Diagnosis is the process of finding the underlying cause of a health problem. If cancer is suspected, the healthcare team will confirm if it is present or not, and what type of cancer it is. The process of diagnosis may seem long and frustrating, but it is important for the doctor to rule out other possible reasons for a health problem before making a cancer diagnosis.
Diagnostic tests for colorectal cancer are usually done when:
- the symptoms of colorectal cancer are present
- the doctor suspects colorectal cancer after talking with a person about their health and completing a physical examination
- screening tests suggest a problem with the colon or rectum
Many of the same tests used to initially diagnose cancer are also used to determine the stage (how far the cancer has progressed). Your doctor may also order other tests to check your general health and to help plan your treatment. Tests may include the following.
Medical history and physical examination
The medical history is a record of present symptoms, risk factors and all the medical events and problems a person has had in the past. The medical history of a person's family may also help the doctor to diagnose colorectal cancer.
In taking a medical history, the doctor will ask questions about:
- a personal history of
- polyps in the colon or rectum
- inflammatory bowel disease
- colorectal cancer
- a family history of
- signs and symptoms
A physical examination allows the doctor to look for any signs of colorectal cancer. During a physical examination, the doctor may:
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Fecal occult blood test (FOBT)
A stool test called a fecal occult blood test (FOBT) checks a stool (feces) sample for blood that cannot be seen with the naked eye. This test may be done if it was not included as a screening test.
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Complete blood count (CBC)
A complete blood count (CBC) measures the number and quality of white blood cells, red blood cells and platelets. A CBC is done to check for anemia from long-term (chronic) bleeding.
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Blood chemistry tests
Blood chemistry tests measure certain chemicals in the blood. They show how well certain organs are functioning and can also be used to detect abnormalities. They are used to stage colorectal cancer as higher levels of some chemicals may indicate that the cancer has spread to the kidneys or liver.
- Urea (blood urea nitrogen or BUN) and creatinine measure kidney function.
- Liver enzymes, such as alanine aminotransferase (ALT), aspartate transaminase (AST) and alkaline phosphatase, measure liver function.
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Tumour marker tests
Tumour markers are substances – usually proteins – in the blood that may indicate the presence of colorectal cancer. Tumour marker tests are used to check a person's response to cancer treatment, but they can also be used to diagnose colorectal cancer.
The tumour markers that may be measured are:
- Carcinoembryonic antigen (CEA) levels may or may not be higher in people with colorectal cancer. It is helpful to compare CEA levels before and after surgery. A rising CEA level after surgery could be a sign that the cancer has returned. Higher CEA levels may also be caused by smoking, pancreatic or liver disease and other non-cancerous conditions, such as benign polyps. High CEA levels may mean the cancer is advanced, but not everyone with colorectal cancer will have increased CEA levels.
- Cancer antigen 19-9 (CA19-9) is a tumour marker that is sometimes higher in cases of colorectal cancer, but also may be increased because of other conditions.
- Cancer antigen 125 (CA125) is a tumour marker that could be higher in colorectal cancer. However, like CA19-9, other conditions can cause CA125 to rise. Its use in diagnosing colorectal cancer is debatable.
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Flexible sigmoidoscopy
A flexible sigmoidoscopy is a procedure that lets the doctor look at the lining of the rectum and lower part of the colon (sigmoid colon) using a flexible tube with a light and lens on the end (an endoscope).
A sigmoidoscopy can be done in a doctor's office or clinic. The doctor gently inserts a sigmoidoscope (a type of endoscope) through the anus and slowly moves it into the rectum and lower part of the colon. This can be uncomfortable but not painful. Drugs for relaxation and pain relief are usually not needed.
If polyps or abnormal areas are found, they can be removed (biopsied) with instruments inserted through the sigmoidoscope.
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Colonoscopy
A colonoscopy is a procedure that lets the doctor look at the lining of the colon using a flexible tube with a light and lens on the end (an endoscope). A colonoscopy is preferred over a flexible sigmoidoscopy because the entire colon can be checked for polyps or abnormal areas.
A colonoscopy is done in a hospital on an outpatient basis. The doctor gently inserts the colonoscope (a type of endoscope) through the anus and slowly moves it into the rectum and colon. The colon is inflated with air to stretch out the lining so the doctor can look at the entire surface. This can be uncomfortable, so drugs are given to help the person relax during the procedure.
