Blood tests can detect signs of cancer, but no single blood test reliably catches every cancer, and the ones that exist vary wildly in what they find and what they miss. Routine lab work like a complete blood count or liver panel can raise suspicion, and older tumor markers such as PSA or CA-125 can flag specific cancers, though they carry notoriously high false-positive rates. A newer generation of tests, including liquid biopsies and multi-cancer early detection panels, is pushing the field forward by reading fragments of tumor DNA or methylation patterns shed into the bloodstream. But even the most advanced options still struggle with early-stage disease and certain tumor types, and none has yet replaced standard screening methods like colonoscopy or mammography.
What Routine Blood Work Can and Cannot Tell You
A standard blood draw at your annual physical includes tests like a complete blood count, a metabolic panel, and liver enzymes. These are not designed to find cancer, but they can drop hints. A persistently dropping hemoglobin might point toward a slow-bleeding colon tumor. A spike in liver enzymes might mean cancer has spread to the liver, or it might mean nothing more than a few too many drinks. A systematic review looking at blood test trends in people later diagnosed with cancer found that increasing or decreasing trends in routine blood markers were associated with multiple cancer types, but the associations were broad and nonspecific.1PubMed Central. The Association between Blood Test Trends and Undiagnosed Cancer: A Systematic Review and Critical Appraisal A falling platelet count, rising calcium, or unexplained anemia might prompt your doctor to dig deeper, but these shifts could just as easily be caused by infection, medication, or a dozen other non-cancerous conditions.
The takeaway is that routine blood panels are useful as a first tripwire, not a diagnostic tool. If your doctor spots something unusual, the response is typically imaging or a biopsy, not a diagnosis of cancer based on blood chemistry alone.
Traditional Tumor Markers and Their Limitations
For decades, clinicians have used specific proteins released by certain tumors as blood-based cancer markers. The best known include PSA for prostate cancer, CA-125 for ovarian cancer, CEA and CA 19-9 for gastrointestinal cancers, and AFP for liver cancer. These markers have genuine clinical value, but they share a common weakness: they are unreliable as standalone screening tools in people without symptoms.
PSA is the poster child for this problem. A large cohort study looking at real-world PSA testing found false-positive rates around 47% in both symptomatic and asymptomatic patients, with a positive predictive value of only about 13%.2PubMed Central. Variables Associated with False-Positive PSA Results: A Cohort Study with Real-World Data In plain terms, roughly half the time a PSA test comes back elevated, there is no cancer. Benign prostate enlargement, infections, and even vigorous exercise can push the number up.
CA-125 follows a similar pattern. While it has been the primary ovarian cancer marker for four decades, no professional society recommends using it to screen average-risk women who have no symptoms, because it has not improved survival when used that way.3PubMed Central. CA125 and Ovarian Cancer: A Comprehensive Review Endometriosis, fibroids, and even menstruation can elevate CA-125, making it a poor tool for catching early ovarian cancer in the general population.
For pancreatic cancer, CA 19-9 is elevated in roughly three-quarters of patients at diagnosis, and CEA in about 39%. CEA correlates with tumor stage and prognosis, with patients who had normal CEA surviving considerably longer than those with elevated levels.4PubMed Central. Serum CA 19-9 and CEA levels as a prognostic factor in pancreatic adenocarcinoma But these markers are most useful once cancer is already suspected or diagnosed. They help track treatment response and recurrence. They are far less useful for catching a cancer you did not know about.
AFP, used for liver cancer, has a sensitivity around 70% and specificity around 89% according to a diagnostic meta-analysis, but it misses roughly a third of liver cancers.5BMC Cancer. Golgi protein 73 versus alpha-fetoprotein as a biomarker for hepatocellular carcinoma: a diagnostic meta-analysis Chronic hepatitis and cirrhosis can also elevate AFP in the absence of cancer. The broader pattern holds across all traditional markers: useful for monitoring, unreliable for detection on their own.
Liquid Biopsies and Circulating Tumor DNA
The idea behind liquid biopsies is that tumors leave traces in the blood. Cancer cells shed fragments of their DNA, called circulating tumor DNA or ctDNA, into the bloodstream. Because ctDNA carries the same mutations as the tumor itself, a blood draw can in principle reveal what is happening inside a tumor without the need for a tissue biopsy.6PubMed Central. Circulating tumor DNA: a promising biomarker in the liquid biopsy of cancer
Beyond DNA fragments, liquid biopsies can also look for circulating tumor cells, which are whole cancer cells that have broken off from a tumor and entered the bloodstream. These two signals complement each other: different tumors shed DNA and cells at different rates and stages, meaning one signal may be present when the other is not.7PubMed Central. Circulating Tumor Cells Versus Circulating Tumor DNA in Colorectal Cancer: Pros and Cons Combining both biomarkers in a single analysis offers a more complete picture of tumor biology and treatment response than either one alone.8PubMed Central. Dual Biomarker Strategies for Liquid Biopsy: Integrating Circulating Tumor Cells and Circulating Tumor DNA for Enhanced Tumor Monitoring
The clinical applications today are strongest in patients who already have a known cancer. Liquid biopsies can guide treatment decisions by identifying targetable mutations, track whether a therapy is working by monitoring ctDNA levels over time, and detect recurrence before imaging can see it. For population-wide screening of people without symptoms, ctDNA-based tests face a harder challenge because early-stage and slow-growing tumors shed very little DNA into the blood, sometimes below the detection threshold.
