A blood test alone cannot tell whether you have cancer: an abnormal result does not always mean cancer, and a normal result does not rule it out [1]. A complete blood count (CBC) mainly helps to find blood cancers such as leukaemia [1]; sometimes iron-deficiency anaemia or a high platelet count leads the doctor to look for cancers such as bowel or lung cancer [3, 4]. To find bowel cancer early, a colonoscopy or a stool test works better than a blood test [10], and new blood tests that look for several cancers at once are still being studied [2, 7].
Key points
- A routine blood test neither proves nor rules out cancer; in most cases a biopsy (a tissue sample) is needed to be sure [1].
- A CBC is mainly useful for blood cancers such as leukaemia; in many other cancers it can be completely normal [1].
- Iron-deficiency anaemia at any age, and anaemia after the age of 60, are reasons to check the bowel with a stool test [3].
- Tumour markers do not work well for check-ups in healthy people [2]; the exceptions are PSA, after talking it through with your doctor [11], and AFP in people with cirrhosis of the liver [12].
- New blood tests for several cancers found about 3 in 10 cancers in a large study in England and did not reduce the number of advanced cancers [7, 8]; they do not yet replace the usual screening tests.
Can a routine blood test (CBC) detect cancer?
A complete blood count (CBC) measures the number of red blood cells, white blood cells and platelets and the amount of haemoglobin. It is part of a routine check-up and helps to diagnose some cancers, especially leukaemia (blood cancer) [1].
But normal values differ from one person to another, and even from one day to the next. Someone can have cancer and a normal result, or be healthy and have a result slightly outside the normal range [1]. This is why a doctor always reads blood results alongside symptoms, examination and scans. If you have your CBC report in front of you, see how to read a CBC.
| Cancer | What the blood test may show | Next step |
|---|---|---|
| Leukaemia (blood cancer) | The number of white cells, red cells or platelets is often abnormal, and a CBC helps with the diagnosis [1] | Further blood and bone marrow tests [1] |
| Multiple myeloma | A routine CBC is not enough; special blood protein tests (protein electrophoresis and free light chains) are needed [3] | If the results are suspicious, referral to a blood specialist (haematologist) [3] |
| Bowel (colorectal) cancer | Iron-deficiency anaemia can be a sign of hidden bleeding from the bowel [3] | A stool test for hidden blood and, if needed, a colonoscopy [3] |
| Lung and bowel cancer | A high platelet count is sometimes the only sign [4] | If platelets are still high on a repeat test, further tests [4] |
| Liver cancer | An AFP test is sometimes done alongside regular ultrasound scans in people with cirrhosis of the liver [12] | A liver ultrasound every 6 months in people at high risk [12] |
| Prostate cancer | The PSA test; a high PSA does not always mean cancer [11] | Decide whether to have it after talking with your doctor [11] |
| Most other cancers (such as breast, ovarian and pancreatic cancer) | There is no blood test that reliably finds them in healthy people [2] | Screening suited to your age, and taking symptoms seriously |
This table does not replace an examination by a doctor; most abnormal blood results have simpler causes.
Does cancer always show up in blood work? My results are normal
Not necessarily. Someone can have cancer and still have a normal routine blood test [1]. In a large study in England, markers of inflammation such as ESR and CRP were raised in fewer than half of the people who had cancer. So normal results do not rule cancer out [5].
On the other hand, some abnormal results are worth following up. In another large study in England, among people aged 40 or over with a high platelet count, about 1 in 9 men and 1 in 16 women were diagnosed with cancer within a year, most often lung or bowel cancer. The chance was higher when platelets were high again on a second test [4]. So most of these people did not have cancer, but they did need to be checked.
If you have a symptom that worries you, such as blood in your stools, a lump or unexplained weight loss, do not wait for a blood test: see a doctor [3]. Read more: symptoms of bowel cancer and symptoms of leukaemia.
Do tumour markers in a blood test show cancer?
Tumour markers are substances made by cancer cells, and sometimes by healthy cells in benign conditions. Researchers hoped they could find cancer in people without symptoms. But studies showed they do not work well for this: they miss many people who have cancer, and they are raised in many healthy people [2]. Their main use is in following the treatment of a cancer that has already been diagnosed [2].
Two important exceptions:
- PSA for prostate cancer: for men aged 55 to 69, the decision about this test is made after talking with a doctor about its benefits and harms [11]. Read: does a high PSA mean prostate cancer?
