title: Influence of central corneal thickness on measured intraocular pressure differentials: Nidek RKT-7700, Topcon CT-80 NCTs and Goldmann Tonometer.
abstract: OBJECTIVE
  We sought to compare the intraocular pressure (IOP) measured by RKT-7700 and CT-80 noncontact tonometers (NCTs) with that measured by Goldmann applanation tonometry (GAT). We also examined the influence of central corneal thickness (CCT) on the agreement between both NCTs and GAT in a sub-population of healthy, young normals.
  
  
  METHODS
  Triplicate IOP and CCT measurements were obtained twice from one randomized eye of 49 subjects (28 males and 21 females) aged 22.2 ± 1.3 (mean ± S.D.) years. Goldmann tonometry was performed subsequent to assessment with the RKT-7700 and CT-80, to negate the 'ocular massage effect', followed by ultrasound pachymetry. The results from each method were compared and assessed for repeatability and between-observer reproducibility. Analysis was performed to determine the correlation between the differences in IOP measurements and corneal thicknesses.
  
  
  RESULTS
  The mean differences (±S.D.) in sessions 1 and 2 respectively between RKT-7700 and GAT (2.6 ± 2.0 and 2.7 ± 1.4 mmHg), between CT-80 and GAT (2.8 ± 2.0 and 3.2 ± 1.3 mmHg) were statistically significant (p < 0.0001). The repeatability coefficients in sessions 1 and 2 respectively were; ±1.2 and ±1.0 mmHg (GAT), ±1.3  and ±1.6 mmHg (CT-80), ±2.3  and ±1.7 mmHg (RKT-7700) and inter-observer reproducibility was; ±1.9 (RKT-7700), ±2.3 (GAT) and ±2.6 mmHg (CT-80). Between the corneal thickness and the differences in GAT-measured and NCT-measured IOPs, there was a significant negative correlation and a trend for a larger difference in thicker corneas.
  
  
  CONCLUSIONS
  Neither the RKT-7700 nor the CT-80 can be used interchangeably with the Goldmann tonometer, although all three tonometers give repeatable measurements of IOP, in this sub-population. IOP differences between GAT and the NCTs significantly correlated with CCT, with the possibility of even greater differences in thicker corneas.
authors.affiliation: Corneal Research Chair, Department of Optometry and Vision Sciences, King Saud University, Riyadh, Saudi Arabia.
authors.forename: Kelechi C
authors.initials: KC
authors.lastname: Ogbuehi
authors.forename: Sven
authors.initials: S
authors.lastname: Mucke
authors.forename: Uchechukwu L
authors.initials: UL
authors.lastname: Osuagwu
mesh_terms: D003315:Cornea / Q000033:anatomy & histology*; D063171:Corneal Pachymetry; D005260:Female; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D015588:Observer Variation; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D014065:Tonometry, Ocular / Q000295:instrumentation / Q000592:standards*; D055815:Young Adult