Uses of tumor marker are -a) Screening of a cancerb) Follow up of a cancer patient, especially for knowing about recurrencec) Confirmation of a diagnosed cancerd) For monitoring the treatment of a cancer
Tumor markers 

Some tumor produce or elicite the production of markers that can be measured in the serum or urine or other body fluids.
Tumor markers are biochemical indicators of the presence of a tumor.
Markers may be cell surface antigen, enzymes, hormones, tumor associated protein, or oncofetal antigens.

Uses of tumor marker

Screening of cancer.
To see the response of therapy → decreasing levels may suggest that tumor is responding to treatment.
For follow up → Reappearance of marker may suggest recurrence.
To determine the prognosis preoperatively → higher levels suggest high tumor burden.
To determine the growth of tumor → increasing levels suggest increase in growth.
To support the diagnosis → But, tumor markers are not specific enough to permit a diagnosis.
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