RATIONALE: Drugs used in chemotherapy work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Measuring blood levels of tumor cells may help in learning how well chemotherapy works to kill metastatic breast cancer cells and allow doctors to plan better treatment. When blood levels of tumor cells are high while receiving chemotherapy, it is not yet known whether it is more effective to change chemotherapy regimens at that time or wait until disease progression.