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You probably got a blood test to check your levels of PSA (a protein called prostate-specific antigen) before your doctor told you that you have prostate cancer. You'll still get those tests now that your cancer has spread beyond your prostate.

The results are important, because if they show that your PSA level rises quickly, you may need different treatment.

Your prostate makes PSA. So do most prostate cancer cells. During prostate cancer treatment, changes in your PSA levels help show whether your treatment is working.

When you take medicines (whether it's chemo, hormone therapy, a treatment vaccine, or a combination of those) for advanced prostate cancer, your PSA levels should drop and stay low. If you have surgery to remove your prostate, you shouldn't have any detectable PSA levels.

Your doctor will closely track how quickly your PSA levels change over many months.  He may call this your "PSA velocity." It can be a sign of how extensive and aggressive your cancer is.

PSA levels can be confusing. They can go up and down for no obvious reason. They can rise after treatment. And levels tend to be higher in older men and those with large prostates.

Plus, the PSA blood test is not precise. That’s why doctors track your results over time, instead of focusing on one test result. 

Your doctor will also consider other things, including what your PSA levels were before you had cancer, your overall health, and whether you've had radiation therapy, which can raise your PSA levels for 1-2 years.

Each case is different, so ask your doctor about what your numbers mean. You'll want that perspective so you have the big picture of how you're doing.