Tear film break -up time:
The tear film break-up time(BUT)is an index of precorneal tear film stability. It is measured as follows :
a. Fluorescien is instilled into the lower forix.
b. The patient is asked to blink several times and then stop.
c. The tear film is examined with a broad beam and a cobalt blue filter . After an interval of time, black spots or line indicating the formation of dry areas will appear .
The BUT is the interval between the last blink and the appearance of the first randomly distributed dry spot. The development of dry spots always in the same location should be ignored because this is caused by by a local corneal surface abnormality and not by intrinsic instability of the tear film. A BUT of less than 10 seconds is abnormal.
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