target or when clinical-trial matching is important.
The correct choice depends on what information is needed. If the clinical question is whether a known resistance mutation has appeared, a focused assay may be sufficient. If the question is comprehensive genomic characterization, broader coverage may provide more opportunities but can also produce more findings of uncertain clinical significance.
Panel size should also be separated from usable tumor signal. A 523-gene assay cannot recover information that is not represented in the circulating DNA molecules collected from the patient. When ctDNA fraction is very low, broader genomic territory may still yield a sparse or non-informative report. Conversely, when tumor shedding is adequate, broader coverage can reveal uncommon alterations, co-mutations, and potential trial biomarkers that a smaller panel was not designed to assess. The report therefore needs two readings: first, what alterations were found; second, how much confidence there is that the plasma sample adequately represented the cancer.
This distinction matters when comparing results across time. A later blood draw may show fewer variants because treatment lowered ctDNA, not because the cancer genetically “lost” every earlier mutation. Likewise, a new alteration can reflect true tumor evolution, expansion of a previously minor clone, or simply better sampling of a metastatic site that is now shedding more DNA.
The 2025 analytical validation of xF+ described a 523-gene panel spanning about 1.8 megabases, with SNV and indel detection in 522 genes, copy-number gains in selected genes, rearrangements in selected genes, and bTMB calculation. That validation supports the assay’s analytical performance but does not mean every detected alteration has an approved therapy.
When a Tempus xF liquid biopsy may be useful
A plasma-based assay is especially useful when tissue is limited, risky to obtain, old, or too slow for the treatment decision.
ASCO’s 2026 guideline supports ctDNA testing for tumor genetic alterations when tissue testing is challenging or not feasible, when biopsy risk is unacceptable, when tissue results may not return in a clinically useful timeframe, or when a drug’s approved indication allows or requires ctDNA testing.
Common clinical situations include:
Advanced cancer with an urgent need for genomic profiling
A patient with newly diagnosed metastatic cancer may need a molecular result before treatment begins. Blood can often be collected immediately, while arranging a biopsy, pathology review, and tissue sequencing can take longer.
In some cases clinicians order tissue and liquid profiling together. This is not redundant: the two sample different biological compartments and can find non-overlapping alterations.
A 2024 multicancer cohort study of 3,209 patients with stage IV non-small cell lung, breast, prostate, or colorectal cancer compared tissue and ctDNA profiling obtained within 30 days. Among patients with guideline-based targeted variants, some actionable findings were unique to plasma and others were unique to tissue. This illustrates why “blood versus tissue” is often a complementary rather than either-or decision.
Monitoring acquired resistance
Targeted therapy creates selective pressure. A resistant clone may expand and release new mutations into plasma. Serial xF testing can therefore provide a current molecular snapshot when a cancer progresses.
Examples can include EGFR or ALK resistance mechanisms in lung cancer, ESR1 mutations in breast cancer, BRCA1/2 reversion changes, or resistance alterations after anti-EGFR treatment in colorectal cancer. The specific alteration must be interpreted according to the current treatment landscape.
When a new tissue biopsy is difficult
Bone lesions, deep abdominal metastases, small lung nodules, or medically fragile patients can make repeated tissue biopsy difficult or unsafe. Blood sampling is less invasive and can be repeated more easily.
The tradeoff is that plasma contains only the tumor DNA that has been shed into circulation. A technically easy blood draw is not necessarily a biologically adequate tumor sample.
How to read Tempus xF mutation results
A Tempus xF report can contain several kinds of findings, and they do not all carry the same weight.
Pathogenic or likely pathogenic alterations
These are variants with evidence that they affect cancer biology. The next question is whether the alteration is actionable for this cancer type. Actionability can mean:
- an FDA-approved targeted therapy for that tumor type;
- an approved therapy in another tumor type;
- a guideline-supported biomarker;
- a clinical-trial option;
- a resistance marker that argues against a particular drug.
A detected mutation should not be treated as an isolated command. Gene, exact variant, cancer type, variant allele fraction, prior therapy, other co-occurring mutations, and regulatory context all influence interpretation.
For example, a KRAS mutation in liquid biopsy can have very different treatment implications depending on whether the cancer is colorectal, lung, pancreatic, or another solid tumor and which KRAS codon is altered.
Variant allele fraction
Variant allele fraction (VAF) is the proportion of sequenced DNA molecules carrying a variant. A VAF of 1% means roughly 1 in 100 DNA molecules at that genomic position contained the variant.
VAF is not a direct percentage of cancer in the body. It is affected by tumor shedding, total cell-free DNA, copy-number changes, clone size, treatment, specimen timing, and technical factors. One mutation may also have a different VAF from another mutation in the same sample.
Serial VAF changes can sometimes help describe molecular dynamics, but ASCO’s 2026 guidance cautions against using ctDNA fractional, percentage, or concentration measures as a universal surrogate for disease burden.
MSI-H and bTMB
MSI-H and tumor mutational burden can be biomarkers relevant to immunotherapy in selected settings. In plasma, their reliability depends strongly on having enough tumor-derived DNA. A low-shedding sample can make these composite biomarkers harder to call even if tissue is positive.
The current Tempus page also notes laboratory-specific reporting conditions for bTMB, so the exact report should be read rather than assuming every xF specimen receives identical biomarker output.
What a negative result and low ctDNA can mean
A negative xF result is one of the most important findings to interpret correctly.
