Irish Journal of Medical Science (1971 -)

, Volume 188, Issue 1, pp 321–325 | Cite as

Difference in intraocular pressure measurements between non-contact tonometry and Goldmann applanation tonometry and the role of central corneal thickness in affecting glaucoma referrals

  • Khalid KamelEmail author
  • Edward Dervan
  • Kevin Falzon
  • Colm O’Brien
Original Article



Patients at glaucoma risk are commonly identified by optometrists and subsequently referred to glaucoma specialists. Optometrists mainly use non-contact tonometry (NCT) for intraocular pressure (IOP) measurement.


To investigate the role of differences in IOP measurement between NCT and Goldmann applanation tonometry (GAT) and the effect of central corneal thickness (CCT) on these differences in optometrist referrals


Details of the initial clinical visit of patients referred with IOP > 21 mmHg in either eye as measured by NCT to a consultant glaucoma specialist were retrospectively reviewed. Demographic and referral data, IOP, CCT, and glaucoma diagnosis were obtained. The main outcome measure was the IOP measurement differences between NCT and GAT.


Of the 98 patients referred, only 23% had IOP > 21 mmHg when measured by GAT. NCT (Nidek NT400, Reichert Puff, Pulsair Easy Eye) measured the IOP greater than GAT by a mean of 5.8 mmHg (NCT 24.1 ± 3.5, GAT 18.3 ± 3.0). The effect of CCT on IOP measurement was less for GAT (R2 0.034, p = 0.067) than for NCT (R2 0.088, p = 0.003). The NCT/GAT IOP differences increased with increasing CCT (R2 0.166, p < 0.0001). The NCT/GAT differences decreased with patient age (R2 0.048, p = 0.03). Patients were classified as normal 67% (66/98), ocular hypertension 11% (11/98), glaucoma suspect 14% (14/98), and glaucoma 7% (7/98).


The difference in IOP measurement between NCT and GAT leads to a possible increase in glaucoma referrals, particularly in patients with thicker corneas. Repeat IOP using GAT and CCT measurement would help in triaging referrals.


Central corneal thickness Goldmann tonometry Intraocular pressure Non-contact tonometry Optometry 


Compliance with ethical standards

This article does not contain any studies with human participants or animals performed by any of the authors.

Conflict of interest

The authors declare that they have no conflicts of interest.


  1. 1.
    Bowling B, Chen SD, Salmon JF (2005) Outcomes of referrals by community optometrists to a hospital glaucoma service. Br J Ophthalmol 89(9):1102–1104. CrossRefGoogle Scholar
  2. 2.
    Tuck MW, Crick RP (1991) Efficiency of referral for suspected glaucoma. BMJ 302(6783):998–1000CrossRefGoogle Scholar
  3. 3.
    Cook JA, Botello AP, Elders A, Fathi Ali A, Azuara-Blanco A, Fraser C, McCormack K, Margaret Burr J, Surveillance of Ocular Hypertension Study G (2012) Systematic review of the agreement of tonometers with Goldmann applanation tonometry. Ophthalmology 119(8):1552–1557. CrossRefGoogle Scholar
  4. 4.
    Bhan A, Browning AC, Shah S, Hamilton R, Dave D, Dua HS (2002) Effect of corneal thickness on intraocular pressure measurements with the pneumotonometer, Goldmann applanation tonometer, and Tono-Pen. Invest Ophthalmol Vis Sci 43(5):1389–1392Google Scholar
  5. 5.
    Brubaker RF (1999) Tonometry and corneal thickness. Arch Ophthalmol 117(1):104–105CrossRefGoogle Scholar
  6. 6.
    Ko YC, Liu CJ, Hsu WM (2005) Varying effects of corneal thickness on intraocular pressure measurements with different tonometers. Eye (Lond) 19(3):327–332. CrossRefGoogle Scholar
  7. 7.
    Tonnu PA, Ho T, Newson T, El Sheikh A, Sharma K, White E, Bunce C, Garway-Heath D (2005) The influence of central corneal thickness and age on intraocular pressure measured by pneumotonometry, non-contact tonometry, the Tono-Pen XL, and Goldmann applanation tonometry. Br J Ophthalmol 89(7):851–854. CrossRefGoogle Scholar
  8. 8.
    Whitacre MM, Stein RA, Hassanein K (1993) The effect of corneal thickness on applanation tonometry. Am J Ophthalmol 115(5):592–596CrossRefGoogle Scholar
  9. 9.
    Stevenson R (1999) Automated perimetry by optometrists in patients at low risk of glaucoma. Br J Ophthalmol 83(6):759–760CrossRefGoogle Scholar
  10. 10.
    Bell RW, O'Brien C (1997) The diagnostic outcome of new glaucoma referrals. Ophthalmic Physiol Opt 17(1):3–6CrossRefGoogle Scholar
  11. 11.
    Bell RW, O'Brien C (1997) Accuracy of referral to a glaucoma clinic. Ophthalmic Physiol Opt 17(1):7–11CrossRefGoogle Scholar
  12. 12.
    Sheldrick JH, Ng C, Austin DJ, Rosenthal AR (1994) An analysis of referral routes and diagnostic accuracy in cases of suspected glaucoma. Ophthalmic Epidemiol 1(1):31–39CrossRefGoogle Scholar
  13. 13.
    Theodossiades J, Murdoch I (1999) Positive predictive value of optometrist-initiated referrals for glaucoma. Ophthalmic Physiol Opt 19(1):62–67CrossRefGoogle Scholar
  14. 14.
    Vernon SA (1998) The changing pattern of glaucoma referrals by optometrists. Eye (Lond) 12(Pt 5):854–857. CrossRefGoogle Scholar
  15. 15.
    Azuara-Blanco A, Burr J, Thomas R, Maclennan G, McPherson S (2007) The accuracy of accredited glaucoma optometrists in the diagnosis and treatment recommendation for glaucoma. Br J Ophthalmol 91(12):1639–1643. CrossRefGoogle Scholar
  16. 16.
    Vernon SA, Jones SJ, Henry DJ (1991) Maximising the sensitivity and specificity of non-contact tonometry in glaucoma screening. Eye (Lond) 5(Pt 4):491–493. CrossRefGoogle Scholar
  17. 17.
    Henson DB, Spencer AF, Harper R, Cadman EJ (2003) Community refinement of glaucoma referrals. Eye (Lond) 17(1):21–26. CrossRefGoogle Scholar
  18. 18.
    Bland JM, Altman DG (1994) Regression towards the mean. BMJ 308(6942):1499CrossRefGoogle Scholar
  19. 19.
    Henson DB, Harper RA (1998) Do non contact tonometers read high? Ophthalmic Physiol Opt 18(3):308–310CrossRefGoogle Scholar
  20. 20.
    Liu J, Roberts CJ (2005) Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis. J Cataract Refract Surg 31(1):146–155. CrossRefGoogle Scholar
  21. 21.
    Kotecha A, Elsheikh A, Roberts CR, Zhu H, Garway-Heath DF (2006) Corneal thickness- and age-related biomechanical properties of the cornea measured with the ocular response analyzer. Invest Ophthalmol Vis Sci 47(12):5337–5347. CrossRefGoogle Scholar

Copyright information

© Royal Academy of Medicine in Ireland 2018

Authors and Affiliations

  1. 1.Institute of OphthalmologyMater Misericordiae University HospitalDublin 7Republic of Ireland
  2. 2.Ophthalmology DepartmentYork HospitalYorkUK

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