Goldmann Applanation Tonometry

Goldmann applanation tonometry (GAT) is the reference standard for measuring intraocular pressure (IOP) in clinical practice, underpinning glaucoma screening, diagnosis, treatment monitoring, and surgical decision-making.¹ The technique applanates a small, fixed area of the cornea with a calibrated force while the examiner visualizes the fluorescein-stained tear film meniscus under cobalt blue illumination; the two semicircular mires are aligned into an "S" pattern, and the drum reading (multiplied by 10) gives IOP in mmHg.

Fluorescein is not optional. Omitting it produces significantly lower readings — averaging 7.01 mmHg lower — with the error growing larger at higher IOP, precisely where accurate measurement matters most clinically.³ Excessive fluorescein creates thick, fuzzy mires that are hard to align accurately, so the goal is a thin, well-defined tear film image rather than maximal dye.

Corneal properties are the dominant source of measurement error. Central corneal thickness (CCT) significantly influences readings: thicker corneas cause overestimation of true IOP, thinner corneas cause underestimation, because GAT was calibrated assuming an average corneal thickness.²⁴ Corneal curvature matters too — steeper corneas tend to yield higher readings.² This is why CCT should be measured in glaucoma suspects: a high GAT reading in a thick-corneaed eye may overstate true risk, while a "normal" reading in a thin-corneaed eye may understate it. Previous corneal refractive surgery (LASIK, PRK) causes a separate, systematic underestimation of IOP due to altered corneal biomechanics and thickness; multiple correction formulas have been proposed for post-refractive-surgery eyes, but none is universally accepted, so unadjusted GAT values in these patients should be interpreted cautiously — using them uncorrected risks delaying a glaucoma diagnosis.⁴

GAT has been compared against newer tonometry technologies including rebound and non-contact tonometry, with agreement varying depending on patient population and corneal characteristics — in one study of Chinese adults with non-pathologic high myopia, non-contact tonometry overestimated IOP relative to GAT by roughly 3.75 mmHg on average, a population-specific finding rather than a universal correction factor.¹² Despite these newer alternatives, GAT remains the clinical and research reference standard.¹

Practically: measure IOP before dilating pupils or performing gonioscopy (both can transiently affect pressure), avoid excessive applanation force (overestimates IOP) or insufficient contact (underestimates it), and remember that repeated tonometry attempts can artificially lower subsequent readings through aqueous suppression — document which eye was measured first for this reason. Irregular corneas (keratoconus, scarring, edema) make GAT unreliable and should prompt consideration of an alternative tonometry method.