Your Visual Field Test Results Explained: What the Numbers Actually Mean. You have just had a visual field test for glaucoma. The printout is in your hands. It is full of numbers, grey squares, symbols, and probability plots, and nobody explained any of it.
This guide will change that.
What Is a Visual Field Test?
A visual field test, also called perimetry, measures what you can see without moving your eyes. It maps your entire field of vision, from centre to periphery.
In glaucoma, this matters enormously. Glaucoma damages the optic nerve. That damage shows up as blind spots in your visual field, often long before you notice any problem with your eyesight.
The most commonly used machine in India is the Humphrey Visual Field analyser. Most reports you receive will be from this machine or a similar automated perimeter.
Why Is the Report So Confusing?
The visual field printout was designed for doctors, not patients. It packs multiple analyses onto a single page.
But each section is telling you something specific. Once you understand the key numbers, the report becomes readable.
Let us go through them one by one.
The Key Numbers on Your Visual Field Report
1. Mean Deviation (MD)
This is the single most important number on the page.
Mean Deviation tells you how much your overall visual field differs from a person of your age who does not have glaucoma. It is expressed in decibels (dB).
- 0 to −2 dB — normal range
- −2 to −6 dB — mild loss
- −6 to −12 dB — moderate loss
- worse than −12 dB — severe loss
A negative number is expected, everyone has some minor variation. A number worse than −2 dB, especially combined with a significant p-value, warrants attention.
The p-value next to MD tells you how likely it is that this result occurred by chance. A p-value of < 5% means fewer than 5 in 100 people without glaucoma would score this poorly. A p-value of < 0.5% is a strong signal of real damage.
What patients often ask: “My MD is −4 dB. Is that bad?”
Not necessarily on its own. Your doctor looks at MD in the context of your optic nerve appearance, OCT, and critically, whether the number is getting worse over time. A stable −4 dB is very different from a −4 dB that was −2 dB six months ago.
2. Pattern Standard Deviation (PSD)
While Mean Deviation tells you about overall loss, Pattern Standard Deviation tells you about the shape of that loss.
PSD measures how uneven your visual field is, how much variation exists from point to point across the field.
- A high PSD (worse than −2 dB with a low p-value) suggests focal, patchy loss. This is typical of glaucoma.
- A low PSD with a poor MD suggests diffuse, uniform depression, more consistent with a poorly done test, media opacity (like cataract), or systemic causes.
In plain terms: glaucoma tends to eat away at vision in specific patterns, not uniformly. PSD detects that patchiness. If your PSD is significantly elevated, your doctor takes it seriously even if your MD looks only mildly abnormal.
3. The Grey Scale Map
This is the visual picture of your field, the grid of grey squares on the left side of the printout.
Darker squares = areas where you saw less. Black squares = areas of significant loss.
Do not rely on this map alone. It is a visual aid, not a precise measurement. It can look dramatic even when the actual numbers are only mildly abnormal, and vice versa.
Your doctor uses it for a quick overview, not for decision-making.
4. The Total Deviation and Pattern Deviation Plots
These are the two sets of small number grids and probability symbol plots in the middle of the printout.
Total Deviation compares each point in your field to the normal value for your age at that location. A negative number means you saw less light than expected at that spot.
Pattern Deviation adjusts for your overall sensitivity level and shows only the focal, localised defects, filtering out generalised depression from cataract or a poorly dilated pupil.
The probability plots (the symbols: squares, triangles) tell you which points are statistically abnormal. The darker the symbol, the more abnormal that point.
In glaucoma, a cluster of abnormal points in the Pattern Deviation plot, especially in the upper or lower arcuate zone, is a meaningful finding.
5. The Glaucoma Hemifield Test (GHT)
The GHT is a line near the top of the printout. It compares the upper and lower halves of your visual field.
Glaucoma typically damages one half before the other, because nerve fibres from the upper and lower retina travel in separate bundles to the optic nerve.
The GHT result will say one of the following:
- Within Normal Limits no significant asymmetry detected
- Outside Normal Limits significant asymmetry between upper and lower halves; clinically important
- Borderline mild asymmetry; warrants monitoring
- General Reduction of Sensitivity uniform depression across the field; often a test quality issue
- Abnormally High Sensitivity rare; usually a test artefact
Outside Normal Limits on the GHT is one of the clearest signals on the entire printout.
6. The Reliability Indices: VFI, FL, FP, FN
These tell your doctor whether to trust the test result.
