Evaluation of the Accuracy of Air-puff Tonometer Compared to Goldmann and Schiotz Tonometers among Egyptians

Abstract

Purpose

To determine the precision of the air-puff tonometer compared to the Goldmann applanation tonometer (GAT) and Schiotz tonometers.

Methods

The study is a descriptive, cross-sectional study that included 410 eyes from 205 primary open-angle glaucoma patients. Intraocular pressure (IOP) was measured with the air-puff tonometer, followed by the GAT, and finally, the Schiotz tonometer. Pachymetry was performed for all patients.

Results

IOP was 20.21 ± 3.63 mm Hg for the air-puff tonometer, 18.73 ± 3.41 mm Hg for GAT, and 18.29 ± 3.92 mm Hg for the Schiotz tonometer, and this difference was not statistically significant. In both eyes, the correlation values between GAT and the air-puff tonometer were greater than 0.8, signifying a strong correlation. Additionally, the Bland-Altman analysis revealed greater agreement between the GAT and the air-puff tonometer than with the Schiotz tonometer.

Conclusion

We can be confident that the air-puff tonometer provides accurate measurements close to those obtained by the GAT. Therefore, it can be reliably used for diagnosing, screening, and monitoring glaucoma patients, particularly in rural areas and during epidemics. Additionally, the values obtained from the Schiotz tonometer can be trusted, making it a reliable option for patients under general anesthesia or when other tonometers are unavailable.

How to cite this article

Khalil KM, Ghoneim EM, Gab-Alla A, et al. Evaluation of the Accuracy of Air-puff Tonometer Compared to Goldmann and Schiotz Tonometers among Egyptians. J Curr Glaucoma Pract 2025;19(2):75–78.

Introduction

Glaucoma is considered the world's leading cause of permanent blindness.¹ Reducing intraocular pressure (IOP) is the primary objective of glaucoma treatment, aimed at preventing its onset or slowing disease progression. Tonometers are the quickest method for diagnosing glaucoma, among many new devices used for screening and diagnosis, such as optical coherence tomography (OCT). Therefore, an accurate and rapid technique for measuring IOP is required.²

The Goldmann applanation tonometer (GAT) is considered the gold standard for measuring IOP in modern clinical practice because of its historical use in most clinical research studies to date and the numerous publications on its validity, reliability, and reproducibility. GAT has relatively low intra- and interobserver variability.³

The Schiotz tonometer operates on the indentation theory. It was the world's most utilized tonometer until GAT took over in the last quarter of the 20th century. Despite the fact that it requires the patient to lie down, it is portable, inexpensive, and durable. There are no complex electronics in it. As a result, it is appropriate for screening, especially in areas with limited resources and remote places.⁴

Another effective approach for measuring IOP is air-puff tonometry. The patient's cornea is flattened using a tiny airburst. Being a noncontact approach for measuring IOP is its main benefit.⁵ Ophthalmological screening can also benefit from using it because it is simple to use, quick, and carries a low risk of contamination, but it occasionally gives over-reading results.⁶

Previous studies have proved that the GAT is the most precise tonometer, but due to the need for quick and noncontact devices, such as the air-puff tonometer—especially with the current wave of COVID-19—it has become essential to depend on air-puff tonometry completely.⁷ This study will help provide critical data comparing the IOP measured by the three devices, which will allow us to determine if the over-reading measured by the air-puff tonometer can be neglected or if it would affect our judgment dramatically.

Patients and Methods

This study includes 410 eyes from 205 glaucoma patients who visited the Ophthalmic Outpatient Clinic at Port Said Ophthalmology Hospital from November 2023 to April 2024. The inclusion criteria for this descriptive, cross-sectional study included a history of primary open-angle glaucoma in patients aged 18–80 years. Exclusion criteria included a history of secondary glaucoma, such as steroid-induced glaucoma, pigmentary glaucoma, pseudoexfoliation glaucoma, corneal disorders such as keratitis, corneal ulcer, exposure keratopathy, corneal dystrophy, corneal degeneration, use of contact lenses within 24 hours, and silicon-filled eyes.

