Basal precorneal tear turnover in the human eye.

We studied 51 normal subjects with a simple method that permits measurement of the rate of fluorescein loss from the central precorneal tear film. In 15 (29%), no measurable dye remained 30 minutes after application of 1 microliter of a 10% solution of sodium fluorescein into the lower culde-sac. In 31 (61%) of the subjects, an exponential decay of the dye was observed between 15 and 30 minutes after application. In 5 (10%) of the subjects, a prolonged increase in dye concentration was observed, followed by a steady decrease that began 30 to 45 minutes after application. One eye of one subject had a steady increase in dye concentration for over an hour, and this eye was excluded from the analysis. The mean value for the tear elimination coefficient in all subjects analyzed was 15%/min. The logarithm of the tear elimination coefficient appeared to be normally distributed within the population sample. Analysis of the frequency distribution permitted an estimate that the 95% confidence limits for the tear elimination coefficient, as measured by this method, was 5 to 30%/min. No statistically significant correlations were found between tear elimination coefficient and sex, eye color, or contact lens use.