title: Agreement and reliability in measuring central corneal thickness with a rotating Scheimpflug-Placido system and ultrasound pachymetry.
abstract: OBJECTIVE
  We compare the agreement and the reliability in measuring central corneal thickness (CCT) using two different technologies.
  
  
  METHODS
  The right eyes of 35 healthy individuals who had a negative history of ophthalmic disease, or ocular surgery were examined. The CCT was determined sequentially with a rotating Scheimpflug camera (Sirius; CSO), and an ultrasound pachymeter (P-1; Takagi). For statistical analysis, we used the methods suggested by Bland and Altman.
  
  
  RESULTS
  The mean values of CCT obtained from Sirius, and ultrasound were 537±28μm, and 550±35μm, respectively. There was a high correlation between Sirius and ultrasound (r=0.92; p<0.001), but the difference between the two measurements was statistically significant (t=-5.7; p<0.00001). The precision of Sirius and ultrasound were 9.4 and 15.9μm; repeatability 13.3 and 22.4μm, and coefficient of variation 0.9% and 1.5%, respectively. The intraclass correlation coefficient was 0.97 for Sirius and 0.95 for ultrasound.
  
  
  CONCLUSIONS
  The average difference between corneal thickness measured with Sirius and ultrasound pachymetry was small but clinically significant. This means that the two instruments cannot be used interchangeably. Sirius showed precision and repeatability almost twice as much as ultrasound pachymetry. Confidence interval of 13.3μm for Sirius can show variations in corneal thickness with an uncertainty value lower than 2.5% in 95% of cases. The simplicity of use, the possibility to obtain pachymetric maps, and less invasiveness make this instrument potentially useful in contact lens practice.
authors.affiliation: Degree Course in Optics and Optometry, Faculty of Mathematics, Physics and Natural Sciences, Roma TRE University, Rome, Italy. Electronic address: nunziomaresca88@hotmail.com.
authors.forename: Nunzio
authors.initials: N
authors.lastname: Maresca
authors.affiliation: Degree Course in Optics and Optometry, Faculty of Mathematics, Physics and Natural Sciences, Roma TRE University, Rome, Italy.
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Zeri
authors.affiliation: Degree Course in Optics and Optometry, Faculty of Mathematics, Physics and Natural Sciences, Roma TRE University, Rome, Italy; Ospedale "Grassi" Roma, Italy.
authors.forename: Paolo
authors.initials: P
authors.lastname: Palumbo
authors.affiliation: Department of Physics (Optics and Optometry), University of Florence, Italy.
authors.forename: Antonio
authors.initials: A
authors.lastname: Calossi
mesh_terms: D000328:Adult; D003315:Cornea / Q000000981:diagnostic imaging* / Q000502:physiology*; D063171:Corneal Pachymetry / Q000295:instrumentation* / Q000379:methods*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D006801:Humans; D008297:Male; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014463:Ultrasonography / Q000295:instrumentation* / Q000379:methods*; D055815:Young Adult
keywords: Agreement; Central corneal thickness; Reliability; Scheimpflug corneal tomography; Ultrasound pachymetry