Tear break-up time also known as tear film break-up time is the time taken to appear first dry spot on cornea after a complete blinking. TFBUT measurement is an easy and fast method used to assess the stability of tear film. It is a standard diagnostic procedure in the dry eye clinics. The volume of tear in the eye depends on two factors, drainage through the lacrimal passages and evaporation. Factors like decreased tear production, increased evaporation rate, tearfilm instability, tear hyperosmolarity, inflammations, ocular surface damages etc. can cause dryness to the eyes. TBUT test was first described by Norn MS, and then revised by Lemp and Holly. Conventional and most common TBUT measurement method is using slit lamp and sodium fluorescein. Noninvasive instruments are used for automatic non-invasive tear breakup time measurements. In non invasive procedure, a grid or concentric ring pattern is projected onto cornea and patient is asked to blink. the rings will appear distorted when cornea becomes dry. Tearscope is a commercially available instrument used to measure NIBUT.
Fluorescein 2% is instilled into the lower fornix of patient's eye. Alternatively, impregnated flourescein strip moistened with preservative free saline solution can also be used.
Examine the eye under slit lamp with a low magnification and broad beam covering whole cornea. Switch to cobalt blue filter in slit lamp.
Ask the patient to blink once and keep the eyes open.
Due to fluorescein, tear film will appear green in color.
Black spot indicating the dry area appear few seconds after each blink.
TBUT is the time interval between last blink and appearance of first randomly distributed dry spot.
If the patient blinks before 10 seconds, test must be restarted.
Taking two or more measurements and calculating its average may give much accurate measurement of TBUT.
Non invasive procedure
In non invasive tear breakup time measurement procedure, a grid or concentric ring pattern is projected onto cornea and patient is asked to blink. the rings will appear distorted when cornea becomes dry. The time interval between last blink and distorted ring pattern gives the measurement of NIBUT.
Normal values
Generally, TBUT value 10-35 seconds are considered as normal. A value less than 10 seconds is usually suspicious about tear film instability. Even though TBUT is under normal limit, if the ocular protection index is less than 1.0, dryness and discomfort can occur.
Ocular protection index
Ocular protection index is used to quantify the interaction between tear film break-up time and blink intervals of a person. It is based on the idea that, even when the TBUT is normal, if the blink interval is too slow it may cause breakage of tear film due to increased evaporation. The ocular Protection Index can be calculated by dividing tear break-up time by inter blink interval. If the inter blink interval and TBUT are same dividing both give a value of 1.0. If TBUT is more than IBI, value will be more than 1.0, so dryness will not occur. A patient is at risk of developing dry eye and if the OPI is < 1.0. Dry eye may occur in certain environmental conditions or while performing certain visual tasks like computer use movie watching etc. Calculating ocular protection index may be useful in these conditions also. For example, If a person's TBUT is 15 seconds which is thought to be normal and his blink interval is 20 seconds, here TBUT is less than IBI, so ocular protection index will be less than 1.0 and is abnormal. Since computer use affect blink rate, OPI is a clinically relevant in detecting dryness related to computer vision syndrome.
Contraindications
Benzalkonium chloride, a preservative used in saline solution can increase tear break up speed, so it is important to choose preservative free saline solutions for moistening the flourescein strip.
Localised corneal surface abnormalities can produce dry spots in that location and can give a falsely low break-up time. To avoid this error, repeat measurements should be taken. Appearance of dry spots in the same location in repeated procedures can indicate a surface abnormality in that area.
Touching the cornea with flourescein strip can cause excessive tearing and will affect the TBUT.
