A new drug, erdafitinib, became available for some patients with bladder cancer that has spread to other organs. To qualify, patients must have specific genetic changes in their tumors. Currently, doctors use tumor tissue samples to check for these genetic changes, but these samples might not accurately reflect the current state of the patient's cancer.

In this study, Investigators will test the patient's blood for these genetic changes in addition to the tumor tissue samples. It is thought that the blood test will give a more accurate result.

Investigators hope this study will help to find out if more patients can benefit from erdafitinib than the ones identified by tissue testing only.

Recently, the first targeted therapy for patients with metastatic urothelial cancer (mUC) received approval, i.e. erdafitinib. Erdafitinib is a pan-FGFR small molecule inhibitor. Approximately twenty percent of mUC patients' tumors harbor qualifying alterations in FGFR2 or FGFR3. Currently, standard testing uses archival tumor tissue. However, this type of testing may not be representative of a patient's clinically dominant tumor clone at the time of treatment initiation due to temporal and spatial biopsy bias of archival tissue testing.

In this study, patients will undergo circulating tumor DNA testing, in addition to conventional tissue testing, for treatment eligibility. Investigators hypothesize that blood-based ctDNA testing will provide a more accurate assessment of somatic FGFR status than same patient archival primary tissue.

This study's objectives are:

Objective 1 (Primary): To evaluate the diagnostic value of ctDNA testing against the "gold standard" of tissue testing.

Objective 2 (Secondary): To evaluate whether ctDNA may be a superior predictive marker by identifying actionable FGFR alterations that are currently missed on tissue assays.

Procedures:

• Medical history will be reviewed, including all previous anti-cancer treatments.
• A blood sample will be collected for ctDNA testing at the time of screening for FGFR alterations and at progression under erdafitinib therapy. Test results will be compared to archival tissue testing.

Inclusion Criteria:

• Patients with metastatic bladder cancer who are about to undergo tissue testing for FGFR mutations and who have blood samples drawn during the management of their disease are eligible to be included in this analysis.

Exclusion Criteria:

• Metastatic bladder cancer patients who will not have tissue sent for FGFR testing will be excluded from this study.

Patients with metastatic urothelial cancer who are screened for erdafitinib eligibility as standard of care.

No plans for future use of the data at this point. Only the study Principal Investigator will have access to the data in the future.