Responsa על נדה 118:15

Q. After urinating Leah usually finds blood on her "examining rag" (ed); the Rabbis of Cologne are of the opinion that she is permitted to cohabit with her husband, since the ruling of R. Jose is accepted that a woman who finds blood in her urine is ritually clean (Niddah 59 b).
A. Even R. Jose would admit that upon finding blood on her "examining rag", after urination, the woman would be ritually unclean. (For R. Jose's opinion is based on the belief that blood found in urine does not come from the interior of the womb, but from a wound or sore; cf. Pr. 630; Mord. Niddah 735. That blood on the "examining rag" after urination however usually comes from the interior of the womb, is universally accepted.) I have often wondered at the compilers of some codes who decide in accordance with the view of R. Jose, yet fail to make the above distinction.
SOURCES: Am II, 51; cf. Pr. 630; L. 403; Hag. Maim. to Issurei Biah, 5, 2; Maharil, Responsa 173.

We have carefully investigated and found that this woman has a wound in her womb; for every time she urinates she suffers severe pain. Occasionally, but not always, she finds blood in her urine; but no blood is found immediately after urination, neither upon wiping herself nor on the "examining rag". I, therefore, agree with you that this woman is permitted to cohabit with her husband, for, when a woman says she has a wound in her womb from which the blood exudes, she is to be believed.
This responsum is addressed to Rabbi Yekutiel; the Mordecai Hagadol, however, ascribes the question to Rabbi Asher.
SOURCES: Am II, 52; Mordecai Hagadol, p. 339a.

Q. While urinating Leah sees on the urine particles of blood of a pale color. May she cohabit with her husband?
A. If she examines herself after urinating and finds blood on the examining rag, she is not permitted to cohabit with her husband. But if examining herself after urination she finds no blood for three consecutive times, she no longer has to examine herself and is permitted to cohabit with her husband.
SOURCES: Pr. 630; L. 403; Mord. Sheb. 735. Cf. Israel Bruno, Responsa 248; ibid. 250.