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REF: http://breastcancer.about.com/od/tumormarkers/f/ca_15-3.htm

Question: What is the cancer antigen 15-3 (CA 15-3) blood test for breast cancer?
Cancer Antigen 15-3 (CA 15-3) is a blood test that is given during or after treatment for breast cancer. It is most useful in monitoring advanced breast cancer and your response to treatment. CA 15-3 is not a blood test that screens for breast cancer. It is a tumor marker test that is helpful in tracking cancers that overproduce CA 15-3.
Answer:
About Cancer Antigen 15-3 (CA 15-3)
About Cancer Antigen 15-3 (CA 15-3)
CA 15-3 is a protein that is a normal product of your breast tissue, and it does not cause breast cancer. If a cancerous tumor (cells growing out of control) is present in your breast, though, your levels of CA 15-3 may increase as the number of cancer cells increase. Tumor cells will shed copies of the CA 15-3 protein, which can be measured by this blood test and by a related test of cancer antigen 27.29 (CA 27.29).
Should I Have a CA 15-3 Tumor Marker Test?
Not every breast tumor causes a rise in CA 15-3, so for patients with tumors that do not produce CA 15-3 or with early-stage breast cancer, this test is not useful. Only about 30% of patients with localized breast cancer (cancer limited to the breast) will have increased levels of CA 15-3. In patients with metastatic breast cancer, CA 15-3 can be found in 50 to 90% of all cases.

Reasons to Have a CA 15-3 Tumor Marker Test
If you have completed treatment for breast cancer, your doctor may test your blood for CA 15-3 on a regular schedule to see if your levels of this antigen are rising or remaining steady. Rising levels of CA 15-3 may indicate a recurrence of breast cancer, but since other conditions can cause higher levels of this antigen, the test results must be taken in to consideration with the results of imaging studies, your symptoms and other tests for hormone sensitivity, HER2/neu and BRCA genes.

Benefits of the CA 15-3 Blood Test
Your levels of CA 15-3 will help your doctor determine if:

Your treatment is working or needs adjustment
Your cancer has made an early recurrence, without noticeable symptoms
Your cancer has progressed
You have other conditions
Other Benign Conditions Test High for CA 15-3
Breast cancer is only one condition that may cause high levels of CA 15-3. Pregnancy and lactation also increase your levels of CA 15-3. Several noncancerous conditions (benign breast or ovarian disease, endometriosis, pelvic inflammatory disease and hepatitis) can bring up your levels of CA 15-3.
CA 15-3 and Other Cancers
Ovarian, lung and prostate cancers may also produce CA 15-3.

Sources:
American Cancer Society. Tumor Markers. What Are Tumor Markers? Revised: 01/04/2006.

American Association for Clinical Chemistry. Lab Tests Online. CA 15-3. Last reviewed on June 24, 2006.

There are multiple blood test markers which may change after treatment for cancer. If the marker test is normal, it may mean nothing. If the marker test changes, and goes up it would clue your oncologist to look further for possible reoccurance of your disease.