Many people have the misconception that they need to do cancer or tumour marker tests to screen for cancer.

A blood-cancer marker is a substance produced in the body by cancerous cells. It is also made by some normal cells, although the levels tend to be much higher when there is cancer.

However, such tests are generally considered to be of uncertain value.

Dr Ng Lee Beng, a consultant at the Singapore General Hospital's family medicine and continuing care department, said cancer markers are not sensitive as they do not pick up cancer most of the time. Also, a positive result does not usually mean that a person has cancer, she said.

Dr Lim Siew Eng, a senior consultant at the haematology- oncology department at the National University Cancer Institute, Singapore, said blood tests should preferably not be used in place of recommended screening tests for cancer.

“Blood tests are convenient. But they have not been shown to reduce cancer mortality and may not detect cancer earlier, and could also give rise to a false sense of well-being.”

Instead, Dr Lim advises people to do recommended age-specific tests – mammography for breast cancer, Pap smears for cervical cancer, and stool blood tests or colonoscopy for colon cancer. These tests should be done even if one does not have a family history of the cancers.

Dr Winston Ho, the medical director of Parkway Shenton, said that if cancer markers are used and cancer is suspected, other tests such as scans, scopes or biopsies are needed to confirm the diagnosis.

Non-cancerous conditions can also give rise to an elevated cancer marker, he added.

For instance, a rise in cancer antigen 125 or CA125 levels, an indicator of ovarian cancer, may be caused by endometriosis, and the CEA (carcinoembryonic antigen) indicator for colon cancer may be elevated due to inflammatory bowel disease.

But though there are limitations to using blood cancer markers as a general screening tool, they are non-invasive and more affordable, Dr Ho said.

He said cancer markers are most useful when used to monitor response to cancer treatment and to screen for recurrence.

They are also useful for older people or at-risk persons with predisposed conditions, he added. For instance, hepatitis B carriers are advised to check their liver-cancer marker every six months as this liver infection is the major cause of liver cancer.