- AFP for liver cancer: in people with cirrhosis of the liver, a liver ultrasound every 6 months is recommended, and AFP is sometimes measured alongside it [12].
Read more: what is a tumour marker and when is it useful? and what does a high CEA mean?
Is there a blood test for colon cancer?
A new blood test for bowel cancer screening looks for fragments of tumour DNA in the blood. In a large study of nearly 8,000 people, it found about 8 in 10 bowel cancers. But it found only about 1 in 8 advanced polyps (growths that may later turn into cancer). And about 1 in 10 people who had no advanced growth got a positive result [9].
Stool tests find advanced polyps and stage 1 cancers better than the blood test, and they are expected to prevent more bowel cancers and more deaths from it. This is why the American Cancer Society, in its 2026 guideline, suggests the blood test only for people who decline or do not complete a colonoscopy or a stool test. Any positive result must be followed by a colonoscopy [10].
Read more: at what age is a screening colonoscopy needed? and my faecal occult blood test was positive: what should I do?
How accurate are multi-cancer blood tests such as Galleri?
"Multi-cancer" tests look for DNA signals of several types of cancer at once in a single blood sample, and guess which organ the cancer comes from. One of them is the Galleri test, which has been studied in large trials [6, 8].
- In a study in the United States of more than 6,000 people aged 50 or over without symptoms, the test found a cancer signal in about 1 in 70 people. About 4 in 10 of these people really had cancer; the rest had no cancer after months of tests and scans [6].
- In a much larger study in England of more than 140,000 people, testing once a year for 3 years did not reduce the number of advanced (stage 3 and 4) cancers. There were signs of fewer stage 4 cancers, but this is not certain yet and follow-up continues [7].
- In the same study, the test detected about 3 in 10 of the cancers found in these people. When the result was positive, about half of the people really had cancer [8].
So these tests miss most cancers, and a negative result does not mean you have no cancer. It is not yet known whether finding cancer with them saves lives [2]. If you are thinking about one, talk with your doctor and do not give up the usual screening tests [2, 7].
So what should I do to find cancer early?
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Have the screening tests suited to your age
For bowel cancer, a colonoscopy or a stool test is the best choice [10]. See which screening tests you need at your age: which screening tests do I need?
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Follow up an abnormal blood test
Follow up unexplained anaemia, a high platelet count or a change in white cells with your doctor; sometimes a repeat test is enough and sometimes more tests are needed [3, 4]. Read: what leukaemia looks like on a blood test (CBC).
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If you are at high risk, tell your doctor
People with cirrhosis of the liver need regular liver ultrasound scans [12]. Men aged 55 to 69 should talk with their doctor about the PSA test [11]. Read more: 5 simple steps to find cancer early.
When should I see a doctor?
Most abnormal blood results have a simple cause. But in these cases, see a doctor soon:
- Iron-deficiency anaemia with no known cause, or any anaemia after the age of 60 [3]
- A high platelet count that is still high on a repeat test [4]
- Paleness, lasting tiredness, unexplained fever or one infection after another, unexplained bruising or bleeding, tiny red spots under the skin, or swollen lymph nodes; these need a CBC within a day or two [3]
- Lasting bone or back pain, or an unexplained fracture, after the age of 60 [3]
- Blood in your stools, a lasting change in bowel habit or unexplained weight loss [3]
Bleeding that will not stop, a high fever with shivering, or shortness of breath needs urgent care; call your local emergency number.
Frequently asked questions
Can a simple blood test (check-up) tell me if I have cancer?
My CBC is abnormal; does that mean I have blood cancer?
Most of the time, no. An abnormal result is not a sure sign of cancer, and a value slightly above or below the range is also seen in healthy people [1]. Your doctor will usually repeat the test and look for more common causes. In other cancers, a CBC may also be normal or show only anaemia [1, 3]. Read: what leukaemia looks like on a blood test (CBC) and how to read a CBC.
My ESR or CRP is high; does that mean I have cancer?
Most of the time, no. These tests are signs of inflammation in the body and are not specific to cancer. In a large study, fewer than 4 in 100 people with raised results were diagnosed with cancer within a year. The chance was higher in older people, in men and when a repeat test was also raised [5]. If the cause of a raised result is not clear, follow it up with your doctor.
Is there a blood test for breast cancer?
No. No blood test is recommended for finding breast cancer in healthy women; blood tumour markers are not accurate enough for this [2]. For breast cancer screening, read: at what age is a mammogram needed?
