"[Tear breakup time] typically assesses tear film instability," Ahmad told the Eye Care Network. "We use a little bit of fluorescein dye on the ocular surface, and we basically examine the patient, looking to see in between blinks how long it takes for that tear film to start to evaporate."
Ahmad noted that a social media demonstration of the test used an exaggerated amount of dye applied to the sclera to make the process visible to viewers. In clinical practice, he recommends a very small amount of fluorescein placed on the palpebral conjunctiva, allowed to sit for a minute or two before examination.
Why do results vary between exams?
Despite decades of published data supporting TBUT, results can vary meaningfully between examiners and even between visits for the same patient. Ahmad attributed this variability to several factors: the amount of fluorescein instilled, the subjectivity involved in identifying the earliest point of tear breakup, and environmental conditions such as humidity.
"Within the same patient, within the same exam, you can get some interim measurement variability as well," Ahmad said.
When is TBUT most useful in practice?
Ahmad said fluorescein staining, and by extension TBUT, should be part of every ophthalmic and optometric exam rather than reserved for high-volume or primary eye care settings alone. The test helps identify ocular surface disease and corneal dystrophies such as epithelial basement membrane dystrophy, and informs whether a patient may benefit from ocular surface optimization ahead of surgery.
What follows a positive TBUT screen?
A positive TBUT result should prompt additional testing rather than an immediate diagnosis, according to Ahmad. He recommends pairing TBUT with a symptom questionnaire such as SPEED or the Ocular Surface Disease Index, along with corneal staining to assess for an inflammatory component. Point-of-care tests, including MMP-9 and tear film osmolarity, along with an evaluation of meibomian gland health, round out a comprehensive dry eye workup.
