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Applanation tonometry: interobserver and prism agreement using the reusable Goldmann applanation prism and the Tonosafe disposable prism
  1. Csilla Ajtony1,
  2. Ahmed Elkarmouty1,
  3. Keith Barton1,2,3,
  4. Aachal Kotecha1,2
  1. 1Glaucoma Service, Moorfields Eye Hospital NHS Foundation Trust, London, UK
  2. 2NIHR Biomedical Research Centre for Ophthalmology, UCL Institute of Ophthalmology and Moorfields Eye Hospital NHS Foundation Trust, London, UK
  3. 3Department of Epidemiology and Genetics, UCL Institute of Ophthalmology, London, UK
  1. Correspondence to Dr Aachal Kotecha, NIHR BRC for Ophthalmology, Institute of Ophthalmology, 11-43 Bath Street, London EC1V 9EL, UK; Aachal.Kotecha{at}


Purpose To evaluate the levels of agreement between the standard reusable prism and a disposable prism, and to examine the agreement between ophthalmologists, nursing and technical staff when measuring intraocular pressure (IOP) using the Goldmann applanation tonometer.

Methods Three hundred eyes of 300 patients were recruited. IOP measurements were made in a randomised order by three observer groups consisting of ophthalmologists and ophthalmic technicians/nurses taken from a pool of clinicians working within a busy outpatient clinic. Agreement was calculated by Bland–Altman analysis, showing the mean difference and 95% limits of agreement (LoA) of measurements.

Results The mean difference between the reusable and disposable prism IOP measurements was <0.5 mm Hg. The LoA ranged from ±3.1 to ±4.9 mm Hg, depending on the observer group. The interobserver variability was <1 mm Hg across all observer groups; the LoA was slightly higher for observers using the reusable prism (range between ±4.3 and ±5.6 mm Hg) compared with using the disposable prism (range between ±3.7 and ±5.4 mm Hg) across observer groups.

Conclusions There is an acceptable agreement between IOP measurements made with the reusable Goldmann tonometer prism and the disposable Tonosafe prism. Interobserver variability in IOP measurements within an outpatient setting is larger than that found within a research setting, and may be of a level that impacts on clinical decision-making.

  • Intraocular pressure
  • Glaucoma

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