The interaction of the time between blinks, or the interblink interval (IBI), and tear film breakup time (TFBUT) helps to regulate the integrity of the ocular surface. A protected surface exists when the TFBUT matches or exceeds the IBI. In contrast, an unprotected surface exists when TFBUT is less than the IBI. This is clinically relevant because repeated intermittent exposure of a tear film-deficient cornea can lead to ocular discomfort and the development of clinical signs, such as keratitis and redness. The relationship between TFBUT and IBI has been quantified by the Ocular Protection Index (OPI), which is calculated by dividing TFBUT by IBI. If the OPI is <1.0, the patient has an exposed ocular surface, putting them at risk for the development of the signs and symptoms of dry eye, whereas if the OPI is >or=1.0, the patient's ocular surface is tear film protected. The OPI has proven to be useful in assessing the factors that may cause or exacerbate dry eye. This review discusses the development and use of the OPI model, its relationship to dry eye, and factors that are known to alter blink rate and tear film integrity.

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