Clinical significance[edit]
Tear film break-up time is abnormal in several conditions like aqueous tear deficiency,[2] keratoconjunctivitis sicca, mucin deficiency, and meibomian gland disorders.[6] Evaporative dryness can also be seen in conditions like lagophthalmos, proptosis, Parkinson's disease, computer vision syndrome, contact lens use, drugs (antihistamines, beta blockers, antispasmodics, diuretics) and vitamin A deficiency, amongst others.[2]
Normal values[edit]
Generally, a TBUT value of 10–35 seconds is considered normal.[7] A value of less than 10 seconds is usually suspicious and may indicate tear film instability. Even if TBUT value is under the normal limit, if the ocular protection index is less than 1.0, dryness and discomfort can occur.[8]
Ocular protection index[edit]
Ocular protection index (OPI) is used to quantify the interaction between tear film break-up time and blink intervals of a person.[9] 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 (IBI).[9] 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.[8]
Dry eye may occur in certain environmental conditions or while performing certain visual tasks like computer use movie watching etc.[9][10] 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.[11]