Date: 1800
From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer rules that when a woman experiences hargashah and subsequently checks with an unchecked cloth finding blood, she is fully impure as a niddah regardless of the blood's size. He further rules that hargashah requires the specific sensation of the opening of the uterine os, not merely a general sensation of liquid flow, a position he reports was confirmed by the tradition of the Noda BiYehudah.
Source: Shu"t Chasam Sofer YD §167