Thin corneas can lead to underestimation, whereas thick corneas can lead to overestimation of the IOP. Corneal factors such as corneal thickness and elasticity can be a source of error and affect the accuracy of measurements obtained. There are several limitations associated with GAT. Another study including various tonometers found GAT to have the best repeatability followed by non-contact tonometry, ocular blood flow tonography, and Tono-Pen, respectively. In another study comprising 420 eyes, Phelps and Phelps reported an inter-observer variability of 3 mmHg or more in 30% of the eyes. In the inter-observer variation study, the mean difference was 1.79 (SD 2.41) mmHg between the first measurements and 1.60 (SD 2.15) mmHg between the medians of the three measurements. In the intra-observer variation study, the mean difference was 1.64 (SD 2.07) mmHg between the first measurements and 1.50 (SD 1.96) mmHg between the medians of the three measurements. Median values for the three measurements were calculated for both series of measurements. After a wait of 10 min, the three measurements were obtained again by the same observer in the intra-observer variation study and a different observer in the inter-observer variation study. In a population-based study including two parts: an inter-observer variation study ( n = 40) and an intra-observer variation study ( n = 22), subjects had their IOP measured by GAT thrice in a consecutive manner. GAT has relatively low intra and inter-observer variability. ![]() GAT is considered the reference standard for IOP measurement in current clinical practice due to its historical use in most clinical research studies to date and the extensive publications on its validity, reliability, and reproducibility. Moreover, at this area, the flattening force(g) multiplied by 10 is equivalent to the IOP (mmHg). At this selected area, the forces of tear surface tension pulling the tonometer tip toward the eye and the corneal elasticity pushing the tip away are almost equal and opposite. GAT uses an applanating force to flatten a constant area of 7.35 mm² (diameter of 3.06 mm). The moment of applanation is ascertained when the arcs are aligned such that their inner margins just touch. The double prism splits the formed image of the tear film meniscus into a superior and inferior arc. ![]() Fluorescein dye is instilled in the eye to highlight the tear film of the patient. The Goldmann applanation tonometer (GAT) consists of a biprism mounted on a slit lamp. There are several instruments that use an applanating force to flatten an area of the cornea. Applanation tonometry is based on the Imbert–Fick law, according to which, the pressure inside a sphere surrounded by an infinitely thin and flexible membrane can be measured by the force required to flatten a certain area of the membrane.
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