Fluorophotometric measurements of tear turnover rate in normal healthy persons: evidence for a circadian rhythm.

The determination of tear turnover rate in a total of 16 normal healthy volunteers is described. A continuous reading fluorophotometer is used to measure the fluorescence resulting from the instillation of 1 microliter of 2 per cent fluorescein into the eye. A mean (+/- s.d.) turnover rate of 14.9 +/- 5.6 per cent per minute is found, in agreement with previously reported values. Morning and afternoon measurements on the same subject show that morning tear turnover rates are significantly higher than those in the afternoon (p less than 0.01), suggesting a circadian rhythm in tear flow. It is also concluded that use of a 10 per cent fluorescein solution can lead to a significant underestimation of tear turnover rates and that any resulting errors are likely to be larger for light coloured irises.