Some cancers produce specific substances that can be measured in the blood. Many readers will be familiar with the relationship of PSA to prostate cancer. After surgery for prostate cancer, the blood levels of PSA should decrease to zero.
By periodically rechecking the level of PSA in the blood, we can monitor for a recurrence of the prostate cancer. Tumors from some other organs have different markers that can be followed by blood tests. Bladder cancer, unfortunately, does not have a useful marker like PSA.
There is of course great interest in discovering and developing a blood test that could give us this information, but it has not yet been identified. We rely instead on other clinical information to check for the spread of cancer, especially X-rays and CT scans.