Measuring accurate IOPs: Does correction factor help or hurt?

Goldmann applanation tonometry has enjoyed the status of being the gold standard in tonometry for many years and is the most widely used tonometer in clinical practice. The Goldmann applanation tonometer (GAT) measures the intraocular pressure (IOP) by applanating a surface area of 3.06 mm of the central cornea. However, central corneal thickness (CCT) in a population varies widely and ranges from 440 to 640 μm. Due to this wide variation, the measured IOP is often erroneous, particularly in the eyes in which the CCT is significantly different when compared with the mean CCT of the population. Ehlers et al1 by using intracameral IOP (manometry) experiments showed that there was a systematic variation in the IOP measured using the GAT with the variation of CCT. Ehlers et al1 observed that IOP is measured erroneously higher in eyes with CCT more than normal, whereas the IOP is measured erroneously lower in eyes with CCT less than normal. Ehlers et al1 proposed a nomogram that is used to date in clinics to correct the IOP measured using GAT (Goldmann IOP) for the errors induced due to variation in CCT. The PASCAL® dynamic contour tonometer (DCT; Ziemer Ophthalmology, Port, Switzerland) is a contact tonometer that is reported to be repeatable2–5 and measures IOP independent of the effects of the CCT.6–13 It is shown with manometry experiments that unlike in the GAT, IOP measured using the PASCAL DCT (PASCAL IOP) does not vary systematically with CCT and closely replicates the IOP measurements obtained with manometry.14,15 The aim of the study was to evaluate the validity of the Ehlers nomogram in correcting the Goldmann IOP. If the Ehlers correction nomogram works well in aiding to eliminate the errors due to the variation of CCT, the following will be expected: 1) the Ehlers-corrected GAT IOP (Ehlers IOP) would have good agreement with the PASCAL IOP. 2) Further, the agreement between Ehlers IOP and PASCAL IOP will be better than the agreement between Goldmann IOP and PASCAL IOP. Ehlers algorithm was primarily derived from a population that was composed of Caucasians.1 CCT and corneal biomechanics are reported to vary among different races, and African Americans on an average have thinner central cornea when compared with Caucasians.16–18 The study participants included both Caucasian and African American individuals. Thus, an additional aim was to stratify the data on the basis of self-reported race and examine the validity of the Ehlers nomogram as a function of race.