Fig. 2A shows the results of the comparison between the iCare and Diaton. The width of the 95% LOA of was large (16.09 mm Hg). The mean difference between the iCare and Diaton showed that there were no significant correlations with the IOP values (r = 0.014, p = 0.910).
Fig. 2B shows the results of the comparison between the iCare and Tonopen XL. The mean difference between the iCare and Tonopen XL showed that there were no significant correlations with the IOP values (r = −0.004, p = 0.590).
Fig. 2C shows the results of the comparison between the iCare and Kowa HAT. The iCare exhibited a weak, but significant, tendency (r = 0.305, p = 0.001) to underestimate the IOP in eyes with a lower IOP (<13 mm Hg) and overestimate the IOP in eyes with a higher IOP (>13 mm Hg) compared with the Kowa HAT.
Fig. 2D shows the results of the comparison between the Diaton and Tonopen XL. The width of the 95% LOA was largest in this comparison (19.19 mm Hg). The mean difference between the Diaton and Tonopen XL showed that there were no significant correlations with the IOP values (r = −0.068, p = 0.567).
Fig. 2E shows the results of the comparison between the Diaton and Kowa HAT. The width of the 95% LOA was large (16.27 mm Hg). The Diaton exhibited a weak, but significant, tendency (r = 0.241, p = 0.041) to underestimate the IOP in eyes with a lower IOP (< 20 mm Hg) and overestimate the IOP in eyes with a higher IOP (> 20 mm Hg) compared with the Kowa HAT.
Fig. 2F shows the results of the comparison between the Tonopen XL and Kowa HAT. The Tonopen XL exhibited a weak, but significant, tendency (r = 0.326, p = 0.005) to underestimate the IOP in eyes with a lower IOP (< 17 mm Hg) and overestimate the IOP in eyes with a higher IOP (> 17 mm Hg) compared with the Kowa HAT.
Intertonometer differences of more than ±3.0 mm Hg were observed between the iCare and Diaton in 35 eyes (48.6%), between the iCare and Tonopen XL in 49 eyes (68.1%), between the iCare and Kowa HAT in 50 eyes (69.4%), between the Diaton and Tonopen XL in 35 eyes (48.6%), between the Diaton and Kowa HAT in 37 eyes (51.4%), and between the Tonopen XL and Kowa HAT in 48 eyes (66.7%).
We investigated differences in IOP measurements obtained in the supine position between four portable tonometers. Many previous studies have estimated agreement tendencies among the GAT and other tonometers; however, such tendencies have only been investigated in measurements taken with the patient in the sitting position. One metaanalysis reported differences between the GAT and other tonometers when the IOP was measured in the sitting position as follows: HAT and GAT (−1.2 mm Hg, 95% LOA; −5.3–2.8 mm Hg); Tonopen and GAT (−0.2 mm Hg, 95% LOA; −6.2 to 5.8 mm Hg); and Diaton and GAT (−0.5 mm Hg, 95% LOA; −6.9 to 5.9 mm Hg).11 However, the IOP is similarly elevated in the supine position in primary open angle and normal tension glaucoma patients, and in normal subjects.12–15 The difference in IOP between the sitting and supine positions is called the ΔIOP. This value has been found to be associated with visual field progression in patients with glaucoma.12 Furthermore, the ΔIOP is not significantly different between the diurnal and nocturnal periods,16 although 24-hr IOP rhythms have been observed to differ between younger and older subjects.17 Therefore, obtaining accurate measurements of the supine IOP is also important for evaluating 24-hr IOP control.
One may expect the agreement between instruments to increase or decrease in the supine position if there is reason to believe that additional factors may influence the measurement of IOP in some, but not all, instruments. We believe that one factor that affects the IOP measurements is the weight of the tonometer tip or probe. When using the Diaton tonometer and iCare, the IOP is measured according to the same method as that used in the sitting position. Therefore, the gravitational force against the tonometer does not differ between the sitting and supine positions. However, when using the Kowa HAT and Tonopen XL, the weight of the tonometer must be supported differently from that required in the sitting position. When using the Kowa HAT, care must be taken not to load the applanation tip weight (approximately 1.6 g) on the cornea. Furthermore, when using the Tonopen XL, the effects of the weight of the small tip (the weight has not yet been made available to the public) cannot be avoided in the supine position. These factors may be associated with the nature of the discrepancies observed in IOP measurements obtained using different tonometers in the supine position.
The GAT remains the clinical standard for IOP measurement. Calibration errors are possible, yet this does not negate the tool’s clinical usefulness. Previous studies have considered various factors (e.g., astigmatism, central corneal thickness, and heart rate and rhythm) and concluded that ±3.0 mm Hg is the maximum clinically acceptable error.18–20 The obvious step in further investigation then is to determine the clinically acceptable level of IOP differences between tonometers.
The best agreement (i.e., interchangeability) is achieved when the mean difference is zero and the SD is under ±3.0 mm Hg. Therefore, the 95% LOA should be within 3.0 × 1.96 mm Hg = 5.88 mm Hg. However, clinically acceptable agreement levels varied in early reports4–8,21 and could not be quantified in a metaanalysis.
