title: The orbscan acoustic (correction) factor for central corneal thickness measures of normal human corneas.
abstract: BACKGROUND
  Orbscan scanning-slit optical pachymetry was introduced over a decade ago and yielded higher central corneal thickness (CCT) values to the "gold standard" of contact ultrasound pachymetry (U/S). An acoustic correction factor (AF) was introduced later to compensate for this difference. The goal of this review was to assess the magnitude and consistency of the difference, as well as to assess how useful the AF had been.
  
  
  METHODS
  Using PubMed (Medline)-sourced citations, published articles were identified that included data on CCT from U/S and Orbscan, with the latter data checked to see whether an AF had been applied. Main comparisons were made between (1) Orbscan data without AF and U/S, and (2) Orbscan data with a 0.92 AF applied and the U/S data.
  
  
  RESULTS
  From 46 studies involving a total of 6136 eyes (average number per study of 133, range 6-1214), the average CCT values by U/S ranged from 0.520 to 0.580, for a group mean of 0.545 mm. For Orbscan without AF, the average CCT values ranged from 0.557 to 0.624 mm, for a mean of 0.582 mm, a net difference of 0.037 mm from U/S, with all Orbscan data (average values from any particular study) being higher than U/S. With a 0.92 AF applied, the net difference was -0.009 mm. The calculated limits of agreement between the two methods ranged from 0.004 to 0.073 without AF, but from -0.041 to + 0.023 mm with the AF. The overall outcome was essentially the same if weighted for cohort size and sample variability, or if only studies reporting on one eye were considered.
  
  
  CONCLUSIONS
  Orbscan pachymetry can be expected to yield CCT data that is approximately 7% higher than U/S. The global application of a 0.92 AF does not robustly align the Orbscan CCT data to that of U/S and, in fact, can easily result in the data being as much as 7% lower. Overall, the level of agreement between Orbscan and U/S is limited, and Orbscan data should simply be reported as measured without any adjustment.
authors.affiliation: Department of Vision Sciences, Glasgow-Caledonian University, Scotland, United Kingdom. m.doughty@gcal.ac.uk
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Doughty
authors.forename: Sven
authors.initials: S
authors.lastname: Jonuscheit
mesh_terms: D003315:Cornea / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D019781:Corneal Topography / Q000379:methods*; D006801:Humans; D012016:Reference Values; D014463:Ultrasonography