Goldmann applanation tonometry (GAT) is the gold standard method for measuring intraocular pressure (IOP). It is the reference technique against which all other tonometers are calibrated, and it appears consistently on the COA exam within the Assessments domain. Understanding its physical principle, setup procedure, mire alignment endpoint, and sources of error is essential for both the exam and clinical practice.
Unlike non-contact tonometry, GAT requires topical anesthesia and fluorescein instillation, direct contact with the cornea, and careful technique to obtain accurate readings. For COA candidates, mastering GAT means understanding not just what to do, but why each step matters and how errors affect results.
The Applanation Principle (Imbert-Fick Law)
GAT operates on the Imbert-Fick principle: the pressure inside a thin-walled sphere equals the force required to flatten (applanate) its surface divided by the area flattened. Goldmann refined this for the eye by discovering that when exactly a 3.14 mm diameter circle of cornea is flattened, two opposing forces — tear film surface tension (which pulls the prism toward the eye) and corneal rigidity (which pushes back) — precisely cancel each other out. This elegant cancellation makes the measurement largely independent of these confounders for a cornea of average thickness and curvature.
The force (in grams) needed to achieve this exact flattening area is read from the calibration drum and multiplied by 10 to convert to mmHg. So a drum reading of 1.6 corresponds to an IOP of 16 mmHg.
Applanation Area
3.14 mm
Diameter flattened at endpoint
Normal IOP Range
10–21 mmHg
Statistical norm (95th percentile)
CCT Assumption
520–540 µm
Calibrated for average CCT
Why 3.14 mm?
Goldmann determined through experimentation that at exactly 3.14 mm of applanation diameter, the error introduced by surface tension (which causes overestimation) is exactly offset by the error introduced by corneal rigidity (which causes underestimation). This sweet spot makes the measurement self-correcting for a cornea of average biomechanical properties.
Setup and Patient Preparation
Proper setup before the measurement is as important as the measurement itself. Errors in preparation — wrong positioning, too much fluorescein, skipping calibration — degrade accuracy before the prism ever touches the cornea.
Verify calibration
Before each clinical session, check the tonometer calibration with the calibration bar. At the 0 setting, the arm should be in neutral; with the calibration weight applied, the arm should deflect to the appropriate mark. A tonometer out of calibration by more than 0.5 mmHg must not be used clinically until serviced.
Clean and disinfect the prism
Wipe the prism with an alcohol-based disinfectant and allow it to air dry completely (at least 5 minutes) before patient contact. Never use a wet prism — residual disinfectant on the eye causes corneal epithelial toxicity. Disposable prisms eliminate this concern entirely and are preferred when disease transmission is a risk.
Instill topical anesthetic and fluorescein
Apply one drop of topical anesthetic (proparacaine 0.5% or tetracaine 0.5%) to each eye and wait 30 seconds for full effect. Then apply fluorescein: either a fluorescein strip moistened with one drop of anesthetic, or a combination fluorescein-anesthetic drop (e.g., Fluress). The goal is a thin, even fluorescein layer — enough to create visible mires, but not so much that mires appear excessively thick.
Position the patient at the slit lamp
Seat the patient comfortably at the slit lamp with the chin in the chin rest and forehead against the forehead strap. Ask the patient to fixate on a distant target with the eye not being tested to minimize accommodation-driven eye movement. Set the cobalt blue filter on the slit lamp illumination arm. Ask the patient to breathe normally — Valsalva (breath-holding) significantly raises IOP.
Set the drum and advance the prism
Set the drum to approximately 1.0 (10 mmHg) as a starting point. Turn on the cobalt blue light. Gently advance the joystick to bring the prism into contact with the central cornea while watching from the side. Once contact is made, view through the slit lamp oculars in low magnification to see the two fluorescent semicircles.
Reading the Mires: The Applanation Endpoint
The split prism of the GAT head creates two fluorescent semicircles (mires) when the prism contacts fluorescein-stained tear film. The measurement endpoint is reached when the inner borders of these two semicircles just touch — not overlapping, not separated. This visual endpoint is called inner-border touching or inner-edge alignment.
Mires Separated
Inner borders have a gap between them. Force is too low.