Previous studies have shown IOP measurements obtained in the sitting position to differ among tonometers. These differences have been reported to be 3.35 ± 2.80,4 1.40 ± 2.19,6 and 0.6 ± 3.27 mm Hg (calculated by us), respectively, between the iCare and HAT or GAT;8 0.00 ± 2.44 mm Hg between the iCare and Tonopen XL6 and −0.6 ± 3.32 mm Hg(calculated by us);5 and 2.78 ± 2.53,4 3.54 ± 2.47,7 and 0.8 ± 3.67 mm Hg between the Tonopen XL and Perkins HAT or GAT, respectively.8
Our results revealed differences of 1.49 ± 2.90 mm Hg between the iCare and Kowa HAT, of 1.47 ± 3.52 mm Hg between the iCare and Tonopen XL; and of 0.02 ± 3.61 mm Hg between the Tonopen XL and Kowa HAT. Between the iCare, Tonopen XL, and HAT, similar results were obtained in the supine patients. The agreement between the Diaton tonometer and other tonometers was poor, with a large SD being observed (±3.39 to ±4.95 mm Hg). These data coincide with those of previous studies, which reported Diaton tonometer differences of −2.32 ± 3.93 and −0.48 ± 4.50 mm Hg against GAT,7,21 −2.37 ± 3.77 mm Hg against the Perkins HAT,7 and −1.22 ± 3.77 mm Hg against the Tonopen XL.7
In our results, the Pearson correlation coefficients calculated for the mean IOP values of the Diaton and other tonometers indicated weak or no correlations (r = 0.041–0.286), and the width of the 95% LOAs in the Bland-Altman analysis indicated a wide bias (16.09–19.19 mm Hg). Therefore, we suggest that the Diaton tonometer should be used only when other tonometers are unavailable.
The Bland-Altman plots revealed relatively good agreement between the iCare, Tonopen XL, and Kowa HAT at the clinical practice level; however, the agreement was not sufficient to be used interchangeably. There were no significant correlations, as calculated in the Bland-Altman analysis, between the measurements obtained from the iCare and those obtained from the Tonopen XL (r = −0.004, p = 0.590). However, we should be careful to note that both the iCare and Tonopen XL significantly overestimated higher IOP measurements compared with the Kowa HAT, as observed in the Bland-Altman analysis (r = 0.305, p = 0.001, and r = 0.326, p = 0.005, respectively).
Early reports have shown a similar tendency to that observed in the Bland-Altman analysis among the iCare, Tonopen XL, and hand-held applanation tonometers when used in the sitting position. García-Resúa et al.’s4 findings are compatible with our results; they also observed that the iCare and Tonopen XL significantly overestimate higher IOPs compared with the Perkins HAT when used in the sitting position. Nakamura et al.6 reported no significant agreement between the iCare and GAT, although the iCare significantly overestimated higher IOPs compared with a noncontact tonometer and the Tonopen XL when analyzed in a Bland-Altman analysis.
However, in this study, all of the Pearson correlation coefficient values between the tonometers shown in Table 2 were less than 0.6. These results were relatively lower than the results of previous reports of measurements obtained in the sitting position. For instance, the correlation coefficient (r) between the iCare and Perkins HAT has been reported to be 0.94322 and 0.81,4 whereas that between the Perkins HAT and Tonopen XL has been reported to be 0.71.4 These differences may be associated with increases in the episcleral venous pressure or gravitational force against the tonometer.
A limitation of our study is that the iCare was used in the left lateral position and not in the supine position. However, Lee et al.9 reported that right eye IOP measurements obtained at 5 and 30 min after changing to the left lateral decubitus position from the supine position are not significantly different from those obtained in the supine position. Therefore, we consider our data to be theoretically valid despite this limitation. The second limitation of our study is that the iCare rebound tonometer has an updated version, the ICare PRO, which allows for the measurement of the IOP in the supine position. Further investigations are warranted to assess the efficacy of the ICare PRO and its measurements of supine IOP. The third limitation of this study is that our subjects were all healthy adults; therefore, our results may not directly apply to glaucoma patients, children, or patients with higher IOPs.
In summary, we measured the IOP in patients in the supine position using four portable tonometers and found that the results are not interchangeable. Relatively good agreement was observed between the iCare, Tonopen XL, and Kowa HAT; however, the iCare and Tonopen XL significantly overestimated the IOP values in the supine position in patients with higher IOPs compared with the Kowa HAT.
Department of Ophthalmology
Saneikai Tsukazaki Hospital
68-1 Aboshi Waku
The authors have no financial disclosures and no conflicts of interest to declare, did not receive any financial support from any public or private sources, and have no financial or proprietary interest in a product, method, or material described herein.
SN, HT, and YK were involved in the design of the study; SN, EM, MY, and YT conducted the study; SN and HT showed statistical expertise; and SN, EM, MY, YT, HT, and YK were involved in the final approval of the article.
Clinical trial registration; Japan Clinical Trials Register; number: UMIN000008034.
The authors thank Norio Sugimoto for the advice on the statistical analyses performed in this paper.
Received August 25, 2012; accepted April 8, 2013.
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Keywords:© 2013 American Academy of Optometry
iCare; Diaton tonometer; Tonopen XL; hand-held applanation tonometer; glaucoma