Visual Field Index (VFI) expresses your overall visual field as a percentage of normal. 100% is perfect. Used to track progression over time.
Fixation Losses (FL) how often you looked away from the central target. If FL is greater than 20%, the test may be unreliable.
False Positives (FP) how often you pressed the button when no light was shown. High false positives (> 15%) make the test look better than it is; artificially good results.
False Negatives (FN) how often you missed a light in an area you already saw. High false negatives can mean fatigue, inattention, or genuine severe loss.
If reliability indices are poor, the test result may not reflect your true visual field. Your doctor will often repeat the test rather than make decisions based on an unreliable one. Sometimes, doctors miss this in busy clinics. Which is why an independent structured glaucoma second opinion can bring clarity to decision making.
What Does Glaucoma Damage Look Like on a Visual Field?
Glaucoma has characteristic patterns of visual field loss. Knowing these helps you recognise what your doctor is looking for.
Nasal step a sharp difference between the upper and lower visual field on the nasal (nose) side. Often an early sign.
Arcuate scotoma a curved arc of loss following the path of the nerve fibres above or below fixation. Classic glaucoma pattern.
Paracentral scotoma small isolated patches of loss near the centre of vision. Can appear early in some types of glaucoma.
Advanced loss only a central island or a temporal island of vision remains. Late-stage disease.
Early glaucoma defects often appear in the upper visual field first, because the inferior optic nerve is typically more vulnerable.
The Most Important Thing Your Report Cannot Tell You
A single visual field test tells you very little on its own.
Trends matter more than single results.
Your doctor needs at least 3 to 5 reliable tests to assess whether your glaucoma is stable or progressing. The rate of change (how fast MD is worsening per year) is what determines whether treatment needs to be intensified.
A worsening rate of −0.5 dB per year is very different from −2 dB per year. Both may look similar on a single printout.
This is why regular perimetry testing for glaucoma, typically every 6 to 12 months, is not optional. It is how your doctor protects your vision over the long term.
Questions to Ask Your Doctor After a Visual Field Test
- What is my MD, and has it changed since my last test?
- Is my Pattern Deviation showing any focal defects?
- Was this test reliable enough to act on?
- What is my rate of progression?
- When should I repeat this test?
- Does my visual field match what my OCT is showing?
Frequently Asked Questions
Can I fail a visual field test?
No. There is no pass or fail. The test measures what is there. Reliability indices flag if the result is trustworthy. If you perform poorly due to fatigue or nerves, your doctor will simply repeat the test.
My visual field was normal last year but abnormal now. Is my glaucoma getting worse?
Possibly, but one abnormal result after a normal result needs confirmation. Fatigue, poor fixation, or a bad testing day can all affect results. Your doctor will likely repeat the test before changing your treatment.
I have cataract. Will it affect my visual field test?
Yes. Cataract causes generalised depression of the visual field. It makes the overall result look worse. Pattern Deviation partially corrects for this, but it is important to tell your doctor if your cataract has worsened since your last test.
My friend’s MD is −3 dB and mine is −5 dB. Does that mean my glaucoma is worse?
Not necessarily. Two patients with different optic nerve structures, ages, and rates of progression cannot be directly compared. Your own trend over time is what matters.
Is the visual field test available in Gurgaon?
Yes. Perimetry is part of a full glaucoma evaluation with Dr Shibal Bhartiya in Gurgaon.
Related Reading
- Why Do I Need a Visual Field Test?
- Understanding Your OCT Report in Glaucoma
- Glaucoma Diagnosis in Gurgaon
- Glaucoma Progression: What It Means and How to Slow It
- Get a Glaucoma Second Opinion in Gurgaon
Read the research articles
This article has been written by Dr Shibal Bhartiya, a glaucoma specialist in Gurgaon known for ethical, patient-centred glaucoma care and independent glaucoma second opinions. She is also a research collaborator with Mayo Clinic, Jacksonville, Florida, USA. This article has been updated in March, 2026.
She has published peer-reviewed research on glaucoma laser and surgeries, examining how treatment decisions should balance medical evidence, patient preferences, and long-term vision outcomes.
These peer-reviewed article discussing glaucoma treatment are benchmarks for glaucoma surgeons globally, and can be accessed on PubMed and Google Scholar
If you would like a structured glaucoma risk assessment or second opinion:
+91 88826 38735
drshibalbhartiya.com