All patients are evaluated starting with history taking, which includes personal data such as name, hospital ID, age, sex, occupation, and phone number; ocular history, including a history of glaucoma and medications taken, previous eye surgery, corneal diseases or previous anterior uveitis, recent contact lens wear; and medical history, including a history of systemic diseases and medications taken.

A full ophthalmic examination was conducted for all patients, which included the anterior and posterior segments of the eye using a slit-lamp biomicroscope (Topcon) and a Volk +90 biconvex lens.

IOP assessment was done by measuring the central corneal thickness (CCT) with a pachymetry device (Compact Touch), followed by IOP testing. The IOP was measured with the air-puff tonometer (Topcon CT-1), then the GAT, and finally the Schiotz tonometer. The assessments were spaced about 5 minutes apart, and the tonometers were calibrated after each measurement. To eliminate diurnal variations, all assessments were carried out around the same time each day.

Results

The mean age of the cases is 51.37 ± 16.35 years, ranging from 18 to 79 years; 62.4% are 50 years or older. Of the studied cases, 61.4% are females and 38.6% are males.

Table 1 illustrates no statistically significant relation between intraocular pressure by air-puff tonometer and the age of the studied cases for the right eye (p = 0.749) and the left eye (p = 0.561). It also illustrates no statistically significant relation between intraocular pressure by GAT and the age of the studied cases for the right eye (p = 0.730) and for the left eye (p = 0.727). It shows no statistically significant relation between intraocular pressure by Schiotz and the age of the studied cases for the right eye (p = 0.977) and the left eye (p = 0.400).

Table 1:

Mean intraocular pressure by air-puff, GAT, and Schiotz tonometers according to age-group

Device	Eye	Age/years	IOP		Test of significance	Mean difference (95% CI)
			Mean ± SD	Range
Air-puff	OD	<50 ≥50	19.81 ± 3.10 19.66 ± 3.51	13–28 11–29	t = 0.320 p = 0.749	0.153 (–0.792, 1.09)
	OS	<50 ≥50	19.92 ± 3.25 20.21 ± 3.63	13–28 12–29	t = 0.582 p = 0.561	0.289 (0.69–1.27)
GAT	OD	<50 ≥50	18.38 ± 3.02 18.53 ± 3.21	12–27 11–27	t = 0.345 p = 0.730	−0.155 (−1.04, 0.727)
	OS	<50 ≥50	18.57 ± 3.06 18.73 ± 3.41	12–28 10–27	t = 0.349 p = 0.727	−0.163 (−1.08, 0.758)
Schiotz	OD	<50 ≥50	17.84 ± 3.92 17.85 ± 3.83	11.2–29 10.2–29	t = 0.029 p = 0.977	−0.016 (−1.10, 1.071)
	OS	<50 ≥50	17.83 ± 3.89 18.29 ± 3.92	9.4–29 10.2–29	t = 0.843 p = 0.400	−0.470 (−1.57, 0.629)

Table 2 illustrates a statistically significant higher mean IOP by air-puff tonometer among females than males for the right eye (p = 0.023) and the left eye (p = 0.024). It also illustrates a statistically significant higher mean IOP by GAT among females than males for the right eye (p = 0.049) and for the left eye (p = 0.049). It shows no statistically significant relation between mean IOP by Schiotz tonometer among females and males for the right eye (p = 0.072), and a statistically significant higher mean IOP by Schiotz tonometer among females than males for the left eye (p = 0.049).

