Colorectal Cancer Diagnosis: Workup, Tests and Staging

Colorectal cancer diagnosis rests on four tests: colonoscopy with biopsy to confirm the cancer, MRI of the pelvis to map a rectal tumour's local extent, CT of the chest and abdomen (or PET-CT) to look for spread to the liver and lungs, and a blood test for the tumour marker carcinoembryonic antigen (CEA). Together they give the stage, and the stage decides the treatment.

Dr. Kaushal Kishor Yadav, currently Director & Unit Head – Surgical Oncology, Max Hospital, Gurugram, India, described this as the ideal workup when he conducted a Jivo Masterclass for international doctors in June 2025. Much of the discussion that followed concerned what to do when parts of it are not available. This article covers both: the complete workup and the way to sequence tests where resources are limited. The wider pathway, from warning signs to stage 4 treatment, is set out in the complete guide to the diagnosis and management of rectum cancer.

When to start investigating for colorectal cancer

Investigation begins with a warning sign, not a confirmed diagnosis. Dr. Yadav named three that should prompt evaluation for colorectal cancer: bleeding from the bowel, either fresh blood or a change in the colour of the stool; a fall in haemoglobin that no other cause explains; and constipation or diarrhoea that is not relieved and persists for more than three weeks.

His threshold for acting is low. Whenever there is doubt, colonoscopy and CT scan should be done, and when there is suspicion they should be done without delay. Waiting for a symptom to become unmistakable usually means waiting for the tumour to advance: as he noted, 60 to 70% of patients already have stage 3 or stage 4 disease at presentation.

Colonoscopy and biopsy: confirming a colorectal cancer diagnosis

Colonoscopy is the central test. A flexible camera examines the full length of the colon and rectum, shows the tumour directly, and allows tissue samples (biopsies) to be taken for the pathologist. No treatment plan for colorectal cancer should begin without a tissue diagnosis.

Colonoscopy does three jobs at once:

• It confirms the cancer through biopsy and histopathology.
• It locates the tumour, which matters for surgical planning. A tumour of the caecum or ascending colon calls for a different operation from a tumour of the sigmoid colon or the low rectum.
• It examines the rest of the large bowel for a second cancer or for polyps that should be removed.

In some health systems colonoscopy is available only at tertiary hospitals, so the patient travels for the procedure and brings the results back. One general surgeon in the audience described exactly this arrangement. It adds time, which is why the referral for colonoscopy should be made as soon as the warning signs appear rather than after a long trial of symptomatic treatment.

Pelvic MRI or CT for rectal cancer staging?

For rectal cancer, MRI of the pelvis is the standard test for local staging. It shows how deeply the tumour has grown into the wall of the rectum, whether the surrounding lymph nodes look involved, and the relationship of the tumour to the mesorectal fascia, the envelope of tissue that surrounds the rectum and its lymph nodes and that defines the surgeon's plane of dissection.

A doctor from Ethiopia raised a practical difficulty during the session: the MRI scanners available to him do not produce images of good enough quality, so his team relies on CT even for rectal tumours, although textbooks recommend MRI. Dr. Yadav's answer set out when MRI genuinely changes management. It is most helpful when the cancer lies very low in the rectum. In that situation the MRI shows whether the muscles of the pelvis are free of tumour, and therefore whether the patient will need a stoma or can keep normal bowel passage. In the rest of the cases, in his view, a CT scan is good enough where good-quality MRI is unavailable, although guidelines recommend pelvic MRI for every rectal cancer because it alone reliably shows the circumferential resection margin.

That distinction is useful for any doctor working with limited imaging. A good-quality pelvic MRI should be prioritised for the patient with a low rectal tumour, because the decision between sphincter-preserving surgery and a permanent stoma depends on it. The surgical side of that decision is explained in the companion article on sphincter-preserving surgery for low rectal cancer.

CT or PET-CT: checking for spread to the liver and lungs

Colorectal cancer most often spreads to the liver and the lungs. The workup therefore includes CT of the thorax and abdomen to check whether these organs are clear. Where PET-CT is available, it can be used instead; a PET-CT combines a CT scan with an injected tracer that highlights metabolically active tissue, which helps to identify spread elsewhere in the body.

This step decides whether the disease is stage 4. That distinction changes everything that follows, because treatment for stage 4 colorectal cancer is led by chemotherapy, with surgery reserved for limited liver or peritoneal disease that can be removed completely.

CT also has a role at the start of the pathway. Dr. Yadav advised that a patient with warning signs and abdominal symptoms that are not settling should have either a colonoscopy or a CT scan, whichever is easily available, so that any doubt can be confirmed or excluded. A general surgeon at a district general hospital described his own team's sequence: staging with CT, colonoscopy, operating on the colon cancers his team can manage, and referring distal rectal tumours that would need an abdominoperineal resection, which removes the rectum, anal canal and sphincters and leaves a permanent colostomy, to a colorectal surgeon.

