A blood test for bowel cancer can give doctors useful clues about your health, but no routine blood test can diagnose or rule out bowel cancer on its own. Doctors often check your blood count, iron levels, liver and kidney function alongside FIT, examination and other investigations.
For example, long-term bleeding from the bowel can lead to iron-deficiency anaemia. Blood tests may reveal that anaemia before a person notices visible blood in the stool. However, many other conditions can also cause anaemia.
The most important point
A normal blood test does not rule out bowel cancer. Likewise, an abnormal result does not mean that you have cancer. Your GP interprets blood results together with your symptoms, examination, FIT result and, when necessary, further tests.
What can a blood test for bowel cancer show?
Doctors can request several blood tests when they investigate bowel symptoms. Each one answers a different question.
| Blood test | What doctors look for |
|---|---|
| Full blood count | Red blood cells, haemoglobin and signs of anaemia |
| Iron studies | Iron deficiency, including ferritin and related measurements |
| Liver function | How well the liver works and whether further assessment may help |
| Kidney function | General health and suitability for some scans or treatments |
| CEA | A tumour marker that doctors may monitor after a bowel cancer diagnosis |
These results can guide the next step. Nevertheless, doctors need other investigations to confirm or exclude bowel cancer.
Full blood count and bowel cancer
A full blood count, often shortened to FBC, measures different types of cells in your blood. In particular, doctors look at your red blood cells and haemoglobin.
If haemoglobin falls below the expected range, you may have anaemia. Anaemia has many causes. However, slow bleeding from the digestive tract can contribute to it.
A bowel tumour can sometimes bleed in amounts that remain too small to notice in the toilet. Over time, continued blood loss may reduce iron stores and haemoglobin.
Therefore, unexplained anaemia can prompt your GP to investigate further rather than simply treating the low haemoglobin.
Iron-deficiency anaemia and bowel cancer
Iron-deficiency anaemia deserves particular attention because chronic gastrointestinal blood loss can cause iron levels to fall.
Doctors may check ferritin and other iron measurements to understand whether your body has enough stored iron. Low ferritin can support a diagnosis of iron deficiency, although inflammation and other medical conditions can affect the result.
Current NICE guidance recommends quantitative FIT when adults have iron-deficiency anaemia and doctors need to assess the possibility of colorectal cancer. NICE also recommends FIT for people aged 60 or over who have anaemia even without confirmed iron deficiency.
Anaemia needs a cause, not just a supplement
Iron tablets can improve iron levels, but they do not explain why iron became low. If your doctor finds unexplained iron-deficiency anaemia, they may investigate for gastrointestinal blood loss and other possible causes.
Research also supports the link between iron deficiency and colorectal disease. Recent scientific reviews continue to examine ferritin and other markers as part of the assessment of patients who may need further bowel investigation.
Can a blood test for bowel cancer detect the cancer itself?
No standard blood test can reliably tell a doctor that bowel cancer is present or absent.
A person with bowel cancer can have normal haemoglobin, normal iron levels and otherwise unremarkable blood results. Conversely, abnormal blood results can occur for many reasons that have nothing to do with cancer.
Therefore, doctors do not use a blood test for bowel cancer as a substitute for FIT, colonoscopy, biopsy or appropriate imaging.
Read our complete guide to tests for bowel cancer, including FIT, colonoscopy, biopsy and scans.
What is the CEA blood test?
CEA stands for carcinoembryonic antigen. Some colorectal cancers release increased amounts of CEA into the blood.
However, CEA does not work well as a stand-alone test for diagnosing early bowel cancer. Some people with cancer have a normal CEA result. In addition, non-cancerous conditions and smoking can raise CEA.
Doctors therefore use CEA more often after they already know that a person has bowel cancer. For example, they may follow changes in CEA alongside scans and other clinical information during or after treatment.
A systematic review of CEA monitoring found that the marker misses a meaningful proportion of colorectal cancer recurrences when doctors use it alone. This supports the need to interpret CEA alongside other investigations.
Why might doctors check liver and kidney function?
Blood tests can also show how well your liver and kidneys work. These results give doctors useful information about your general health.
