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Blood Tests and Cancer Diagnosis
Cancer arises from mutational changes in our DNA, leading to abnormal, uncontrolled proliferation of cells. A definitive diagnosis of cancer usually requires examination of the cells; either by histology (biopsy/tissue sample) or cytology (cell sample). Tumor cells can be distinguished from normal cells by their morphology (shape, size) when viewed under a microscope. If a tumor is suspected in a certain organ, cells can be removed from the organ and sent to the laboratory. Such tests will usually be carried out if cancer is suspected from a physical examination.
Patients often ask 'would cancer show up in a blood test'. The answer is that blood tests may not provide a definitive answer but often form part of a general examination to assess health. Common tests include measurement of liver function and hemoglobin levels, abnormalities in these can point to the possibility of cancer.
Leukemia arises in the bone marrow and tumor cells usually enter the bloodstream. Therefore malignant cells will be visible if the blood cells are examined in a blood film. Cells from non blood cancers such as breast, colon, lung etc are generally not easy to detect in the bloodstream. Therefore in these tumors (known as solid tumors), tissue from the diseased organ must be examined before a diagnosis of cancer can be given. Even in the case of leukemia, a bone marrow biopsy may still be required to confirm a diagnosis made from a blood sample.
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Detection of Tumour Markers in the Bloodstream
Due to the mutations they carry, many tumor cells contain abnormally high levels of certain proteins called tumor markers. In some cases, these proteins can be measured in the blood, theoretically providing a ‘test for cancer’. The best example of this at present is in the diagnosis of prostate cancer. Prostate specific antigen (PSA) is produced exclusively by the prostate and is normally present at low levels in the bloodstream. Levels are increased in men with prostate cancer and its detection is now widely used in diagnosing this condition. However, elevated PSA can also result from benign prostate disease such as prostatitis (inflammation of the prostate). Therefore PSA levels alone cannot be taken as a definitive answer, and will only form part of the diagnosis.
The fact that tumor markers can be elevated in benign conditions is their main limitation in cancer diagnosis. However, PSA levels in the blood often show a good correlation with the severity of the disease. In addition, measurement of PSA levels can provide a convenient way to monitor therapy, since a reduction in PSA often occurs when prostate tumors shrink.
Other tumor markers that can be tested for in the blood are: CA125 (ovarian cancer), C19-9 (pancreatic cancer), CEA (breast cancer), and AFP (liver cancer) however their use in diagnosis is controversial. It is worth remembering that even if a patient tests negative for a tumor marker, it does not mean they are definitely free of cancer. Would cancer show up in a blood test ? Not necessarily.
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Detection of other Substances
Substances such as hormones are sometimes raised in cancer patients and can show up in a blood test. For example a thyroid tumor can result in increased calcitonin levels and a pancreatic tumor to increased insulin production. A common finding in multiple myeloma, a B cell tumour that arises in the bone marrow, is increased immunoglobulin (antibody protein) and calcium in the bloodstream.
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A Blood Test for Cancer - Future Possibilities
There have been recent press reports of an 'amazing new blood test to detect cancer'. Unfortunately this is a classic case of research findings being over-hyped by the mainstream media. This technology, whilst exciting, is a long way from being a routine test for malignancy.
The new test utilizes a chip containing thousands of tiny 'bristles' linked to multiple diagnostic antibodies. Theoretically the chip can detect a single malignant cell in a blood sample and give information on its molecular characteristics. This has potential to detect cancer at an early stage and in a non-invasive manner. Information regarding the genetics of a tumor could help determine the best treatment for a patient. However, this research is at at an early stage and a great deal of work is required to prove the effectiveness of the test.
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British Medical Journal Archive http://archive.student.bmj.com/issues/08/01/education/035.php
The Mayo Clinic www.mayoclinic.com/health/cancer-diagnosis/CA00028
Cancer Research UK www.cancerresearchuk.org/