Table 2:

Mean intraocular pressure by air-puff, GAT, and Schiotz tonometers according to sex group

Device	Eye	Sex	IOP		Test of significance	Mean difference (95% CI)
			Mean ± SD	Range
Air-puff	OD	Male Female	19.05 ± 3.55 20.13 ± 3.18	11–29 12–28	t = 2.29 p = 0.023*	−1.082 (−2.01, −0.152)
	OS	Male Female	19.42 ± 3.58 20.53 ± 3.37	12–28 12–29	t = 2.28 p = 0.024*	−1.12 (−2.08, −0.149)
GAT	OD	Male Female	17.94 ± 3.13 18.81 ± 3.10	12–27 11–26	t = 1.98 p = 0.049*	−0.88 (−1.75, −0.005)
	OS	Male Female	18.11 ± 3.38 19.02 ± 3.17	10–28 12–27	t = 1.98 p = 0.049*	−0.912 (−1.83, −0.003)
Schiotz	OD	Male Female	17.24 ± 3.78 18.23 ± 3.87	10.2–29 10.2–29	t = 1.81 p = 0.072	−0.985 (−2.05, 0.088)
	OS	Male Female	17.45 ± 3.98 18.54 ± 3.81	9.4–29 9.4–29	t = 1.98 p = 0.049*	−1.09 (−2.18, −0.003)

*statistically significant

Table 3 shows a statistically significant positive correlation between the air-puff tonometer and GAT in the right eye (r = 0.886) and the left eye (r = 0.890).

Table 3:

Correlation between GAT and air-puff tonometer readings

	GAT
Air-puff tonometer	r	p-value
OD	0.886	<0.001*
OS	0.890	0.001*

r, Spearman correlation coefficient; *statistically significant

Table 4 shows a statistically significant positive correlation between the air-puff tonometer and Schiotz tonometer in the right eye (r = 0.723) and the left eye (r = 0.755).

Table 4:

Correlation between Schiotz and air-puff tonometer readings

	Schiotz tonometer
Air-puff tonometer	r	p-value
OD	0.723	0.001*
OS	0.755	0.001*

r, Spearman correlation coefficient; *statistically significant

As illustrated in Figures 1 and 2, the Bland−Altman plot revealed that the measurements shown are extremely strongly grouped around the mean 0 line, so the values between the GAT and air-puff measurements for the right and left eyes are coherent. Hence, the two are in good agreement.

Fig. 1:

Bland−Altman analysis for agreement between GAT and air-puff measurements for the right eye

Fig. 2:

Bland−Altman analysis for agreement between GAT and air-puff measurements for the left eye

Discussion

There are numerous methods for glaucoma diagnosis and screening, but IOP measurement is the most important parameter in the diagnosis, screening, and follow-up of glaucoma patients.⁸ Humans have created numerous instruments and devices for measuring IOP. Newer devices designed for IOP measurement have lower error factors than older devices. However, before using these devices to replace older instruments, we must feel confident in their efficacy, accuracy, and safety.⁹

Newer devices are continuously evaluated by comparing them to the gold standard device. This evaluation can aid in analyzing and improving technique and accuracy, making newer devices faster, safer, and more accurate. This could speed up and simplify the diagnosis, screening, and follow-up of glaucoma cases, which is useful in remote and crowded clinics, as well as during epidemics where using touchable devices aids in the transmission of infection.¹⁰

This study is based on the continual evaluation of newer devices and compares the air-puff tonometer with the Goldmann applanation tonometer, which is the gold standard device for better glaucoma management. The Schiotz tonometer, despite being an old tonometer, remains in use in many hospitals today because it is suitable for examining unconscious patients or those under general anesthesia.¹¹

The air-puff tonometer is a recently used device for IOP assessment, which is noncontact, easy-to-use, and quick. It can help in the screening of glaucoma cases in rural areas with little experience needed compared to optical coherence tomography. To determine its dependability, it was compared to the gold standard, the Goldmann applanation tonometer.¹²

The Goldmann applanation tonometer is the gold standard tonometer. Besides its accuracy, it is time-consuming, uncomfortable for patients, and requires experience to be used. Furthermore, it requires contact with the patient's cornea, making it an unsuitable choice during epidemics.¹³