For example, kidney function can influence decisions about contrast-enhanced scans. Meanwhile, liver tests can help doctors decide whether they need further investigation.
Abnormal liver results do not prove that bowel cancer has spread to the liver. Many common conditions affect liver blood tests. Doctors interpret the result in context and arrange imaging when appropriate.
What happens after an abnormal blood test?
The next step depends on the type of abnormality and your symptoms.
If blood tests show unexplained anaemia or iron deficiency, your GP may arrange FIT and other investigations. Current NICE guidance uses FIT to help guide colorectal cancer referral in adults with iron-deficiency anaemia and several other bowel-related symptoms.
A symptomatic FIT result of at least 10 µg Hb/g faeces meets the NICE threshold for a suspected colorectal cancer pathway referral. However, NICE also advises doctors not to delay referral when strong clinical concern remains despite a lower result.
Read our detailed guide to FIT stool tests, results and what happens next.
What if your blood test is normal?
A normal blood test can reassure you about some aspects of your health, but it cannot exclude bowel cancer.
If you have persistent rectal bleeding, an unexplained change in bowel habit, abdominal pain, weight loss or other concerning symptoms, discuss them with your GP even when your blood results look normal.
Your symptoms and clinical history matter. Therefore, doctors may arrange FIT or further investigation despite normal blood results.
Read about bowel cancer symptoms and warning signs.
Can blood tests show whether bowel cancer has spread?
Blood tests can provide clues about organ function, but they cannot accurately map where a cancer has spread.
If doctors diagnose bowel cancer, they usually use imaging such as CT or MRI to assess the cancer more fully. They may also test tumour tissue for molecular changes that can influence treatment.
What about colon hydrotherapy?
Colon hydrotherapy serves a completely different purpose from cancer testing. It cannot diagnose, rule out, prevent or treat bowel cancer. Therefore, anyone with unexplained bleeding, unexplained anaemia or symptoms that need medical investigation should complete the appropriate medical assessment first.
However, once a doctor has investigated concerning symptoms and there is no medical reason to avoid treatment, some people choose colon hydrotherapy as a separate bowel-wellbeing treatment.
At Parkland Natural Health in Holborn, we provide a private consultation before treatment and screen every client for suitability. We use a closed-system, gravity-fed method and tailor the appointment to the individual rather than taking a one-size-fits-all approach.
If we find that colon hydrotherapy is not suitable for you, we do not proceed with treatment. This screening helps keep the focus where it belongs: your safety, comfort and appropriate care.
Frequently asked questions about blood tests and bowel cancer
Can bowel cancer cause low haemoglobin?
Yes. Slow blood loss from a bowel tumour can contribute to anaemia and low haemoglobin. However, many other conditions also cause low haemoglobin, so doctors need to investigate the cause.
Does low iron mean bowel cancer?
No. Iron deficiency has many possible causes. Nevertheless, unexplained iron-deficiency anaemia can lead doctors to investigate the gastrointestinal tract and offer FIT under current NICE guidance.
Can you have bowel cancer with normal blood tests?
Yes. Normal blood results cannot exclude bowel cancer. Doctors assess symptoms, FIT results and other investigations rather than relying on blood tests alone.
Is CEA a blood test for bowel cancer?
CEA is a blood tumour marker associated with colorectal cancer, but doctors do not rely on it to diagnose bowel cancer. They more commonly use it alongside other tests after diagnosis and during follow-up.
Trusted medical and scientific sources
For further information, these UK clinical and scientific sources explain how doctors use blood tests during bowel cancer investigation and follow-up.
- NICE: Suspected cancer – colorectal cancer recommendations
- Cancer Research UK: Tests for bowel cancer
- Cancer Research UK: Blood tests
- PubMed: Serum ferritin and risk of colonic neoplasia
- PubMed: Iron-deficiency anaemia in colorectal cancer
- PubMed: CEA systematic review and meta-analysis
Medical information notice: This article provides general information about blood tests and bowel cancer. It does not provide a diagnosis and cannot replace assessment by a GP, gastroenterologist or colorectal specialist. If you have persistent or unexplained symptoms, seek appropriate medical assessment.
