Loss of fluorescein across the conjunctiva

The rate of disappearance from the tear film of fluorescein and rhodamine dextran instilled together into the human eye was compared. In many cases fluorescein disappeared more rapidly, which was attributed to its penetration across the conjunctival surface. A corresponding mean fluorescein permeability across this surface of 2.5 x 10(-5) cm min-1 was calculated. This route of loss of fluorescein from the tears leads to an average overestimate of tear turnover of 25%.