Binding of fluorescein to vitreous in vitro

When the blood-retina barrier is estimated by fluorophotometry, it is usually assumed that fluorescein is not bound in the vitreous. If a certain amount of fluorescein is not free it will influence the precise estimation of the permeability. To investigate a possible binding of fluorescein, vitreous from 25 eyes was aspirated post mortem, and an indirect estimate of the free fraction of fluorescein was made. The mean ratio between free and total amount of fluorescein was found to be 0.78 +/- 0.15 SD. The implication of the present result on previously published data on the blood-retina barrier is that the values for permeability may have been estimated erroneously. The direction of the error on the estimated permeability can not be simply predicted.