From: Rabbi Moshe Sofer
To: Rabbi Simcha Noson Ellenberg
A woman with a regular veses also finds blood when urinating, but the Maharil/Mahariv diagnostic test established it as non-uterine. The Chasam Sofer affirms the lenient ruling that once this test was performed and her normal veses is intact, the blood found during urination does not invalidate her hefsek taharah or seven clean days. He distinguishes this from the Avnei Miluim's stringent case, where the woman has no menstrual blood other than with urination and thus requires the test to be repeated at hefsek taharah.
Source: Shu"t Chasam Sofer YD §177