Did anyone have access to the original publication and can tell me, if they explain how they determined it being the first study and what other liquids have been used before in studies? The Guardian article only says “Manufacturers have traditionally used saline or water”, but that does not tell you much, as these are not scientists with independent studies and manufacturers usually do not publish their full internal testing methods.
I only have access to its abstract and curiously it does not mention it being the first published study with actual blood, so the authors themselves did not find it very noteworthy.
I can easily imagine, that a published, standardized, reproducible (model) menstrual fluid for such an analysis does not exist yet, but I am not that involved in medical publishing. If this is the case, that would be really infuriating. It might exist as some vendors sell artificial menstrual fluid.