title: Evaluation of tonometric correction factors.
abstract: OBJECTIVE
  To investigate the efficacy of currently available correction factors in correcting intraocular pressure (IOP) measurements for the errors induced by the normal variations in corneal structural characteristics.
  
  
  METHODS
  Central corneal thickness (CCT) and corneal radius of curvature were measured on 324 individuals (175 normal: group 1 and 149 had either open angle glaucoma or ocular hypertension: group 2). IOP was measured in all normal subjects with the Goldmann applanation tonometer and the highest recorded IOP was obtained from patient charts for subjects with either open angle glaucoma or ocular hypertension. Regression analysis was performed on IOP, CCT, and corneal radius of curvature. The corrected IOP was also calculated using the models proposed by Ehlers and Orssengo and Pye. Linear regression analysis was used to calculate the residual association between corneal parameters and corrected IOP.
  
  
  RESULTS
  There was a significant positive correlation between IOP measured using Goldmann applanation tonometer and the CCT in both groups. There was no significant correlation between corneal radius of curvature and IOP in either group. There was a significant negative correlation in both the groups between CCT and corrected IOP calculated using the models of Ehlers and Orssengo and Pye. This indicates that the Ehlers and Orssengo and Pye models may significantly overestimate the effect of CCT on IOP measurement.
  
  
  CONCLUSIONS
  The effect of CCT and IOP as observed in the present study and by other studies in literature is less than predicted by both the Ehlers formula and the Orssengo and Pye model. Correcting IOP for the effect of CCT using these models could be erroneous and lead to overcorrection of IOP, thus resulting in erroneously low corrected IOP eyes with thicker cornea and erroneously high corrected IOP in eyes with thinner cornea.
authors.affiliation: Vision Science Group, Department of Psychological and Brain Sciences, University of Louisville, Kentucky 40292, USA. contact@pinakin-gunvant.com
authors.forename: Pinakin
authors.initials: P
authors.lastname: Gunvant
authors.forename: Daniel James
authors.initials: DJ
authors.lastname: O'Leary
authors.forename: Mani
authors.initials: M
authors.lastname: Baskaran
authors.forename: David Charles
authors.initials: DC
authors.lastname: Broadway
authors.forename: Russell Julian
authors.initials: RJ
authors.lastname: Watkins
authors.forename: Lingam
authors.initials: L
authors.lastname: Vijaya
mesh_terms: D000328:Adult; D003315:Cornea / Q000033:anatomy & histology*; D005260:Female; D005902:Glaucoma, Open-Angle / Q000175:diagnosis*; D006801:Humans; D007429:Intraocular Pressure; D008297:Male; D008875:Middle Aged; D049451:Nomograms; D009798:Ocular Hypertension / Q000175:diagnosis; D014065:Tonometry, Ocular / Q000379:methods*