International comparisons of intraocular pressures, as measured by Tono-Pen and Goldmann applanation tonometry, in healthy adults: A meta-analysis

Author: Keller WJ

Geographical coverage: Korea, Sweden, Switzerland, Turkey, Israel, USA, Lithuania, Canada, UK, Brazil, India, Saudi Arabia, Japan, China, and Nigeria.

Sector: Treatment

Sub-sector: Intraocular pressure

Equity focus: Not explicitly stated

Study population: Healthy adults

Review type: Effectiveness review

Quantitative synthesis method: Meta-analysis

Qualitative synthesis method: Not applicable

Background

Accurate intraocular pressure (IOP) measurement is essential for diagnosing and managing glaucoma, a leading cause of irreversible blindness. Goldmann applanation tonometry (GAT) is the reference standard but requires specialist equipment and training. The Tono‑Pen (TP) is a portable, user‑friendly alternative, although its readings can differ from those of GAT. This meta‑analysis reviewed 22 studies from 15 countries (2011–2021) to compare TP and GAT measurements in healthy adults and to explore whether geography or methodology affects agreement between devices.

Objectives

To determine whether intraocular pressure readings obtained with the Tono‑Pen differ significantly from those obtained with Goldmann applanation tonometry in healthy adults; to establish whether any difference exceeds the clinically acceptable measurement error of ±2 mm Hg; and to investigate the influence of country or clinical setting on measurement differences.

Main findings

Across 2,114 eyes measured in 22 primary studies, intraocular pressure recorded with the Tono‑Pen was, on average, slightly higher than that recorded with Goldmann applanation tonometry. The pooled mean difference was –0.73 mm Hg (95 % CI –1.39 to –0.07 mm Hg; P = 0.03). The 95 % prediction interval (–4.03 to 2.58 mm Hg) indicates that, in some settings, TP may read higher or lower than GAT by more than ±2 mm Hg. Sensitivity analyses showed the result was robust to the removal of individual studies (summary effect range –0.54 to –0.88 mm Hg).

Meta‑regression found that country explained much of the between‑study heterogeneity (R² analogue = 0.75; P = 0.001), whereas measurement setting (hospital, outpatient clinic, office or laboratory) did not (R² analogue = –0.17; P = 0.65).

Methodology

The authors searched PubMed, Embase, Cochrane Library, Scopus, Google Scholar and ProQuest, supplemented by conference abstracts from the American Academy of Ophthalmology and American Academy of Optometry, with no language restrictions. Reference lists were hand‑searched. Eligible studies measured intra‑ocular pressure with both devices in the same participants (≥ 18 years) during one session and excluded people with ocular pathology.

Quality was appraised with the 12‑item Primary Study Quality Assessment Checklist. A random‑effects meta‑analysis was performed; heterogeneity was quantified with I², and potential publication bias was inspected with funnel plots, Rosenthal’s fail‑safe N and Egger’s test.

Applicability/external validity

Findings appear generalisable to healthy adults from diverse regions and clinical contexts, supporting the use of the Tono‑Pen where Goldmann applanation tonometry is impractical. Applicability to people with glaucoma, abnormal corneal thickness or other ocular disease remains uncertain and warrants further research.

Geographic focus

Studies were conducted in Korea, Sweden, Switzerland, Turkey, Israel, the USA, Lithuania, Canada, the UK, Brazil, India, Saudi Arabia, Japan, China and Nigeria.

Summary of quality assessment

We have low confidence in the review’s conclusions. The authors did not state how many reviewers independently screened records or extracted data, nor did they provide a list of excluded studies or report contacting study authors for clarification. Although publication bias was assessed, these omissions limit assurance in the findings.