If polyps or abnormal areas are found, they can be removed (biopsied) with instruments inserted through the colonoscope.
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Double-contrast barium enema
A double-contrast barium enema uses a contrast substance (barium sulphate) and x-rays to create pictures of the entire large intestine. Air is pumped into the intestine after the barium has been put in the intestine to improve the images. This test is used:
- as follow-up to a positive fecal occult blood test (FOBT)
- to find growths or polyps in the colon
- to find the source of unexplained bleeding, unexplained low red blood cell levels (anemia), or pain in the lower abdomen
- in combination with a flexible sigmoidoscopy to diagnose colorectal cancer
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Biopsy
During a biopsy, tissues or cells are removed from the body so they can be tested in a laboratory. The pathology report from the laboratory will confirm whether or not cancer cells are present in the sample and may also identify the type of cancer.
A biopsy is the only definite way to diagnose colorectal cancer. Biopsies of polyps or abnormal areas are taken during a sigmoidoscopy or colonoscopy. A biopsy sample will allow the doctor to find out the type of colorectal cancer and the grade. Biopsy results may also show how far the cancer has grown through the wall of the colon or rectum.
KRAS gene testing
Biopsy samples from people with stage IV or recurrent colorectal cancer may have additional testing to determine if the cells contain the KRAS gene mutation. Tumours that test positive for the KRAS gene mutation are unlikely to respond to the targeted therapy drugs cetuximab (Erbitux) and panitumumab (Vectibix).
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Computed tomography (CT) scan
A CT scan uses special x-ray equipment to make 3-dimensional and cross-sectional images of organs, tissues, bones and blood vessels inside the body. A computer turns the images into detailed pictures. It is used to:
- check if the cancer has spread to other organs in the abdomen or pelvis (small areas of spread [microscopic spread] may not be detected by CT scan)
- check if the cancer has spread to the lymph nodes in the abdomen
- check how far the tumour has grown into the wall of the colon or, especially, the rectum
CT-guided needle biopsy
- CT scans may also be used to help guide a needle to perform a biopsy (CT-guided needle biopsy) to check for cancer cells in a tumour in the colon or a suspected area of metastasis (cancer spread outside of the colon or rectum).
Virtual colonoscopy
- Virtual colonoscopy uses a CT scan to create images of the colon without having to insert an endoscope through the rectum. A virtual colonoscopy is less invasive and more comfortable than a regular colonoscopy. Studies are continuing to examine the effectiveness of this test.
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Ultrasound
Ultrasound uses high-frequency sound waves to make images of structures in the body.
- Endorectal ultrasound (EUS or ERUS) uses a special instrument (transducer) that is inserted into the rectum. It is used to see:
- how far a tumour has grown into the rectal wall
- if the tumour has spread to nearby organs or lymph nodes
- Abdominal ultrasound may be done to see if the cancer has spread to other organs in the abdomen, such as the liver.
- Pelvic ultrasound may be done if doctors suspect that the cancer has spread to the urinary tract.
- An ultrasound may also be used during abdominal surgery. The surgeon can place the transducer directly on the liver to check for metastases.
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Magnetic resonance imaging (MRI)
MRI uses powerful magnetic forces and radio-frequency waves to make cross-sectional images of organs, tissues, bones and blood vessels. A computer turns the images into 3-dimensional pictures. It is used to check for:
- tumours that are too small to be found with a CT scan
- disease spread to other organs, such as the liver or lymph nodes
- disease spread to the brain and spinal cord
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Chest x-ray
An x-ray uses small doses of radiation to make images of the body’s structures on film. A chest x-ray may be done to see if colorectal cancer has spread to the lungs.
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Positron emission tomography (PET) scan
A PET scan uses radioactive materials (radiopharmaceuticals) to detect changes in the metabolic activity of body tissues. A computer analyzes the radioactive patterns and makes 3-dimensional colour images of the area being scanned.
PET scans are not routinely used to diagnose colorectal cancer. They are more commonly used to help stage and check for recurrent disease if a person's CEA level starts to rise following treatment. PET scans are not readily available at all centres.
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Intravenous pyelogram (IVP)
IVP uses x-rays and a special dye (radiopaque) to make images of the kidneys and urinary tract. It is used to see if:
- there are any blockages along the urinary tract
- the cancer has spread to the bladder, kidney or ureters
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See a list of questions to ask your doctor about diagnostic tests.