“No reportable alteration detected” is not the same as “the tumor has no mutations.” It may mean the blood sample contained too little tumor DNA for reliable genotyping.
Several factors can lower ctDNA:
- small tumor burden;
- recent effective treatment;
- tumors with biologically low shedding;
- disease confined mainly to the brain or certain other sites;
- some bone-dominant or indolent disease patterns;
- timing of the blood draw;
- a high background of non-tumor cell-free DNA.
ASCO recommends tissue-based confirmation when ctDNA results are negative, inconclusive, or inconsistent with the clinical picture. That principle is particularly important when a missing result would change treatment—for example, when an actionable driver is common enough that a false-negative plasma test would deny a useful therapy.
A negative blood result should therefore be categorized as either informative negative or possibly non-informative. If the report suggests adequate tumor fraction and no target is found, confidence is greater. If ctDNA is very low or absent, tissue can be substantially more informative.
The opposite issue can also occur: plasma may detect a mutation that tissue missed because the tissue sample was old, came from one site, or did not include a resistant subclone that emerged later.
Performance, clonal hematopoiesis, and limitations
Tempus publishes analytical performance specifications for different variant classes and laboratories. For the current Chicago xF assay, the company reports 98.5% sensitivity for SNVs at or above 0.25% VAF with specificity above 99.9%; performance drops at lower VAF. Other variant classes have their own limits of detection, and the Durham laboratory has somewhat different validated characteristics.
These figures describe analytical conditions. Clinical sensitivity still depends on whether the patient’s cancer released enough ctDNA into the tube.
Clonal hematopoiesis can mimic tumor mutations
As people age, normal blood-forming stem cells can acquire mutations and expand into clones. Their DNA enters plasma and can appear on liquid-biopsy sequencing. Common clonal-hematopoiesis genes overlap with genes altered in cancer, so an unfiltered plasma result can be misleading.
The original xF validation described a dynamic filtering approach using matched data to reduce calls from germline variants and clonal hematopoiesis. xF+ also reports variants potentially associated with clonal hematopoiesis.
This issue is especially important when a detected mutation has a low VAF, occurs in a gene commonly altered in blood cells, or does not fit the tumor’s known biology. A result may need confirmation in tumor tissue or white blood cells before it is treated as a tumor-derived target.
Other limitations
- Not every alteration has a therapy.
- Liquid biopsy cannot show tumor histology, grade, architecture, or protein expression.
- Some fusions and copy-number changes can be harder to detect in low-tumor-fraction plasma.
- A blood sample may underrepresent tumors that shed little ctDNA.
- Genomic reports can become outdated as drug approvals and guidelines change.
For these reasons, molecular findings should be reviewed in the current clinical context, often by an oncologist, molecular pathologist, or molecular tumor board.
How xF compares with tissue NGS and MRD tests
Tempus xF is not a replacement for every other cancer test. It answers a specific genomic question.
| Approach | Best suited to answer | Main weakness |
|---|---|---|
| Tempus xF/xF+ | Which mutations or resistance alterations are detectable in plasma now? | Can be falsely negative when ctDNA shedding is low |
| Solid-tumor tissue NGS | What genomic alterations are present in the sampled tumor tissue? | Requires tissue and may not capture later or distant heterogeneity |
| Tumor-informed MRD test | Is a personalized tumor DNA signal detectable after treatment? | Requires initial tumor sequencing and is not primarily for broad therapy-target discovery |
| Tumor-naive MRD test | Is a residual cancer-associated signal detectable without custom tumor assay design? | Performance varies by platform and cancer type |
A solid tumor NGS panel has the advantage of sequencing DNA directly from a known tumor sample and can often provide more complete tumor content when plasma shedding is low. Plasma has the advantage of speed, repeatability, and the possibility of sampling DNA from multiple metastatic sites at once.
MRD tests such as Signatera or RaDaR are different again. They use highly sensitive strategies to detect tiny residual signals, typically after curative-intent treatment. xF is a genomic profiling assay for treatment-relevant alterations; it should not be interpreted as a dedicated personalized MRD test simply because it measures ctDNA.
The practical rule is straightforward: match the test to the question. Use broad liquid-biopsy profiling when the main question is “What actionable tumor genetics can we detect in blood now?” Use tissue when histology or more complete genomic confirmation is needed. Use an MRD assay when the validated question is “Is molecular residual disease detectable after treatment?”
References
- Circulating Tumor DNA Testing in Solid Tumors and Lymphoma: ASCO Guideline 2026 (Guideline)
- Tempus xF/xF+ liquid biopsy 2026 (Official Test Information)
- Analytical Validation of Next-Generation Sequencing-Based Comprehensive Liquid Biopsy Assay for Therapy Selection. 2025 (Validation Study)
- Concurrent Tissue and Circulating Tumor DNA Molecular Profiling to Detect Guideline-Based Targeted Mutations in a Multicancer Cohort 2024
- Pre-radiotherapy ctDNA liquid biopsy for risk stratification of oligometastatic non-small cell lung cancer. 2023
- Validation of a liquid biopsy assay with molecular and clinical profiling of circulating tumor DNA 2021 (Validation Study)
Disclaimer
This article is for general education and does not replace medical advice or molecular interpretation by an oncology team. Tempus xF findings must be interpreted according to cancer type, disease burden, prior treatment, tissue results, and current therapy guidelines. A negative plasma result may require tissue testing when an actionable alteration is still clinically suspected.
