title: Mathematical Discrepancies of the Tono-Pen Applanation Tonometer.
abstract: OBJECTIVE
  The purpose of the study was to determine if Tono-Pen tonometers use simple average and coefficient of variation (CV) algorithms to calculate intraocular pressure (IOP).
  
  
  METHODS
  IOPs were measured as part of routine ocular examination in 152 client-owned dogs. Using 11 Tono-Pen's, a total of 778 averaged readings were collected. Individual IOP readings, and average IOP and CV displayed by the instrument, were recorded. Average IOP and CV were then manually calculated from individual readings and compared with those displayed by the instrument.
  
  
  RESULTS
  The mean absolute difference between the calculated and displayed average IOP was 1.37±2.01 mm Hg (P<0.001). In 6% of cases, the calculated average IOP was 5 to 15 mm Hg different from the displayed average IOP. The difference between the displayed and calculated average IOP was significantly higher in hypertensive eyes with displayed IOP≥25 mm Hg. Calculated CV was equal to, lower than, or greater than displayed CV in 28.6%, 1.5%, and 69.7% of cases, respectively. In 17.6% of cases, calculated CV was >20%, but displayed CV was <5%. Receiver operating characteristic analysis could not correlate number of individual IOP readings with magnitude of difference in average IOP.
  
  
  CONCLUSIONS
  Calculated average IOP and CV differ significantly from values displayed by the instrument, especially at higher IOPs. A difference of ≥5 mm Hg between calculated and displayed average IOP seen in 6% of cases may impact clinical judgement. Displayed CV<5% does not correlate with accurate IOP measurement based on individual results.
authors.affiliation: *Eye Care for Animals, Tucson, AZ †Koret School of Veterinary Medicine, The Hebrew University of Jerusalem, Rehovot, Israel.
authors.forename: Clinton J
authors.initials: CJ
authors.lastname: Doering
authors.forename: Eugenia
authors.initials: E
authors.lastname: Feldman
authors.forename: Tali
authors.initials: T
authors.lastname: Bdolah-Abram
authors.forename: Reuben E
authors.initials: RE
authors.lastname: Merideth
authors.forename: Ron
authors.initials: R
authors.lastname: Ofri
mesh_terms: D000465:Algorithms; D000818:Animals; D004285:Dogs; D005260:Female; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D009798:Ocular Hypertension / Q000175:diagnosis* / Q000662:veterinary; D011237:Predictive Value of Tests; D012372:ROC Curve; D015203:Reproducibility of Results; D013223:Statistics as Topic / Q000592:standards*; D014065:Tonometry, Ocular / Q000592:standards* / Q000662:veterinary