Purity: Inability to Self-Examine for Istiḥāḍah
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 404:
If a mustaḥāḍah cannot examine herself, she must act according to what is undoubtedly her responsibility. For example, if she does not know whether her istiḥāḍah is slight or medium, she must do the things required of a woman with slight istiḥāḍah; and if she does not know whether she has medium or excessive istiḥāḍah, she must do the things required of a woman with medium istiḥāḍah.[1] However, if she knows which one of the three types it was previously, she must act according to the duties for that type.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)
In these two examples, the mustaḥāḍah knows for sure that her istiḥāḍah is at least the lesser type. It could possibly be more than this, but as that is only a possibility, she must act according to what she knows is certain, i.e. the lesser type.