The effect of instilled fluorescein solution volume on the values and repeatability of TBUT measurements

Purpose: To investigate how the volume of instilled fluorescein solution alters invasive tear breakup time (TBUT) and the standard deviation (SD) of multiple recordings, and to determine the volume of fluid supplied by traditional fluorescein-impregnated strips (FS) and a micropipette calibrated to dispense 1 mu L. Methods: TBUT was measured 3 times in the right eyes of 46 subjects following the instillation of 1, 2.7, and 7.4 mu L of fluorescein solution by micropipette and the use of a wetted FS,. The volume of fluid delivered to an eye by FS and a micropipette set for 1 mu L was estimated by weighing each before and after use. Results: The volume of fluorescein solution instilled before measurement influenced TBUT (P < 0.001). Increasing the delivered volume from 1 to 2.7 mu L lengthened TBUT (P < 0.001), but an additional increase in volume to 7.4 mu L was not associated with a ftirther change in TBUT (P = 0.50). Differences between TBUT values measured after using FS and a micropipette calibrated for 1 mu L were not significant (P = 0.95). No significant differences were found in the repeatability of TBUT recordings between the instillation techniques used in this study (P = 0.18), although measurement variability increased with mean TBUT for all techniques. With the FS instillation technique described herein, the average volume of fluorescein solution they supplied was less than that delivered by a micropipette set for 1 mu L (P = 0.017), but the range of volumes transferred by FS was greater. Conclusions. Values of TBUT are dependent on the volume of fluorescein solution instilled before measurement; thus, attempts should be made to standardize this variable. Accurate estimation of TBUT requires more recordings to be averaged than, are normally taken, unless breakup time is very short.