Multi-Cancer Early Detection Tests
The biggest recent development in blood-based cancer detection is the multi-cancer early detection test, or MCED. Instead of looking for one marker tied to one cancer type, MCED tests analyze patterns in cell-free DNA, typically focusing on methylation, to screen for signals from dozens of cancer types simultaneously. The commercial test furthest along in development, Galleri, reads DNA methylation patterns and attempts to not only detect a cancer signal but also predict where in the body the cancer is located.
A large validation study of this methylation-based approach found an overall sensitivity of about 52% across all cancers combined, with a very high specificity of 99.5%. Sensitivity climbed sharply with stage: roughly 17% for stage I cancers, 40% for stage II, 77% for stage III, and 90% for stage IV.9PubMed. Clinical validation of a targeted methylation-based multi-cancer early detection test using an independent validation set When the test did detect a cancer signal, it correctly predicted the organ of origin about 89% of the time. For the twelve cancer types that together account for roughly two-thirds of U.S. cancer deaths, the combined stage I through III sensitivity was about 68%.
Those numbers reveal the central tension in MCED testing: specificity is excellent, meaning false positives are rare, but sensitivity for the earliest-stage cancers is modest. A separate analysis confirmed this pattern, reporting aggregate sensitivities of 27% at stage I, 53% at stage II, and 60% at stage III. Among cancers that already have established screening programs, prostate cancer was detected at just 6% across stages I through III, while breast cancer reached 31% across all stages. Cervical and colorectal cancers were more consistently detected across stages.10PubMed Central. Multi-cancer early detection test sensitivity for cancers with and without current population-level screening options
Newer approaches are trying to close the early-stage gap. One next-generation MCED assay using serum-based multi-omics reported a stage I sensitivity of 95% in an independent validation, a dramatic improvement if confirmed in larger populations.11Journal of Clinical Oncology. Next-generation multi-cancer early detection (MCED) assay: Independent blinded validation of a locked serum multi-omics NMR test with stage I sensitivity A different proof-of-concept test targeting four common cancers achieved an overall sensitivity of about 60% with 87% specificity, picking up lung cancer most readily at 82% and prostate cancer least at 44%.12PubMed. Multi-cancer early detection via a DNA methylation multiplex ddPCR-based blood test These are still early results, and no MCED test has yet received FDA approval or a recommendation from any major screening guideline body.
Why Blood Tests Miss Certain Cancers
The gap between what blood tests can theoretically detect and what they actually catch in practice comes down to biology. Several factors conspire to make some cancers nearly invisible in the bloodstream.
The first is shedding. Not all tumors release DNA or cells into the blood at the same rate. Small, localized, or slow-growing tumors may shed so little ctDNA that even sensitive assays cannot pick it up. A tumor tucked away in the brain, behind the blood-brain barrier, may barely register in a peripheral blood draw at all. The physical characteristics of ctDNA, including its size, structure, and how quickly the body clears it, vary based on tumor type and location.13PubMed Central. Circulating Tumor DNA as a Cancer Biomarker: An Overview of Biological Features and Factors That may Impact on ctDNA Analysis
The second is heterogeneity. Cancer cells within a single tumor are not genetically identical. Different parts of the same tumor can carry different mutations, and the DNA fragments that enter the bloodstream may not represent the full diversity of the tumor. This molecular variation complicates the picture for any test that relies on spotting specific mutations or patterns.14PubMed Central. Tumour Heterogeneity: The Key Advantages of Single-Cell Analysis A study of circulating tumor cells in metastatic prostate cancer illustrated this vividly: sequenced cells from individual patients showed substantial genomic variation from one cell to the next.15Journal of Clinical Oncology. Performance of a blood-based liquid biopsy test to detect PSMA expression on circulating tumor cells in men with metastatic prostate cancer
The third blind spot is something called clonal hematopoiesis, or CHIP. As people age, blood-forming stem cells can accumulate mutations that have nothing to do with cancer. When a liquid biopsy test sequences cell-free DNA in the blood, it can pick up these age-related mutations and mistake them for a cancer signal. CHIP mutations are a known cause of false positives in liquid biopsy screening and can also interfere with sequencing results used to guide cancer therapy.16PubMed Central. Clinical Significance of Clonal Hematopoiesis of Indeterminate Potential in Hematology and Cardiovascular Disease 17PubMed Central. Clonal Hematopoiesis of Indeterminate Potential in Patients with Solid Tumor Malignancies Newer tests try to filter CHIP signals out, but it remains an active challenge, especially in older patients where CHIP is most common.
