Medical Oncologist
CION Cancer Clinics
Other blood tests at a colon cancer diagnosis, and why each one matters | CION Cancer Clinics
No blood test can diagnose colon cancer. That job belongs to colonoscopy and biopsy. The blood tests you have around diagnosis do something different: they show how your body is coping, whether you are low on iron, and whether your kidneys and liver are ready for scans, surgery and drugs. This page goes through each test, when it is done, and what a result actually changes. At CION Cancer Clinics, our team goes through your reports with you and plans the next test or treatment step.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed October 2026, next review due October 2027
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The short answer
Which blood tests will you have after a colon cancer diagnosis?
Most people have a full blood count, iron studies, kidney and liver tests, a blood sugar check and a CEA level. Before an operation, clotting tests and a blood group are added. Before chemotherapy, a few more follow. None of these finds the cancer. They tell your team how safely, and in what order, it can be treated.
Why a blood test cannot make the diagnosis
Colon cancer is diagnosed by seeing it and sampling it. A colonoscopy finds the tumour, a biopsy takes a small piece of it, and a pathologist confirms the diagnosis under a microscope. Blood-based screening tests do exist, but a positive one still leads to colonoscopy, and a negative one does not rule out cancer when you have symptoms. Read more on blood-based screening tests.
What the blood tests do instead
They answer practical questions. Are you short of haemoglobin from slow bleeding? Can your kidneys handle the dye used in a CT scan? Is your liver working well enough for chemotherapy? Is your blood sugar steady enough for a wound to heal well? Each answer can change the timing or the order of treatment, which is why the tests come early in the diagnostic pathway.
A normal set of blood tests does not mean the cancer is small. An abnormal set does not mean it has spread.Test by test
What does each blood test look for?
These are the tests you are most likely to see on your report, and the plain reason each one is there.
Full blood count
This measures haemoglobin, white cells and platelets. A colon tumour, especially on the right side, can bleed slowly for months without any visible blood, leaving anaemia, a low haemoglobin. That often explains the tiredness and breathlessness that sent you to the doctor.
More on this in anaemia as the first sign.Iron studies
Ferritin, a measure of the body's iron store, shows whether a low haemoglobin comes from lost iron. If you are short of iron before surgery, you may be given iron as tablets or through a drip, so you go into the operation stronger.
Kidney and liver tests
Creatinine, a waste product the kidneys clear, shows how well they are working and guides both the CT dye and several chemotherapy doses. Liver tests show how the liver is coping, but they are a poor guide to spread. They can be normal with cancer in the liver and abnormal without it.
CEA
CEA, short for carcinoembryonic antigen, is a protein some colon cancers release. At diagnosis it is recorded as a baseline for later comparison. Many people with confirmed cancer have a normal level, which is explained in the CEA blood test page.
Not sure whether this applies to you?
Ask an oncologistAlong the pathway
When is each blood test done?
At your first visit
A blood count, iron studies, kidney and liver tests and a CEA level are usually taken together. If you have recent results from another hospital, bring them so nothing is repeated without reason.
Before the CT scan
Kidney function is checked before the contrast dye is given. If it is reduced, the radiologist may give fluids, adjust the dye or suggest a different scan.
Before the operation
Clotting tests, a blood group and cross-match, and an HbA1c, a measure of average blood sugar over recent months, are added. Anaemia is corrected where time allows. CION's surgical oncologists plan the operation, and it is carried out at an accredited partner hospital.
Before chemotherapy
Many centres test for hepatitis B and C, because chemotherapy can wake up a hidden hepatitis B infection. Some also test for DPYD, a gene that affects how the body clears capecitabine and 5-FU. Before irinotecan, a UGT1A1 test may be added.
During treatment
A blood count is checked before each chemotherapy cycle, because the drugs can lower white cells and platelets. A low count usually means a short delay or a dose change, not the end of treatment.
Reading a result
What does an abnormal result usually lead to?
If the report shows
What usually happens next
Low haemoglobin
Iron is checked and replaced; a transfusion only if it is very low or you are unwell
Raised liver tests
A CT or MRI of the liver decides whether there is spread
Normal liver tests
Spread is still possible; the scan rules it in or out
Raised creatinine
The CT dye or some drug doses may be adjusted
High blood sugar
Diabetes is brought under control before surgery where time allows
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When a low haemoglobin cannot wait
If you become breathless at rest, have chest pain, feel faint when you stand, or pass black, tarry stools or a large amount of blood, go to the nearest emergency department today. Tell them you are being investigated for colon cancer and bring your latest blood report. Do not wait for your next appointment.
