To compare the effects of different volumes of fluorescein on tear breakup time (FTBUT) and to investigate if and to what extent the tear breakup time determined by an automated noninvasive instrument (NITBUT) differs from FTBUT.
Twenty-four healthy volunteers were recruited to the study. Fluorescent tear breakup time was measured with different volumes of fluorescein solution delivered by either glass rod or objective directly by fluorescein strip. Noninvasive tear breakup time was measured with a noninvasive instrument (Oculus Keratograph 5M, Germany; K5). The average levels of the tear breakup times (TBUTs) and the variability of the successive recordings for each measurement were compared.
Increasing the volume of fluorescein delivered from 1 to 7 μl lengthened FTBUT by a mean ratio of 1.26-fold (p = 0.019) for the glass rod technique. No significant difference was detected in the FTBUT measured by the fluorescein strip technique when the delivered volume was increased from 4.5 to 7 μl. The variability of successive recordings was stable across the tested volumes for both techniques. Noninvasive tear breakup time determined by K5 was significantly longer than FTBUTs (mean difference: 3.90 seconds, p = 0.003 and 4.12 seconds, p = 0.002, respectively). Although no significant difference was detected in the average SD for successive recordings among K5 and the other two invasive techniques (p = 0.325), the variability of NITBUT was found significantly dependent on the observed TBUT (r = 0.532, p = 0.007).
The Oculus Keratograph 5M usually produced substantially longer TBUT compared with invasive techniques tested. The variability of readings by this novel method increased with the values of NITBUT. Given the small sample size in the current experiment, however, these findings need to be confirmed in a larger study.