TBUT & NIBUT

After a complete blink, the tear film should spread across the exposed surface of the eye. But it does not remain perfectly uniform forever. Eventually, a small area begins to lose its smooth, continuous tear covering. This is called tear-film breakup.

The time between the blink and the first detected breakup is the tear breakup time.

So if someone tells you “Your breakup time is short,” what they are really saying is: “Your tear film is becoming unstable relatively quickly after you blink.”

We spend most of our waking life with our eyes open between blinks. During that time, the tear film needs to protect the ocular surface and provide a smooth optical surface for vision. If it begins to break apart too soon, parts of the ocular surface may be left with poorer tear coverage before the next blink arrives. This can contribute to:

• •dryness or irritation
• •fluctuating vision
• •ocular-surface stress
• •the need to blink more frequently

But a short breakup time tells us something very specific: the tear film is unstable. It does not automatically tell us why. That distinction is extremely important.

You may see several abbreviations on your dry-eye report:

• •TBUT — tear breakup time
• •FBUT — fluorescein breakup time
• •NIBUT or NIKBUT — non-invasive tear breakup time

They are all trying to assess tear-film stability. But they do it differently.

For a traditional fluorescein breakup-time test, a small amount of fluorescein dye is placed into the tears. You blink to distribute it. Then you keep your eye open while the clinician observes the tear film under special light. The clinician measures the time between the last blink and the first visible area of tear-film breakup.

This is simple and widely available. But there is a problem. The fluorescein itself changes the tear film. Adding liquid or dye can alter tear volume and tear-film behaviour. So the test is no longer observing the completely untouched, natural tear film. This is one reason the amount of fluorescein used matters.

NIBUT means non-invasive tear breakup time. Instead of putting dye into the eye, an instrument observes changes in the natural tear film.

Many devices do this by projecting a pattern of light onto the tear surface. As long as the tear film remains smooth, the reflected pattern remains relatively regular. When the tear film begins to break up, the reflected pattern becomes distorted. The instrument detects when this first happens.

So NIBUT asks essentially the same question as TBUT — how long does the tear film remain stable after a blink? — but it does so without first putting fluorescein into the tears.

Current TFOS DEWS III recommendations prefer non-invasive breakup time when it is available. The reason is not that fluorescein TBUT suddenly became useless. It is because NIBUT allows us to observe tear-film stability without first disturbing the tear film with dye. That makes it closer to the natural situation on the eye.

Fluorescein TBUT is still useful, particularly when non-invasive equipment is not available. But the method used should always be considered when interpreting the number.

This is where patients often become confused because different sources — and sometimes different machines — give different numbers.

For the non-invasive breakup time, the current TFOS DEWS III diagnostic framework uses less than 10 seconds as evidence of abnormal tear-film stability. In simple terms: if the first breakup occurs before 10 seconds during a properly performed NIBUT measurement, the tear film is considered insufficiently stable for the current diagnostic framework.

But this does not mean that 9.9 seconds is “terrible” and 10.1 seconds is “perfect.” Clinical cut-offs are tools for interpretation. Biology does not suddenly change at one exact second.

Because fluorescein TBUT is a different measurement. TFOS DEWS III notes that fluorescein itself can reduce measured tear-film stability. When a non-invasive device is not available and fluorescein breakup time is used instead, the report recommends minimizing the amount of fluorescein and uses less than 5 seconds as the positive sign of instability.

So it is important not to compare a NIBUT result and a fluorescein TBUT result as though they were obtained in exactly the same way. They weren’t.

It tells us: your tear film is becoming unstable relatively quickly between blinks. That is useful information. But now comes the next question — why? And TBUT cannot answer that by itself.

No. Meibomian gland dysfunction can certainly contribute to tear-film instability. If the lipid component of the tear film is abnormal, stability may be affected. But it is not the only possibility. Tear-film instability can also be influenced by:

• •aqueous tear problems
• •mucin or glycocalyx dysfunction
• •ocular-surface abnormalities
• •blinking
• •eyelid closure
• •combinations of several mechanisms

So a short TBUT is not an MGD diagnosis. It means tear-film instability has been detected. The cause still needs to be investigated.

Not necessarily. These concepts are related, but they are not identical.

TBUT measures tear-film stability. It does not directly measure the amount of water evaporating from the ocular surface. A tear film can become unstable for several reasons.

So a short breakup time should not automatically be translated into “My tears evaporate too fast.” That may be part of the picture in some people, but the test itself has not directly measured evaporation.

Yes. This is another important distinction. Tear quantity and tear stability are different things.

You may have a reasonable amount of tear fluid on the eye, but that tear film may not remain stable for long between blinks. Conversely, assessing tear volume does not tell us everything about tear-film stability.

This is why tests such as tear meniscus height, Schirmer and breakup time answer different questions.

Tear-film stability is dynamic. It can be influenced by the testing conditions and by what happened before the measurement.

Blinking matters. Environment matters. The test method matters. The amount of fluorescein matters when dye is used. Recent eye drops or other procedures may matter. Different instruments can also measure non-invasive breakup in somewhat different ways.

So if one clinic measures 4 seconds and another measures 7 seconds, it does not necessarily mean that your disease suddenly changed dramatically. Before comparing the numbers, it helps to ask: was the same test performed in the same way?

Some modern devices provide more than one number. You may see something like first NIBUT and average NIBUT.

The first breakup time records when the first area of instability appears. An average measurement may describe tear-film behaviour across a larger area or across multiple breakup events, depending on the device.

These values are not interchangeable. The current TFOS DEWS III diagnostic criterion is based on the first non-invasive breakup time. So when looking at a report, it is useful to know exactly which number the machine is showing.

Not by itself. A very short breakup time clearly tells us that tear-film instability is present. But dry-eye severity is much broader than one measurement.

Symptoms matter. Ocular-surface health matters. Other tear-film abnormalities matter. The eyelids and blinking matter. And neural function can influence how the disease is experienced.

So TBUT should not be turned into a simple scale such as “3 seconds = severe, 7 seconds = moderate, 12 seconds = healthy.” The biology is more complicated than that.

They can tell us how quickly the tear film becomes unstable after a blink. They can help identify tear-film instability, which is an important part of dry-eye diagnosis. And they can help clinicians follow one aspect of tear-film behaviour over time.

By themselves, they cannot tell you:

• •why the tear film is unstable
• •whether you have MGD
• •how much aqueous tear fluid you produce
• •how much of your tears are evaporating
• •what your meibomian glands look like
• •how severe your entire dry-eye condition is

Those questions require other information.