Yes, there are blood tests designed to detect cancer, though no single test catches every type. Some have been used for decades to screen for specific cancers, like the PSA test for prostate cancer or the alpha-fetoprotein test for liver cancer. A newer category called multi-cancer early detection (MCED) tests aims to spot dozens of cancer types from a single blood draw. None of these newer tests have FDA approval for screening yet, but they are commercially available and undergoing large clinical trials.
How Cancer Blood Tests Work
Cancer cells shed biological signals into the bloodstream as they grow. These include fragments of DNA released by tumors, abnormal chemical tags on that DNA (called methylation patterns), specific proteins, and genetic mutations. MCED tests look for combinations of these signals to determine whether cancer is likely present and, in many cases, to predict where in the body it originated.
The Galleri test, made by GRAIL, analyzes methylation patterns on cell-free DNA circulating in your blood. Another test called CancerSEEK (now being developed commercially as Cancerguard) takes a different approach, combining eight protein markers with mutations in 16 cancer-related genes, then running the data through a machine-learning model to calculate the probability of cancer. Each MCED test screens for a different group of cancer types. Some focus on cancers that already have standard screening options, like colorectal and breast cancer, while others include hard-to-detect cancers like ovarian and pancreatic cancer, which currently have no routine screening method.
How Accurate These Tests Are
Accuracy depends heavily on the cancer type and how advanced it is. The Galleri test has a specificity of 99.5%, meaning that out of every 1,000 people without cancer, only about 5 would receive a false alarm. That sounds impressive, but the sensitivity, the ability to catch cancer that’s actually there, varies widely.
For cancers that already have population-level screening (like breast and prostate), the Galleri test’s combined sensitivity across all stages was 34%. For cancers without existing screening options, sensitivity jumped to 66%. The pattern is consistent: early-stage cancers are harder to detect than late-stage ones. Prostate cancer sensitivity was just 6% across stages I through III but rose to 83% at stage IV. Breast cancer followed a similar curve, with 31% sensitivity overall but 86% to 91% at stages III and IV.
This creates a real tension. The whole point of a screening test is to find cancer early, when treatment works best. But these tests are currently better at finding cancers that are already more advanced. For early-stage disease across all cancer types, aggregate sensitivity drops to roughly 27% for cancers that already have standard screening and about 53% for those that don’t.
What Happens After a Positive Result
A positive blood test is not a cancer diagnosis. Every positive result requires follow-up diagnostic work, which may include imaging scans, endoscopy, or a tissue biopsy. These procedures add time, cost, and their own medical risks. In the first large interventional MCED study (called DETECT-A), about 1% of participants received a positive test result that turned out to be a false positive after a full clinical workup found no cancer or precancer within 12 months.
That 1% might sound small, but applied to millions of people, it would mean tens of thousands of individuals going through anxiety-provoking follow-up procedures for a cancer that isn’t there. This is one of the main reasons health agencies have not yet endorsed MCED tests for routine screening. The National Cancer Institute notes that whether these tests are effective for screening people without symptoms is still unknown and needs to be assessed through randomized clinical trials.
Traditional Single-Cancer Blood Tests
Before MCED tests entered the picture, a handful of blood tests were already in clinical use for specific cancers. The PSA test measures prostate-specific antigen levels and has been used alongside physical exams to screen for prostate cancer. However, most expert groups now advise against routine PSA testing for men at average risk because of high false-positive rates and the risk of overdiagnosis, where slow-growing cancers that would never cause harm get treated anyway.
The alpha-fetoprotein blood test is sometimes used alongside liver ultrasound to screen for liver cancer in high-risk individuals, such as people with chronic hepatitis or cirrhosis. These older tests are narrowly targeted and come with their own accuracy limitations, which is part of what motivated the development of broader multi-cancer approaches.
Blood Tests for Guiding Treatment
There’s an important distinction between using a blood test to screen for cancer in healthy people and using one to guide treatment in someone already diagnosed. Liquid biopsies, which analyze tumor DNA in the blood, are increasingly used in place of traditional tissue biopsies for patients with known cancers like non-small cell lung cancer.
In a study of 170 lung cancer patients who received both liquid and tissue biopsies, doctors based 73.5% of their treatment decisions on the liquid biopsy results. The blood-based test returned results an average of 26.8 days faster than tissue biopsy and matched tissue results 95% to 100% of the time for key biomarkers. Survival outcomes were no different between the two approaches. For patients already dealing with a cancer diagnosis, skipping an invasive tissue procedure in favor of a blood draw is a meaningful quality-of-life advantage.
How Early Can Blood Tests Catch Cancer?
One of the most promising findings comes from a recent study at Johns Hopkins, which applied an MCED test to stored blood samples from a large NIH-funded cardiovascular study. Researchers identified cancer signals in blood that had been drawn up to three years before the patients were clinically diagnosed. In four out of six cases where earlier blood samples were available (collected 3.1 to 3.5 years before diagnosis), tumor-derived mutations were already detectable at that earlier time point.
Three years is a significant head start. Tumors found that much sooner are likely to be smaller, less advanced, and more curable. But this is still proof-of-concept research. The practical question, whether acting on a positive MCED result years before symptoms appear actually improves survival, hasn’t been answered by a randomized trial yet. Researchers have also noted that figuring out the right clinical follow-up after an early positive test is an unsolved problem. Telling someone they may have cancer somewhere in their body, without a clear next step, carries real psychological and medical costs.
Should You Get One of These Tests?
MCED tests like Galleri are available by prescription in the United States, typically costing several hundred dollars out of pocket since insurance rarely covers them. They are not FDA-approved for cancer screening, and no major medical organization currently recommends them as part of routine care. The National Cancer Institute is clear that their effectiveness for screening asymptomatic people remains unproven.
That said, these tests may hold particular interest for people at elevated cancer risk due to family history, genetic factors, or prior cancer. Some oncologists offer them as a supplement to, not a replacement for, standard screenings like mammograms, colonoscopies, and low-dose CT scans for lung cancer. If you’re considering one, the most important thing to understand is that a negative result does not rule out cancer, and a positive result does not confirm it. Either way, standard screening methods remain your primary line of defense.