Why is a blood test necessary to find cancer markers?

Blood tests aren't always necessary to find cancer markers, as some can be found in other bodily tissues or in urine, Cancer.net explains. Cancer marker tests look for the presence of certain genes or proteins in bodily fluids and tissues that indicate the presence of cancer, states National Cancer Institute.

Using tumor marker tests to analyze DNA can detect or help determine appropriate treatment for non-small cell lung cancer, colorectal cancer and melanoma. Multigene tumor marker tests can also determine the course of treatment for some kinds of breast cancer. Tests looking for cancer markers are always used in conjunction with other tests, since they are not always reliable.