Action: Increase dial (more force)
Inner Borders Touching
The two semicircles just make contact at their inner edges. This is the endpoint.
Action: Read the drum value
Mires Overlapping
Inner borders cross each other. Force is excessive.
Action: Decrease dial (less force)
Astigmatism Correction for the Prism
In eyes with corneal astigmatism greater than 3 diopters, the mires become elliptical rather than circular, introducing measurement error. The correction: rotate the prism so that the red axis mark aligns with the minus cylinder axis of the patient's refraction. This orientation averages the IOP across the two principal meridians and reduces the astigmatic error significantly.
Corneal Thickness Effect and Sources of Error
Because GAT is calibrated for an average CCT of approximately 520–540 µm, eyes with non-average CCT introduce systematic error. This is one of the most tested concepts related to GAT on the COA exam and is clinically critical for glaucoma risk stratification.
| CCT Value | GAT Effect | Approximate Error | Clinical Implication |
|---|---|---|---|
| Thin (<500 µm) | Underestimates true IOP | ~1 mmHg per 20 µm below average | May miss elevated IOP; glaucoma risk underestimated |
| Average (520–540 µm) | Accurate reading | No correction needed | GAT reading is reliable |
| Thick (>560 µm) | Overestimates true IOP | ~1 mmHg per 20 µm above average | May overdiagnose ocular hypertension |
| Post-LASIK cornea | Significantly underestimates | 2–5 mmHg depending on ablation | Requires correction formula; GAT alone unreliable |
Additional Sources of GAT Error
Valsalva Maneuver
Breath-holding raises episcleral venous pressure, which elevates measured IOP. Instruct patients to breathe normally throughout the measurement. A difference between readings may indicate the patient is holding their breath.
Lid Squeezing (Blepharospasm)
When patients squeeze their eyelids, orbicularis muscle contraction raises IOP. Use a lid speculum if necessary or have the patient open their eye as wide as possible. Never force the lids open with your fingers as this directly transmits pressure to the globe.
Excessive or Insufficient Fluorescein
Too much fluorescein causes thick mires (overestimation of applanation area, underestimation of IOP). Too little fluorescein makes mires hard to see, leading to inaccurate endpoint identification. Aim for mire width of about one-tenth the diameter of the mire ring.
Corneal Edema
Edematous corneas are softer than normal and require less force to applanate, leading to IOP underestimation. This is particularly relevant in patients with Fuchs dystrophy or after intraocular surgery. The noncontact tonometer or Tono-Pen may also be inaccurate in edematous corneas.
Tight Neckwear / Collar
A tight collar or tie compresses the jugular veins, impeding episcleral venous drainage and transiently raising IOP by 2–4 mmHg. Ask patients to loosen tight collars before measurement.
Documenting IOP Readings
Accurate documentation of IOP measurements is both a clinical and medicolegal requirement. The COA exam tests documentation format, and errors in recording can create significant clinical problems in ongoing glaucoma management.
Include OD and OS values
Always document both eyes, even if only one has pathology. Example: IOP OD 16 mmHg, OS 18 mmHg by GAT.
Record time of measurement
IOP fluctuates throughout the day (diurnal variation), typically peaking in the early morning. Time of measurement is essential for comparing serial readings accurately.
Specify the tonometer type
Document whether the reading was obtained by GAT, NCT, iCare, or another method. Different instruments have different accuracy profiles and are not directly interchangeable.
Note medications affecting IOP
Topical beta-blockers, prostaglandin analogs, and other glaucoma medications lower IOP. Steroids raise it. Document current ophthalmic medications when recording IOP.
Practice GAT and IOP Questions on the COA Exam
Opterio includes Goldmann tonometry questions within COA Assessments domain practice, with AI-powered explanations that reinforce the clinical reasoning behind each answer.
Contraindications to GAT
Absolute Contraindications
- Active corneal ulcer or infectious keratitis
- Ruptured globe or penetrating ocular injury
- Known allergy to topical anesthetics or fluorescein
Situations Requiring Special Care
- Soft contact lenses in place (remove before testing; fluorescein stains lenses)
- Active blepharitis or conjunctivitis (infection risk)
- Corneal epithelial defect (causes discomfort and inaccuracy)
- Irregular cornea from scarring or keratoconus