There is no specific blood test that will definitively conclude the presence of cancer. However, blood can be analysed to determine the presence or levels of specific biological or tumour markers, which are proteins or chemicals produced by tumour cells. These blood tests can be useful screening tools for cancers such as prostate cancer, liver cancer, reproductive system cancers and digestive tract cancers, especially when used in conjunction with other diagnostic tests.
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Prostate cancer is the second most common cancer in American men (second to skin cancer). Therefore, reliable screening tests have been developed to help in early diagnosis. The prostate-specific antigen (PSA) test is a blood test that looks at the level of PSA present in the blood. In normal healthy men, the PSA level is low; however, when tumours are present the PSA level increases. This test, along with a digital rectal exam (DRE), can detect the possibility of cancer even in men with no symptoms.
Alpha fetoprotein (AFP) is a protein produced by the yolk sac and liver of a foetus (developing baby). The protein level in adults is very low, as this protein serves no function in adults. However, the level of this protein is often higher when liver cancer is present. The level of AFP can be determined during a blood test.
Reproductive System Cancers
The tumour marker CA125 is often used to determine the presence of cancer affecting the Fallopian tubes, ovaries and uterus. A blood test screening for CA125 may not be conclusive, because an elevated level of CA125 may also indicate other conditions including hepatitis, pericarditis, ovarian cysts, cirrhosis of the liver and peritonitis. However, known high-risk individuals can still benefit from this test being performed.
The tumour marker known as CA 15.3 is often higher in a blood sample of a patient with breast cancer. For patients with a family history of breast cancer, this blood screening test can be an early diagnostic tool, along with a mammogram, to detect the presence of cancer.
Digestive Tract Cancers
Patients exhibiting symptoms of frequent constipation, diarrhoea or bloody stools can benefit from a blood test for carcinoembryonic antigen (CEA). CEA is a protein usually only produced during fetal development; however, its presence in an adult is associated with various digestive cancers including colorectal, pancreatic, gastric and medullary thyroid.
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