From: Rabbi Moshe Sofer
To: Unknown recipient
A woman found small red and black particles on her examination cloths; the particles were tested with saliva and fingernail and did not dissolve. The Chasam Sofer rules unequivocally that she is tehorah, as no authority disputes that particles which do not dissolve in the saliva-and-fingernail test are pure, limiting the Ra'avad's stringency to recognizable pieces of actual blood rather than tiny particles removed with a needle.
Source: Shu"t Chasam Sofer YD §159