The KAT goes digital

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Accuracy

The Goldmann applanation tonometer (GAT) is another of those instruments to which the term ‘gold standard’ is routinely applied. It is the method used in ophthalmology throughout secondary care and is therefore the technique which anyone wishing to monitor intraocular pressure with a view to refer to secondary care should also use. However, to say it is more accurate than the non-contact technique is overstating the case. The technique relies upon a force being applied to an area of cornea such that it flattens it and the force applied may be related to the pressure from within the eye pushing back on the probe. The well-known relationship of force = pressure x area may apply if we assume the cornea has no rigidity (it does) and is dry (it is not). These two sources of error are compensated by choosing an area of flattening (7.354mm2) where the surface tension attraction from the wet cornea may balance the rigidity outward push from a cornea of known thickness. Goldmann seems most accurate for a cornea of 520 micron thickness.1