Being straight with you
What this page cannot tell you
It cannot interpret your own numbers. Normal ranges differ between laboratories, and a value that worries you on paper may mean little in context, or the reverse. Your oncologist reads the blood results alongside your scans and biopsy, and that combined picture is what matters.
Who this page does not apply to
If you have not been diagnosed and are looking for a blood test to check for colon cancer, none of these will do it. A routine blood count can hint at a problem through anaemia, and that should lead to a colonoscopy, but a normal count does not rule cancer out. Bleeding or a change in bowel habit needs a doctor whatever the blood tests say. If the tumour is in the rectum, the blood work is similar but the pathway is not, and the rectal cancer pages cover it.
Genetic blood tests are a separate question
MMR and MSI testing, which looks for a fault in the cell's repair system, is done on the tumour tissue, not the blood. If those results suggest an inherited condition such as Lynch syndrome, a blood test for the inherited gene may follow, with counselling first. See genetic testing after a diagnosis.
If you have results from elsewhere and are unsure what they mean, call the helpline. A CION oncologist can go through them with you.Questions we are asked
Common questions about blood tests at diagnosis
Can a blood test show colon cancer?
No routine blood test can confirm or rule out colon cancer. A low haemoglobin or a raised CEA can raise suspicion, and newer blood-based screening tests exist, but every one of them leads back to a colonoscopy and biopsy. Only those can make the diagnosis.
Why is my haemoglobin low if I have not seen any blood?
Tumours, particularly on the right side of the colon, can bleed in tiny amounts that mix into the stool and are never visible. Over time the body runs short of iron and the haemoglobin falls. This is one of the common ways colon cancer first comes to light.
Will I need a blood transfusion before surgery?
Not usually. Most anaemia is treated with iron, as tablets or through a drip, ideally in the weeks before the operation. A transfusion is kept for a very low haemoglobin, heavy ongoing bleeding, or when surgery cannot wait for the iron to take effect.
Do I need to fast for these blood tests?
Most do not need fasting. A fasting blood sugar does, and some laboratories prefer a morning sample for iron studies. The team will tell you when you book. If you have diabetes, do not skip medicines or meals without asking, because a low sugar is a risk of its own.
My liver tests are normal. Has the cancer stayed out of the liver?
Not necessarily. Liver blood tests often stay normal even when there are small deposits in the liver, because the rest of the liver keeps working. Spread is judged from a CT scan, and sometimes an MRI. Treat normal liver tests as good news about liver function, not about spread.
Why do they test for hepatitis before chemotherapy?
Some people carry hepatitis B quietly without knowing. Chemotherapy weakens the immune system enough to let it flare, which can damage the liver. A simple blood test finds it, and if it is there, a tablet taken alongside chemotherapy keeps it in check.
How often is blood tested during chemotherapy?
Before every cycle, usually a day or two ahead or on the morning of treatment. If your counts are low, the cycle may be put back by a few days or the dose adjusted. You will also be told which symptoms, such as a fever, need a call the same day.
Can I get these tests done near home?
Often, yes. Many routine tests can be done at a reliable laboratory in your own district and the reports sent ahead, which saves a journey to Hyderabad. Ask your CION team which tests must be done at a specific laboratory and which can be done locally.
Your Specialists
Meet CION's colon cancer specialists. Bring your colonoscopy, scan or biopsy report.
Our surgical and medical oncologists plan colon cancer care together: which operation, whether chemotherapy is needed, and the follow-up that comes after.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
Medical Oncologist
Surgical Oncologist
Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Surgical Oncologist
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Sources
- American Cancer Society — Testing for Colorectal Cancer
- NHS — Tests and next steps for bowel cancer
- NHS — Iron deficiency anaemia
- National Cancer Institute — Tumor Marker Tests in Common Use
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
What tests will I need?The full list of scans, scopes and tests in one placeCEA blood test explainedThe one blood marker linked to colon cancer, and its limitsA normal CEA with confirmed cancerWhy a normal level does not cast doubt on the diagnosisTiredness and iron-deficiency anaemia as the first signWhy a low haemoglobin so often leads to the diagnosis
Talk to us
Holding blood reports you are not sure how to read?
Bring them along with any scan and biopsy reports, and a CION oncologist will explain what each result means for your next step. Nothing is repeated without a reason. One helpline serves every CION centre.