False Positives, Overdiagnosis, and What Happens Next
When a blood test flags a possible cancer that turns out not to be there, or catches a cancer that never would have caused harm, the consequences are not trivial. A false-positive result triggers imaging, biopsies, specialist visits, and sometimes months of diagnostic limbo. A modeling study comparing approaches to multi-cancer screening found that while an MCED test detected fewer cancers than running ten individual single-cancer tests side by side, it generated dramatically fewer false positives: roughly 500 versus more than 93,000 unnecessary follow-up investigations in cancer-free people.18PubMed Central. Estimating the Burden of False Positives and Implementation Costs From Adding Multiple Single Cancer Tests or a Single Multi‐Cancer Test to Standard‐Of‐Care Screening The high specificity of MCED tests is a genuine advantage over older markers, but false positives still occur.
Overdiagnosis is the subtler risk. This happens when a screening test finds a real cancer that is so slow-growing it would never have caused symptoms or shortened the person’s life. Since screening programs for breast and prostate cancer were introduced, a surge in early-stage and in-situ diagnoses has been observed without a proportional drop in cancer deaths, suggesting that some of those detected cancers were harmless.19PubMed Central. Cancer overdiagnosis: a biological challenge and clinical dilemma One modeling analysis estimated that roughly 2 to 6% of all cancers found through MCED screening could be overdiagnoses, with the proportion rising steeply with age, from about 1% at age 50 to over 10% of screen-detected cancers by age 75.20medRxiv. Assessing potential harms from screening overdiagnosis and false positives with multicancer early detection tests
There is a counterargument worth noting. Some data suggest that cell-free DNA-based screening may actually skew toward catching more aggressive cancers, the ones most likely to kill, rather than indolent ones. One study found that cancers detected through plasma cfDNA analysis had a worse prognosis than cancers the test did not detect, and that detection carried prognostic significance comparable to clinical staging.21Journal of Clinical Oncology. Prognostic significance of blood-based cancer detection in plasma cell-free DNA (cfDNA): Evaluating risk of overdiagnosis If confirmed, this would mean these tests preferentially catch the cancers you most want to find early, a meaningful advantage over imaging-based screening for some tumor types.
Watching for Recurrence After Treatment
One area where blood-based testing has already proven its value is monitoring patients after surgery or chemotherapy. A personalized ctDNA test, built from the genetic profile of a patient’s own tumor, can detect microscopic residual disease, meaning cancer cells too few for imaging to see. Growing evidence shows that finding ctDNA after treatment strongly predicts relapse, potentially allowing doctors to start or adjust therapy months before a visible recurrence appears.22PubMed Central. Detecting Liquid Remnants of Solid Tumors: Circulating Tumor DNA Minimal Residual Disease
In resected lung cancer, one study found that every patient who tested ctDNA-positive at a post-surgical landmark time point experienced recurrence within two years, a perfect positive predictive value. But ctDNA negativity did not guarantee freedom from recurrence: about a third of patients with negative results still relapsed, often with locoregional or brain-only disease that may not shed detectable DNA into the blood.23Journal of Thoracic Oncology Research and Reports. Prognostic Value of Longitudinal Circulating Tumor Data Analysis in Patients With Resected Non–Small Cell Lung Cancer Using a Personalized Tumor-Informed Assay In liver cancer patients after surgery, a tissue-informed ctDNA assay detected residual disease in about 71% of patients, and the level of ctDNA risk correlated with both recurrence-free survival and overall survival.24Cancer Research. Abstract 1020: Tissue-informed ctDNA MRD assay detects post-surgery minimal residual disease in HCC patients
This post-treatment use case is where blood tests are arguably most transformative right now. Rather than waiting months for a scan to show a growing mass, clinicians can act on a molecular signal. The limitation remains the same: tumors that shed little DNA into the blood, or that recur in locations like the brain, can still be missed.
Emerging Technologies Beyond ctDNA
The next wave of blood-based cancer detection is moving beyond reading DNA mutations or methylation alone. Two areas generating particular excitement are exosome analysis and DNA fragmentomics.
