Illustration of a visual field test grid overlaid on an eye, representing glaucoma-related vision loss.

Navigating Glaucoma: Understanding Changes in Visual Field Testing

Lena Kashyap in Health & Wellness December 2025 • 4 min read.

"A Simple Guide to Visual Field Index (VFI) and What Changes in Your Results Might Mean"

If you're managing glaucoma, you're likely familiar with visual field tests. These tests are crucial for tracking changes in your vision over time. One key measurement from these tests is the Visual Field Index (VFI), which provides a percentage score reflecting the overall health of your visual field.

The VFI is designed to give doctors a clear picture of how glaucoma is affecting your vision. It helps in making informed decisions about treatment and managing the condition effectively. However, it's important to understand that the way the VFI is calculated can sometimes change, and this can lead to fluctuations in your scores.

This article breaks down why your VFI score might change and what factors can influence it. We'll explore the details of how VFI is measured, what happens when the calculation method shifts, and what this means for your glaucoma care.

Why Did My VFI Change? Understanding the Switch Between PDPP and TDPP

Illustration of a visual field test grid overlaid on an eye, representing glaucoma-related vision loss.

The VFI calculation isn't always the same. It depends on the severity of your glaucoma, as measured by something called Mean Deviation (MD). When your MD is better than -20 decibels (dB), the VFI is calculated using the pattern deviation probability plot (PDPP). But, when your MD crosses -20 dB and becomes worse, the calculation switches to the total deviation probability plot (TDPP).

This switch from PDPP to TDPP can cause a noticeable change in your VFI score. It's like changing the lens on a camera – the view is still the same, but the way it's captured is different. This change doesn't necessarily mean your glaucoma has suddenly worsened; it could simply be a result of the different calculation method.

PDPP (Pattern Deviation Probability Plot): This focuses on localized vision loss, pinpointing specific areas where glaucoma is causing damage.
TDPP (Total Deviation Probability Plot): This looks at overall vision loss, including both localized damage and any widespread issues like cataracts.

A study published in Investigative Ophthalmology & Visual Science explored this phenomenon. Researchers found that when the MD crosses -20 dB, the VFI can change anywhere from 3% to 33%. That's a significant range, highlighting just how much this calculation switch can affect your results. The study also found that the number of points within normal threshold sensitivities in PDPP when MD was better than -20 dB compared to TDPP when MD crossed -20 dB significantly influenced the VFI change. To put it simply, the difference in what's considered "normal" between the two plots impacts your final VFI score.

What Does This Mean for Managing Your Glaucoma?

If you notice a significant drop in your VFI score around the -20 dB MD mark, don't panic. Talk to your ophthalmologist about it. Understanding the shift from PDPP to TDPP can provide context for the change. Your doctor can assess whether the change reflects true disease progression or is simply an artifact of the testing method. This ensures you receive the most appropriate and effective glaucoma care.

About this Article -

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Everything You Need To Know

What is the Visual Field Index (VFI), and why is it important in glaucoma management?

The Visual Field Index (VFI) is a percentage score that reflects the overall health of your visual field, as assessed through visual field tests. It's a crucial measurement for individuals managing glaucoma because it provides doctors with a clear picture of how the condition is impacting your vision over time. This allows for informed decisions about treatment strategies and helps in effectively managing the progression of glaucoma.

Why might my VFI score fluctuate during glaucoma management?

Fluctuations in your VFI score can occur due to changes in the calculation method used. Specifically, the method switches between the Pattern Deviation Probability Plot (PDPP) and the Total Deviation Probability Plot (TDPP) based on the severity of your glaucoma, as measured by Mean Deviation (MD). When your MD is better than -20 decibels (dB), the VFI is calculated using PDPP, which focuses on localized vision loss. As your glaucoma progresses and your MD crosses -20 dB, the calculation switches to TDPP, which looks at overall vision loss. This switch can lead to noticeable changes in your VFI score, even if your glaucoma hasn't significantly worsened.

What is the difference between PDPP and TDPP, and how does this impact my VFI?

The Pattern Deviation Probability Plot (PDPP) and Total Deviation Probability Plot (TDPP) are different calculation methods used in visual field testing. PDPP focuses on pinpointing specific areas of localized vision loss, while TDPP looks at overall vision loss, including localized damage and widespread issues like cataracts. When the Mean Deviation (MD) crosses -20 dB, the VFI calculation switches from PDPP to TDPP. This switch can lead to changes in your VFI score, as the two methods assess vision loss differently. Studies have shown that this switch can cause the VFI to change by anywhere from 3% to 33%, highlighting the significance of understanding this aspect of glaucoma management.

If my VFI score drops significantly, should I be concerned, and what steps should I take?

If you notice a significant drop in your VFI score, particularly around the -20 dB MD mark, it's essential not to panic. The change could be an artifact of the switch between PDPP and TDPP calculation methods. The first step is to discuss this change with your ophthalmologist. They can assess whether the drop reflects a genuine progression of the disease or is simply a result of the testing method. Understanding this distinction ensures you receive the most appropriate and effective glaucoma care.

How does the switch between PDPP and TDPP influence the interpretation of my glaucoma progression?

The switch from PDPP to TDPP can significantly influence how your glaucoma progression is interpreted. Since PDPP and TDPP measure visual field differently, with PDPP focusing on localized damage and TDPP encompassing overall vision loss, the change in calculation method can create an illusion of disease progression. When MD crosses -20 dB, the VFI calculation shifts to TDPP, which may show a lower score even if the glaucoma hasn't dramatically worsened. This shift emphasizes the need to consider the calculation method when interpreting VFI changes. Consulting your ophthalmologist is crucial to distinguish between genuine disease progression and changes attributed to the shift in the testing method, ensuring appropriate management of glaucoma.