Liquid biopsy vs tissue biopsy: can a blood test replace a biopsy?
A liquid biopsy is a test on a blood sample that looks for cancer cells or fragments of tumour DNA. It is also used to choose treatment, to see how well treatment is working or to find cancer that has come back [1]. But in most cases, a sample of the tissue itself is needed to be sure of a cancer diagnosis [1]. Read more: liquid biopsy and circulating tumour DNA.
Study and guideline details (for doctors)
For physicians
Blood counts and inflammatory markers in primary care. In a prospective CPRD cohort (40,000 patients aged 40 or over with platelets above 400 × 109/L and 10,000 controls), the 1-year cancer incidence was 11.6% vs 4.1% in men with and without thrombocytosis and 6.2% vs 2.2% in women, rising to 18.1% and 10.1% when a second raised platelet count was recorded within 6 months. Lung and colorectal cancer were more common, and one third of these patients had no other features of malignancy [4]. In a CPRD cohort of 160,000 people, the 1-year cancer incidence was 3.53% with a raised inflammatory marker vs 1.50% with a normal one; sensitivity for cancer was 46.1% for CRP, 43.6% for ESR and 49.7% for PV, so these tests are not useful for ruling cancer out [5].
NICE NG12. Quantitative FIT to guide referral in iron-deficiency anaemia (any age) and in people aged 60 or over with anaemia even without iron deficiency; a very urgent FBC (within 48 hours) in adults with pallor, persistent fatigue, unexplained fever, persistent or recurrent infection, generalised lymphadenopathy, unexplained bruising, bleeding or petechiae, or hepatosplenomegaly; in people aged 60 or over with persistent bone pain (particularly back pain) or an unexplained fracture: FBC, calcium, ESR or PV, serum protein electrophoresis and serum free light chains [3].
MCED tests. PATHFINDER (6,662 participants aged 50 or over without symptoms; 6,621 with analysable results): a cancer signal was detected in 92 (1.4%); 35 (38%) had cancer and 57 (62%) were false positives; median time to diagnostic resolution 79 days (57 days in true positives and 162 days in false positives) [6]. NHS-Galleri (randomised trial, 142,250 people aged 50 to 77, 3 annual rounds): the primary end point, incidence of stage III or IV cancer among 12 prespecified types, did not differ (IRR 1.03; 95% CI 0.92 to 1.14; P = 0.63); the IRR for stage IV after 3 rounds was 0.86 (95% CI 0.74 to 1.00) [7]. In the intervention arm, positive results in rounds 1 to 3 were 1.03%, 0.80% and 0.90%; PPV 58.0%, 50.4% and 45.8%; specificity 99.50% to 99.60%; episode sensitivity 26.7% to 37.2% for all cancers and 47.6% to 63.4% for the 12 prespecified types; cancer signal origin accuracy 91.1% to 93.6% [8].
cfDNA colorectal screening. ECLIPSE (7,861 average-risk people, compared with colonoscopy): sensitivity 83.1% for colorectal cancer (95% CI 72.2 to 90.3), 87.5% for stages I to III and 13.2% for advanced precancerous lesions; specificity 89.6% for advanced neoplasia [9]. ACS 2026 guideline: compared with stool-based tests, blood-based tests have lower sensitivity for advanced precancerous lesions and stage I cancer and should be recommended only to people who decline or do not complete preferred tests; any positive non-colonoscopy test requires follow-up colonoscopy, preferably within 6 months [10].
Tumour markers. Blood tumour markers generally lack the sensitivity and specificity needed for screening [2]. USPSTF: PSA screening at 55 to 69 years is an individual decision (grade C) and is not recommended at 70 or over (grade D) [11]. EASL 2025: HCC surveillance with abdominal ultrasound every 6 months in high-risk groups, particularly cirrhosis, based on individual risk assessment; AFP can be used together with ultrasound [12].
Scientific evidence
Scientific evidence
3 itemsA Cell-free DNA Blood-Based Test for Colorectal Cancer Screening
Screening for Prostate Cancer
EASL Clinical Practice Guidelines on the management of hepatocellular carcinoma
Related cancer types
Related questions
References
12 referencesShow all references, 3 with a study cardClose the reference list
Numbers in the text, such as [1], refer to these sources. Sources with a “Study card” button are summarised in the site’s evidence library.
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- 9A Cell-free DNA Blood-Based Test for Colorectal Cancer Screening
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