The normal range for tumor marker 15-3, also called cancer antigen 15-3, is less than or equal to 30 units per milliliter, states the University of Rochester Medical Center. This blood test is used to monitor treatment for breast cancer, look for its recurrence or determine if it has spread.
CA 15-3 is an antigen that stimulates the immune system, URMC explains. Higher-than-normal levels of CA 15-3 do not always indicate cancer. Not all laboratories use the same procedure to test the blood, so results sometimes vary. Non-cancerous conditions of the breast, ovary and liver also raise levels of the antigen. If results are elevated, other information is collected to determine the cause.