Can blood tests detect rectal cancer?
The short answer, and the most important thing in this article: there is no blood test that rules out rectal cancer. Perfectly normal blood tests are compatible with a rectal tumour that has been bleeding for months.
This belief — “I had blood tests and they were fine, so there is nothing wrong” — is one of the most common causes of delayed diagnosis I see.
Why CEA is not a screening test
CEA (carcinoembryonic antigen) is the best-known marker in colorectal cancer, and the most misused.
Its problem is not that it is useless, but that it has low sensitivity and specificity. Guidelines do not recommend it either for detection in healthy people or for early diagnosis.
The part that matters most: CEA sensitivity rises with stage. That is, it is lowest exactly where it would have been useful — in the early, curable stages. Many early colorectal cancers do not raise CEA at all.
Conversely, CEA can be elevated without any cancer: in smokers, in inflammation, in liver disease, and in some benign lung and digestive conditions.
So a normal CEA should not reassure you, and a raised one does not automatically mean cancer. As a detection test, it does not work.
What CEA is genuinely useful for
Once the diagnosis exists, CEA becomes a real tool:
As a baseline before treatment. A raised preoperative CEA that does not fall after surgery suggests residual disease.
In monitoring. A progressive rise during follow-up can signal a recurrence before it becomes visible on imaging. This is where its value is greatest.
As a prognostic element, alongside the other factors.
I have covered how it is interpreted in the article on tumour markers. In short: CEA is useful after diagnosis, not before it.
What blood tests can still show
Ordinary blood tests do not detect cancer, but they can raise a signal that compels investigation. The most important is this:
Iron-deficiency anaemia. Low haemoglobin together with low ferritin means chronic iron loss. In a man of any age and a woman after menopause, iron-deficiency anaemia is considered digestive bleeding until proven otherwise and requires investigation of the digestive tract. This is the one situation in which a blood test genuinely points towards the diagnosis.
Other changes — raised ESR or C-reactive protein, altered liver enzymes, thrombocytosis — are non-specific. They can occur in cancer, but they also occur in dozens of other situations, and their absence means nothing reassuring.
The confusion with the faecal blood test
Many people get lost here, and it is worth clarifying, because there is a test that genuinely works — but it is not a venous blood test.
The faecal immunochemical test (FIT) looks for human haemoglobin in stool, not in blood. It detects small bleeding invisible to the naked eye. It is validated as a screening test, done at home, and a positive result means colonoscopy follows.
So if someone tells you to “do the test for detecting colon cancer”, they mean this stool test, not CEA from blood. The difference is essential and even medical staff confuse the two.
Remember, though: a negative FIT does not replace colonoscopy if you have visible bleeding or other symptoms. FIT is for people without symptoms.
What is emerging: circulating tumour DNA
Tests looking for fragments of tumour DNA in blood (ctDNA) are the most intensively researched area. They can detect residual disease after surgery, sometimes before imaging, and are being studied to decide who needs adjuvant chemotherapy.
It is worth knowing they exist, and equally worth knowing that they have replaced nothing in standard practice. They are not a detection test for healthy people and they do not substitute for classical surveillance.
What to do in practice
If you have symptoms — bleeding, change in bowel habit, weight loss, a sense of incomplete evacuation — blood tests will not help you decide. The investigation that answers the question is colonoscopy, and it does not depend on what the blood count shows.
If you have no symptoms and want to check — do not ask for CEA. Ask for a FIT test or screening colonoscopy, depending on your age and family history.
If iron-deficiency anaemia has been found — do not settle for oral iron and nothing else. Ask where the lost iron is coming from. Correcting the anaemia without looking for the source treats the symptom, not the disease.
If you have already been treated — CEA makes sense, at the intervals your doctor sets, as part of surveillance alongside imaging and colonoscopy.
The conclusion
Blood tests are useful in following an already diagnosed rectal cancer. They are not useful for finding one.
If you take one thing away from this article, make it this: “my blood tests are fine” is not a reason to postpone a colonoscopy when symptoms exist.
The information in this article is educational and does not replace medical consultation.