CEA blood test: the colorectal cancer tumour marker

Carcinoembryonic antigen (CEA) is the blood tumour marker for colon and rectum cancer. Dr. Yadav noted that a serum CEA test can be done even at the primary level, which makes it one of the more accessible parts of the workup.

CEA is not a diagnostic test on its own: it can be normal in patients with cancer and raised for other reasons, including smoking. Its value lies in a baseline measurement taken before treatment, against which later results during follow-up can be compared. A rising CEA after treatment can be an early signal that the cancer has returned.

Stool tests for blood: FOBT and FIT at the primary level

The faecal occult blood test looks for small amounts of blood in the stool that the eye cannot see; the faecal immunochemical test (FIT) detects human haemoglobin specifically. Dr. Yadav's screening advice is a faecal occult blood test or FIT once a year, together with colonoscopy once every ten years, for people beyond 40.

At primary and general hospitals, a stool test for blood may be the only early test on offer. A general surgeon in the audience explained that in his setting genetic tests are not available, colonoscopy requires referral to a tertiary hospital, and the stool test is what can be done locally. He described its limitation candidly: it is not very specific and may miss some cases, but it helps in others. A positive result, or a negative result in a patient with persistent warning signs, should lead to colonoscopy.

Colorectal cancer staging: the TNM system

The results of these tests are combined into a stage using the TNM system, which scores the tumour (T), the lymph nodes (N) and distant spread (M):

• T (tumour) describes how deeply the primary tumour has grown. T1 tumours are confined to the inner layers of the bowel wall; T2 tumours have grown into the muscle layer; T3 tumours have grown through the muscle wall into the surrounding fatty tissue; T4 tumours have reached the outer lining or adjacent organs.
• N (nodes) describes whether nearby lymph nodes contain cancer. N0 means no nodal spread.
• M (metastasis) describes spread to distant sites, such as the liver, lungs or peritoneum.

These combine into the overall stages 1 to 4. Stage 1 and 2 cancers have not spread to lymph nodes; stage 3 cancers have spread to regional lymph nodes; stage 4 cancers have spread to distant organs.

How the stage sets the colorectal cancer treatment plan

Dr. Yadav's treatment rules depend directly on this staging:

This is why the pelvic MRI matters so much in low rectal cancer. It is the test that tells the team whether the tumour is T3 or node positive and should receive chemotherapy and radiotherapy first, and whether the sphincters are free for a sphincter-preserving operation. A rectal cancer that is operated on without that information may be under-treated.

Diagnosing colorectal cancer with limited resources

Several doctors described working in facilities where a CT scan cannot be ordered at level one, MRI quality is poor, or colonoscopy is available only after travel. Drawing on the session, a workable sequence looks like this:

1. Take the warning signs seriously at first contact. Bleeding, unexplained anaemia or a change in bowel habit lasting more than three weeks warrants investigation at any age; Dr. Yadav now sees colon cancer in patients in their twenties and thirties.
2. Do the tests available locally. A stool test for blood and a serum CEA can usually be done at the primary level. One doctor in the audience described his own first step for a patient with a change in bowel habit and unexplained low haemoglobin: referral for colonoscopy.
3. Secure a tissue diagnosis. Refer for colonoscopy and biopsy without waiting for symptoms to progress.
4. Stage before treating. CT of the chest and abdomen for distant spread, and pelvic MRI for low rectal tumours.
5. Refer when the next step cannot be done locally. Dr. Yadav's advice to a doctor from a region with very limited infrastructure was to do whatever is within his control and, when the tests or treatment are not available, to refer the patient to a centre where they can be done with precision. Cancer, as he put it, gives only one chance.

He also encouraged doctors to contact the treating team at the initial stage, describing the patient and the symptoms, to ask which investigations should be done first. The full exchange, with the follow-up questions raised by doctors in Zambia and the Democratic Republic of the Congo, is on the live discussion page for this masterclass.

Why a complete workup protects the patient

Every element of the workup feeds a specific decision. Colonoscopy and biopsy confirm the disease and locate it. Pelvic MRI decides whether a low rectal tumour needs chemotherapy and radiotherapy first and whether the sphincters can be saved. CT or PET-CT decides whether the disease is stage 4 and, if so, whether liver or peritoneal deposits are limited enough to remove. CEA sets the baseline for follow-up.

Skipping a step rarely saves time. It risks an operation in the wrong plane, an operation that should have been preceded by treatment, or an operation that cannot cure because the disease had already spread. For a patient who will travel for surgery, a complete and recent set of these results also allows the receiving team to plan before arrival. Other cancers' warning signs and diagnostic steps are covered across the oncology specialty page.