text: FDA hydrolysis is often used to measure activity in soil and compost samples; however, it may not give an accurate reading if microbes with lower activity phases, such as esterases, cleave the fluorescein first.
  It is also used in combination with propidium iodide (PI) to determine viability in eukaryotic cells. Living cells will actively convert the non-fluorescent FDA into the green fluorescent compound fluorescein, a sign of viability; while nucleus of membrane-compromised cells will fluoresce red, a sign of cell death. Currently FDA/PI staining is the standard assessment of human pancreatic islet viability with suitability for transplantation when viability score is above 70%.