Abstract: A fluoresceable composition in dosage unit form and method for determining fluid or blood flow are provided for medical, surgical or diagnostic purposes, using 6, 7-dihydroxycoumarin 6-glucoside which in vivo is invisible in natural light and is highly visible as a blue-white color in ultraviolet light. The composition and method enable "reading", visualization, diagnosis, demarcation, assessment, etc., of adequacy of blood supply, vasculature and fluid flow; loss of fluid; viability of tissue, skin and organs or aspects thereof; and the like.