title: The accuracy of continued clinical use of goldmann applanation tonometers with known calibration errors.
abstract: OBJECTIVE
  With normal clinical use, Goldmann applanation tonometers frequently develop calibration errors. Only the manufacturer can perform recalibration. This study aimed to assess whether intraocular pressure (IOP) measured by Goldmann applanation tonometers with known small calibration errors could be adjusted to reflect true IOP to allow continued clinical use.
  
  
  METHODS
  Evaluation of diagnostic test.
  
  
  METHODS
  Patients under regular review who had undergone previous applanation tonometry.
  
  
  METHODS
  Patients with a range of IOPs underwent IOP measurement using a gold standard 0-error tonometer and tonometers with known calibration errors in a randomized blind fashion. The calibration errors of the tonometers ranged 0 to +5 mmHg.
  
  
  METHODS
  Intraocular pressure.
  
  
  RESULTS
  For the first part of the study, 125 eyes of 125 patients with a mean IOP of 18.5 mmHg (range, 8-43 mmHg) were tested. Mean IOP measured by the tonometer with an error of +1 mmHg was +1.0 (95% confidence interval [CI], 0.3-1.7 mmHg; P = 0.0076, compared with gold standard 0-error), with the +2 mmHg error was +1.2 (95% CI, 0.8-1.7 mmHg; P<0.0001), with the +3 mmHg error was +1.6 (95% CI, 1.2-1.9 mmHg; P<0.0001), with the +4 mmHg error was +3.6 (95% CI, 2.9-4.2 mmHg; P<0.0001), and with the +5 mmHg error was +3.3 (95% CI, 2.9-3.8 mmHg; P<0.0001). In the second part of the study, IOP measured by each of the tonometers with +2 mmHg error was +0.6 mmHg (95% CI, 0.1-1.1 mmHg; P = 0.0241), +1.5 mmHg (95% CI, 1.0-2.0 mmHg; P<0.0001), and +1.5 mmHg (95% CI, 1.9-2.1 mmHg; P<0.0001).
  
  
  CONCLUSIONS
  There is a relationship between calibration error and clinical error in IOP measured, but it is not a one-to-one relationship. The error overestimates IOP and is consistent over a clinical range of IOPs. In certain circumstances where resources are limited, it may be clinically acceptable to use tonometers with calibration errors of less than +3 mmHg, because they do not overestimate IOP by more than 2 mmHg.
  
  
  BACKGROUND
  The author(s) have no proprietary or commercial interest in any materials discussed in this article.
authors.affiliation: Department of Ophthalmology, Royal Victoria Infirmary, Newcastle-upon-Tyne, United Kingdom.
authors.forename: Sukhpal S
authors.initials: SS
authors.lastname: Sandhu
authors.forename: Sandip
authors.initials: S
authors.lastname: Chattopadhyay
authors.forename: George A
authors.initials: GA
authors.lastname: Amariotakis
authors.forename: Fillipos
authors.initials: F
authors.lastname: Skarmoutsos
authors.forename: Michael K
authors.initials: MK
authors.lastname: Birch
authors.forename: Neeta
authors.initials: N
authors.lastname: Ray-Chaudhuri
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002138:Calibration / Q000592:standards*; D003951:Diagnostic Errors; D004868:Equipment Failure; D005260:Female; D005901:Glaucoma / Q000175:diagnosis*; D006801:Humans; D007429:Intraocular Pressure; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D011786:Quality Control; D015203:Reproducibility of Results; D014065:Tonometry, Ocular / Q000295:instrumentation / Q000592:standards*