Fluorescein has been used in a standard fashion to aid in the determination of the tear break-up time (BUT) said Dr William Boothe. The BUT is defined as the interval between the last complete blink and the development of the first randomly distributed dry spot in the tear film. To perform the BUT a fluorescein strip is moistened with one drop of distilled water and applied to the inlerotemporal bulbar conjunctiva. Patients are instructed to blink several times to evenly distribute the fluorescein. The patient is then asked to stare straight ahead without blinking. With a 3-mm wide vertical beam though a cobalt filter, the tear film is scanned until the first dry spot ap¬pears .

Lemp has proposed that an abnormal BUT is less than 10 sec . There has been wide variation in BUTs in normal patients, often ranging from 5 to 100 sec. In individual patients there has been marked variation .Hid lack of reproducibility. Thus BUT has been disappointing as a clinical test for diagnosis of diseases resulting in precorneal teat instability.

There are many variables that can influence the BUT. Some of these may be the result of fluorescein itself and the method of application. The concentration of fluorescein has been reported to affect BUT. Increasing concentrations have resulted in shorter BUTs . This may be due to alterations in the tear stability or differences in the ease of detecting dry spots, or both. Standardizing fluorescein concentration with a fluorescein strip is difficult at best. Irritation from touching the strip to the bulbar conjunctiva as well as the fluorescein itself can stimulate reflex tearing. This will affect BUT indirectly by affecting the fluorescein concentration and directly by affecting the quality and quantity of the precorneal tear film Reflex tearing is a highly individualized response and may be responsible for wide variations among individuals and in the same individual. These variables as well as others will need to be controlled before the BUT be-comes a practical test. This is available at Boothe Laser Center