<unk> s are substances found in the blood, urine or body tissues that can be elevated in cancer. There are many different<unk> s. They are used in oncology to help determine the presence of cancer. An elevated level of a<unk>  can indicate cancer, however there can often also be other causes of the elevation.<unk> s can be produced directly by the tumor or by non-tumor cells as a response to the presence of a tumor.
Koepke outlines a hierarchy of clinical laboratory tests, from least to most informative. As used in oncology, they are 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<unk> s can be classified in two groups: Cancer-specific markers and tissue-specific markers.
Tumor marker
Biomarker
Tumor antigen
Cardiac marker
Tumor metabolome
Imaging agent
Early prostate cancer antigen-2
Cluster of differentiation