Stool tests, blood tests or colonoscopy: Which colorectal cancer screening method is right for you?

After her father was diagnosed with colorectal cancer at 60, joining several other relatives with the disease, Emma Chen (not her real name) was advised to undergo a colonoscopy at 38. However, she delayed the procedure until she was 42. 

During the examination, doctors found a large polyp containing early invasive cancer. She subsequently underwent surgery to remove the affected section of her colon.

While screening generally begins at age 50 for those at average risk, people with a family history may need to start earlier, according to HealthHub.

Chen’s colorectal surgeon, Dr Aaron Poh, medical director at Alpine Surgical Practice, explains how earlier screening might have made a difference.

“Given her family history, an earlier colonoscopy at age 40 would have detected the polyp before it became cancerous, potentially allowing it to be removed without extensive surgery. Even a couple of years of delaying the screening can make a difference,” he says.

While blood in the stool, changes in bowel habits, persistent abdominal discomfort and unexplained weight loss are warning signs that warrant medical attention, the disease can also develop without noticeable symptoms, notes Poh.

According to the Singapore Cancer Registry Annual Report 2023, colorectal cancer was the second most common cancer among both men and women, with close to 13,000 new cases diagnosed between 2019 and 2023.

The National Cancer Centre Singapore also notes that cases among adults under 50 have been increasing.

Most colorectal cancers develop from polyps in the colon or rectum. While many are benign, some can become cancerous over time. Screening can detect abnormalities before symptoms appear, while a colonoscopy can also remove large precancerous polyps.

“Screening can also detect asymptomatic tumours in the early stages,” says Poh, adding that earlier diagnosis and treatment generally lead to better outcomes.

Why family history matters in colorectal cancer screening

Age, medical history, family history and previous screening results all influence when and how a person should be screened.

“People with a parent or sibling diagnosed with colorectal cancer, particularly at a younger age, may need to begin screening before 50.”

“I also look at their personal history of cancers and pre-cancerous conditions, and profile them for risk factors like smoking, alcohol intake and obesity,” notes Poh.

“For certain higher-risk individuals, guidelines recommend starting colonoscopy at age 40, or 10 years before the youngest diagnosis in the family, whichever comes first,” he advises.

Inherited conditions such as Lynch syndrome (HNPCC) and familial adenomatous polyposis (FAP) can substantially increase risk and may warrant specialist assessment.

How stool tests, blood tests and colonoscopy differ

Screening methods differ in what they detect, their accuracy and whether polyps can be removed during the procedure. Poh explains three approaches.

• Faecal immunochemical test (FIT)

This simple, non-invasive test checks for hidden blood in stool samples collected at home. In Singapore, annual FIT screenings are recommended for adults aged 50 and above who have no symptoms and are not at increased risk due to family history.

“However, FIT’s detection rate for colorectal cancer or advanced colorectal polyps is lacking compared to other methods,” notes Poh. “A negative result does not rule out cancer, while a positive result requires a follow-up colonoscopy.”

• Blood-based circulating tumour DNA (ctDNA) tests

These newer tests look for tumour-associated DNA fragments in the bloodstream that may indicate cancer.

Their role in routine screening is still being evaluated. Unlike a colonoscopy, they cannot locate abnormalities, collect tissue samples or remove polyps, he explains.

“A positive result requires further investigation, usually with a colonoscopy. A negative result does not rule out cancer, as some tumours may release too little DNA into the bloodstream to be detected,” Poh adds.

• Colonoscopy

This procedure uses a flexible tube with a camera to examine the lining of the colon and rectum. Doctors can identify abnormalities, take tissue samples and remove multiple polyps during the same procedure.

“A colonoscopy can be used both for screening people without symptoms and investigating suspected disease,” says Poh.

For average-risk adults aged 50 and above, a colonoscopy screening may be recommended at intervals of five years, depending on clinical guidance and individual circumstances. Those at higher risk or with previous advanced polyps may require earlier or more frequent examinations.

For those concerned about discomfort, Poh explains that a colonoscopy is typically performed under sedation. Although complications are uncommon, the procedure carries some risks, as with any invasive examination.

CT colonography, which uses CT scans to examine the bowel, may be considered in selected circumstances. Suspicious findings generally require follow-up colonoscopy.

Patients hesitant about colonoscopy can discuss their concerns and screening options with their doctor.

Poh emphasises that screening should be tailored to individual risk. Those experiencing symptoms such as rectal bleeding, persistent changes in bowel habits or unexplained weight loss should first consult a doctor for a medical assessment, rather than rely on routine screening tests. 

This allows the doctor to evaluate their symptoms and determine which investigations, if any, are appropriate.

Maintaining a healthy weight, exercising regularly, avoiding smoking and limiting alcohol and processed meat may also help lower risk, although these measures do not replace screening.

Find out more about the colorectal health screenings at Alpine Surgical Practice.