Galleri Test Accuracy: How Reliable Is the Multi-Cancer Blood Test?
By Dr. Diana Rangaves, PharmD
The Galleri test has gained attention for its potential to revolutionize cancer screening by detecting signals from over 50 types of cancer with a simple blood draw.[1]
But how accurate is it, really?
As more people consider multi-cancer early detection (MCED) tests, understanding Galleri’s sensitivity, specificity, and real-world performance is crucial. Here, we’ll break down how Galleri works, compare its accuracy to other diagnostic tools, and outline what patients need to know before undergoing testing.
Understanding Accuracy: Sensitivity vs Specificity
When assessing cancer detection tests like Galleri, two performance measures are critical:
- Sensitivity refers to the test’s ability to correctly identify people who have cancer (true positives).[2]
- Specificity refers to the ability to correctly identify those who do not have cancer (true negatives).[2]
Together, these measures help determine how often a test gives false positives (detecting cancer when it’s not present) or false negatives (failing to detect cancer that is present).[2]
Galleri’s Specificity: Rare False Positives
One of Galleri’s strengths is its high specificity. In both clinical trials and real-world use, Galleri has consistently shown a specificity of 99.5%, meaning that only 0.5% of individuals without cancer receive a false-positive result.[3]
This is especially important for reducing the burden of unnecessary diagnostic follow-up procedures and anxiety in healthy individuals.
Galleri’s Sensitivity: Stage-Dependent Accuracy
While Galleri is highly specific, its sensitivity varies based on cancer type and stage. According to the Circulating Cell-free Genome Atlas (CCGA) study, Galleri achieved an overall sensitivity of 51.5%, meaning it correctly identified cancer in approximately half of the individuals who had it.[4]
However, detection rates are stage-dependent, increasing dramatically for later-stage cancers:[4]
- Stage I: 16.8% (14.5–19.5%)
- Stage II: 40.4% (36.8–44.1%)
- Stage III: 77.0% (73.4–80.3%)
- Stage IV: 90.1% (87.5–92.2%)
This means Galleri is most effective for detecting cancers in stages III and IV, where tumor DNA is more abundant in the bloodstream, but less reliable for early-stage detection.
What the PATHFINDER Studies Reveal
Two major clinical trials, PATHFINDER-1 and PATHFINDER-2, have evaluated Galleri’s real-world performance. These studies focused on how well the test performs in average-risk adults and how its results are used in clinical workflows.
PATHFINDER-1 Highlights[3]
- Enrolled 6,621 individuals aged 50+
- Cancer signal was detected in 1.4% of participants
- Of those with a detected signal, 43% were confirmed to have cancer after diagnostic evaluation
- Specificity remained at 99.5%
- The test correctly identified the Cancer Signal Origin (CSO) in 88% of cases
PATHFINDER-2 Early Results[5]
- Maintained high specificity and improved positive predictive value (PPV)
- Demonstrated better CSO accuracy and faster time to diagnostic resolution
- Patients reported high satisfaction with minimal test-related distress
These results suggest that Galleri can detect cancer with a high degree of accuracy, particularly when used in conjunction with other clinical tools.
Accuracy of Cancer Signal Origin (CSO)
When Galleri detects a cancer signal, it also predicts where in the body the cancer likely originated, known as the Cancer Signal Origin (CSO). The accuracy of this prediction is crucial for directing follow-up diagnostic tests.
In clinical studies, Galleri’s CSO accuracy has ranged from 88% to 97%, depending on the study population and cancer type.[3,6] This high level of precision enables physicians to focus imaging and biopsies efficiently.
Galleri vs Other Screening Tools
Unlike traditional screening tests that target specific cancers (such as mammograms or colonoscopies), Galleri is designed to detect a wide range of cancers, including those without routine screening protocols such as pancreatic, ovarian, and esophageal cancers.[7]
Compared to other MCED tests, Galleri shows superior performance:
- Positive predictive value (PPV) of ~38%, compared to ~28.3% in the DETECT-A study[8]
- Validated in over 20,000 individuals across diverse populations[3]
However, no MCED test—including Galleri—should replace routine screenings for cancers with established guidelines.
Clinical Strengths and Limitations
Strengths
- Very low false-positive rate (0.5%)[3]
- High CSO accuracy (~88–97%)[3,6]
- Detects over 50 cancer types, including those without standard screening tools[3]
- Useful for adults aged 50+ with no symptoms but elevated risk[3]
Limitations
- Lower sensitivity for early-stage cancers, especially Stage I (~17%)[3]
- False negatives possible—absence of a signal ≠ absence of disease
- Not FDA-approved as a standalone screening tool and not covered by insurance[9]
- Cost may be prohibitive (~$949 without insurance)[10]
What About Patient Experience?
One concern with novel screening methods is the potential psychological burden of false positives. In the PATHFINDER study, most participants reported minimal distress and high satisfaction.[3]
Additionally, the diagnostic resolution time after a positive Galleri test—how long it takes to confirm or rule out cancer—was typically within 3 months.[3]
Frequently Asked Questions (FAQs)
How accurate is the Galleri test? It has 99.5% specificity and stage-dependent sensitivity ranging from 16.3% (Stage I) to 90.1% (Stage IV).[3]
What is the false positive rate? Approximately 0.5%, meaning Galleri rarely misidentifies cancer when there is none.[3]
Can Galleri detect colon cancer? Yes, colon and rectal cancers are among the 50+ types Galleri can detect.[7]
Is Galleri FDA approved? No. It is performed in CLIA-certified labs but is not FDA-approved as a primary screening tool.[9]
Why isn’t Galleri covered by insurance? Because it’s not yet FDA approved and lacks long-term outcome data.[10]
How much does Galleri cost? Around $949 out-of-pocket.[10]
Are GRAIL and Galleri the same? Yes—GRAIL is the company that developed and markets Galleri.
Final Thoughts
The Galleri test represents a major step forward in cancer screening, particularly for hard-to-detect cancers. Its exceptionally high specificity and CSO accuracy reduce false alarms and streamline care. However, lower sensitivity in early stages and lack of FDA approval mean it should complement—not replace—routine screenings.
For adults aged 50 and older or those at elevated risk, Galleri may be a valuable proactive health tool. As with any medical test, it’s important to understand its strengths and limitations before deciding whether it’s right for you.
Ready to explore Galleri for yourself?
Order the Galleri Cancer Screening Test from Everlywell and take the next step in proactive, personalized cancer detection.
References
- Facing Our Risk of Cancer Empowered. Headline hype: One simple blood test to detect 50 cancers? Not yet. 2021.
- Shreffler J, Huecker MR. Diagnostic Testing Accuracy. StatPearls Publishing; 2023.
- GRAIL, Inc. Breakthrough test performance. Accessed July 2025.
- Klein EA, et al. Ann Oncol. 2021;32(9):1167–1177.
- GRAIL, Inc. PATHFINDER 2 study results. 2025.
- GRAIL, Inc. PATHFINDER study published in The Lancet. 2023.
- GRAIL, Inc. Types of cancer detected. Accessed July 2025.
- Singal AG, et al. Future Oncol. 2024;20(34):2603–2607.
- American Cancer Society. Multi-cancer early detection tests. 2025.
- GRAIL, Inc. Test pricing and billing. Accessed July 2025.