Exosomes are tiny vesicles that cells release into the bloodstream, carrying RNA, proteins, and other cargo that reflects their cell of origin. Tumor-derived exosomes carry microRNAs that are detectably different from those in healthy people. For colorectal cancer, a systematic review and meta-analysis found that exosome-based biomarkers showed sensitivity around 80% and specificity around 75% for early-stage disease.25PubMed Central. Extracellular vesicle biomarkers in circulation for colorectal cancer detection: a systematic review and meta-analysis Several specific microRNAs within exosomes have been proposed as diagnostic markers for colorectal cancer, with some showing promise for catching disease before it has spread.26PubMed Central. Circulating Extracellular Vesicle MicroRNA as Diagnostic Biomarkers in Early Colorectal Cancer-A Review For gastric cancer, a multicenter study established a 10-microRNA signature in exosomes that identified early-stage tumors with exceptionally high accuracy.27JAMA Surgery. Exosomal Liquid Biopsy for the Early Detection of Gastric Cancer: The DESTINEX Multicenter Study
Fragmentomics takes a different angle. Instead of reading the sequence or methylation of cell-free DNA, it analyzes the physical patterns of how that DNA is broken up: fragment lengths, the motifs at fragment ends, and the spacing between nucleosomes. These patterns differ between tumor-derived and normal DNA and can reveal the tissue a fragment came from.28PubMed Central. Cell-free DNA fragmentomics: a universal framework for early cancer detection and monitoring Fragmentomics can work as a standalone screening signal or as a supplement to mutation and methylation analysis, adding another layer of information from the same blood draw.29PubMed Central. Circulating DNA fragmentomics and cancer screening Machine learning is accelerating the field: one recent approach integrated thirteen different fragmentation features to improve early cancer detection accuracy.30Briefings in Bioinformatics. Early cancer detection via multi-omics cfDNA fragmentation using early-late fusion neural network with sample-modality evaluation
Neither exosomes nor fragmentomics is ready for routine clinical screening yet. But both represent attempts to extract more cancer-relevant information from the same tube of blood, and they could meaningfully improve sensitivity for the early-stage, low-shedding tumors that current tests struggle with.
The Psychological Toll of Ambiguous Results
One underappreciated dimension of blood-based cancer screening is what happens inside a person’s head when they receive an uncertain or positive result. In the NHS-Galleri trial, participants who received a “cancer signal detected” result described wide variation in the diagnostic journey that followed, and interviews revealed the emotional weight of living with that ambiguity.31PubMed Central. Experience of NHS diagnostic investigation following a multi-cancer early detection (MCED) screening test: qualitative interviews with NHS-Galleri trial participants who had a cancer signal detected One participant described lying awake at three in the morning, mentally sorting through insurance policies and bank accounts, convinced cancer was coming for him.32British Journal of Cancer. The impact of cancer expectations on psychological responses following a cancer signal detected result in asymptomatic multi-cancer detection (MCED) testing
This is not unique to MCED testing. Research on false-positive ovarian cancer screening results found that about a quarter of women who received a false alarm experienced high levels of cancer-specific distress that declined over time but remained elevated four months later.33PubMed Central. Psychological Response to a False Positive Ovarian Cancer Screening Test Result: Distinct Distress Trajectories and Their Associated Characteristics The anxiety is real, and it is one of the costs that gets weighed against the potential benefit of catching cancer early. If you are considering any form of blood-based cancer screening, it helps to go in knowing that a positive result often does not mean cancer, that follow-up testing is the norm rather than the exception, and that the weeks between a flagged result and a final answer can be genuinely difficult.
Access, Cost, and Where Things Stand
As of late 2024, no multi-cancer early detection test has been approved by the FDA. The one commercially available MCED liquid biopsy test, Galleri, is sold under laboratory-developed test regulations at a cost of $949 out of pocket. Few insurers cover it. No clinical practice guideline from the American Cancer Society, the U.S. Preventive Services Task Force, or comparable organizations recommends MCED testing for cancer screening in any population. Randomized controlled trials, the kind of evidence typically needed for insurance coverage and guideline endorsement, are still underway.
What this means practically is that the most talked-about blood tests for cancer remain in a gray zone. You can buy one, but you cannot get it covered, and the medical establishment has not yet decided it helps more than it hurts at the population level. For now, the blood tests with the strongest evidence behind them are the older, humbler ones: PSA monitored in conversation with your doctor, CA-125 tracked during ovarian cancer treatment, CEA watched after colon cancer surgery. These are imperfect, but they are understood. The newer tests are promising and moving fast, but anyone considering them should weigh the cost, the possibility of ambiguous results, and the current lack of guideline support against the potential benefit of catching something early that would otherwise go unnoticed.