Samples collected for cancer blood tests are analyzed in a lab for signs of cancer. The samples may show cancer cells, proteins or other substances made by the cancer. Blood tests can also give your doctor an idea of how well your organs are functioning and if they've been affected by cancer.xamples of blood tests used to diagnose cancer include: 1  Complete blood count (CBC). 2  This common blood test measures the amount of various types of blood cells in a sample of your blood.