A tumor marker is a substance found in the blood, urine, or body tissues that can be elevated in cancer, among other tissue types. There are many different tumor markers, each indicative of a particular disease process, and they are used in oncology to help detect the presence of cancer. An elevated level of a tumor marker can indicate cancer; however, there can also be other causes of the elevation.
Tumor markers can be produced directly by the tumor or by non-tumor cells as a response to the presence of a tumor. Most tumor markers are tumor antigens, but not all tumor antigens can be used as tumor markers.
Uses of tumor markers can broadly be classified as follows:
- Screening for common cancers on a population basis. Example: elevated prostate specific antigen suggests prostate cancer.
- Monitoring of cancer survivors after treatment. Example: elevated AFP in a child previously treated for teratoma suggests relapse with endodermal sinus tumor.
- Diagnosis of specific tumor types, particularly in certain brain tumors and other instances where biopsy is not feasible.
As stated in the BMJ 2009, tumour markers should not generally be used for the purpose of diagnosis of cancers, as opposed to monitoring purposes in certain cancers, or in certain cases, screening purposes. The use of these tests without understanding their utility has resulted in inappropriate use of tumour marker blood tests, which has also resulted in further inappropriate over-investigation for cancers.
Tumor markers can be detected by immunohistochemistry.
If repeated measurements of tumor marker are needed, some clinical testing laboratories provide a special reporting mechanism, a serial monitor, that links test results and other data pertaining to the person being tested. This requires a unique identifier for the person. In the United States commonly a Social Security number & Civil Personal Record (CPR) in Bahrain are used for this. One important function of this mechanism is to ensure that each test is performed using the same assay kit. For example, for AFP many different commercial assay kits, based on different technologies, are available. AFP measurements obtained using different assay kits are not comparable unless special calculations are performed.
Interlaboratory proficiency testing for tumor marker tests, and for clinical tests more generally, is an emerging field.In the United States, New York state is prominent in advocating such research.
Sources of inaccuracy
The high dose hook effect is an artefact of tumor marker immunoassay kits, that causes the reported quantity of tumor marker to be incorrectly low when the quantity is high. An undetected hook effect may cause delayed recognition of a tumor. The hook effect can be detected by analyzing serial dilutions. The hook effect is absent if the reported quantities of tumor marker in a serial dilution are proportional to the dilution.
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