In this study, 205 individuals were assessed using Schiotz, GAT, and air-puff tonometers. Demographic features of the examined patients revealed that females (61.4%) outnumbered males (38.6%). Patients in this study ranged in age from 18 to 79 years. In this investigation, no correlation was found between IOP and age in either eye. All devices showed higher IOP values in the age group older than 50 years. All of the data were statistically insignificant; however, Qureshi found that IOP increased by 0.28 mm Hg every 10 years with age.¹⁴

There was a correlation between IOP and sex. Females had higher IOP values in both eyes in all tonometers compared to males. These results are statistically significant, except for the right eye in Schiotz's IOP values, which were insignificant. However, Bonomi et al.'s study found that IOP is higher in males than in females.¹⁵

An intracoefficient study was conducted to evaluate the consistency and repeatability of several tonometers in producing trustworthy findings in both eyes. It revealed that the correlation values between the air-puff tonometer and GAT were >0.8 in both eyes, indicating a very strong correlation. Pakrou et al. found the same results, showing a high correlation between the air-puff tonometer and GAT.¹⁶ A Bland−Altman plot showed better agreement between the air-puff tonometer and GAT than between the GAT and Schiotz tonometer. Martinez-de-la-Casa et al.'s study gave the same result.¹⁷

Like our results, Yilmaz et al.'s cross-sectional study revealed that the air-puff tonometer measurements were similar to those measured by the GAT in normotensive eyes.¹⁸ Also, Shah et al. revealed high correlation values between the PT100 air-puff tonometer and GAT (>0.7 in both eyes), indicating a significant correlation between the two tonometers.¹⁹ Similarly, the Bang et al. study showed that three different air-puff tonometers (Topcon CT-1P, Canon TX-20P, and Nidek NT-530P) have similar accuracy to GAT.²⁰

In contrast to our results, Vincent et al.'s cross-sectional study showed that the average air-puff tonometer measurements were lower than the average GAT measurements for both eyes.¹⁰ Also, Kouchaki et al.'s cross-sectional study showed that there is limited agreement between the tonometers.⁹ Another study, Lee and Ahn's study, suggested that the air-puff tonometer's IOP values were (8.4% ± 11.3%) higher than the GAT values.²¹ The causes of differences between these results and ours may refer to different races, whether glaucoma patients or healthy subjects were examined, differences in age groups, and the sample size.

According to our results, we can feel confident that the values measured by the air-puff tonometer are accurate and approximately close to the values measured by the GAT. Therefore, we can fully depend on using an air-puff tonometer in the management of glaucoma patients, especially in rural areas and during epidemics. Additionally, we can rely on the Schiotz tonometer values and feel safe using it for patients under anesthesia or when the GAT and air-puff tonometers aren't available.

Conclusion

The study found a high and significant correlation between the air-puff tonometer and GAT in both eyes. Furthermore, the air-puff tonometer and the GAT tonometer showed very good agreement when compared to the Schiotz tonometer. The Bland−Altman analysis demonstrated greater agreement between the air-puff tonometer and the GAT tonometer than the Schiotz tonometer.

Based on these results, we can be confident that the air-puff tonometer provides accurate measurements close to those obtained by the GAT. Therefore, we can rely on using an air-puff tonometer for the management of glaucoma patients, particularly in rural areas and during epidemics. Additionally, we can trust the values obtained from the Schiotz tonometer, making it a reliable option for patients under general anesthesia or when the GAT and air-puff tonometers are unavailable.

Ethical Approval

Port Said University Faculty of Medicine. The study was conducted after approval of the protocol by the Ophthalmology Department Committee, the Research Committee, and the Research Ethics Committee in May 2023 [ERN: MED (2/4/2023)s.no(84) OPT824_001].

Orcid

Khalil M Khalil https://orcid.org/0000-0003-2533-2741

Ehab M Ghoneim https://orcid.org/0000-0003-3312-2998

Amr Gab-Alla https://orcid.org/0000-0002-8005-8178

Moataz A Sallam https://orcid.org/0000-0003-4759-4849