Many new tonometer have been introduced in recent years. They have all been compared with the Goldmann applanation tonometer, which has been considered the gold standard.1–3
When comparing tonometry devices randomisation of methods is necessary because each subsequent measurement lowers the IOP by 0.1–0.4 mm Hg.4,5
A new tonometry method needs to be accurate and reproducible.
In our study the median intraindividual deviation between TGDc-01 and Goldmann was 0 mm Hg (interquartile range −1; 2 mm Hg). Lösch et al
demonstrated a mean intraindividual deviation of 3.6 mm Hg (SD 4.8) between TGDc-01 and Goldmann in 25 patients with intraocular inflammation, with lower IOP values using TGDc-01.6
Rombold et al
also found statistically significant lower IOP values with TGDc-01 compared to Goldmann in 100 eyes.7
Müller et al
, who performed TGDc-01 and Goldmann applanation tonometry in 50 healthy volunteers, did not find statistically significant deviations between both devices: mean IOP with TGDc-01 was 11.2 mm Hg (range 7.5–19.5 mm Hg) and with Goldmann 12 mm Hg (range 9–19.8).8
However, he compared averaged IOP values of a whole population. Thus, interindividual variation of IOP values is to high to detect statistically significant deviations between instruments. Therefore, it is important to evaluate intraindividual device deviations, as we did.
The reason that we did not find a statistically significant deviation between TGDc-01 and Goldmann was not the very good agreement between devices, but the high variability of intraindividual deviations. Deviations between TGDc-01 and Goldmann were more than plus or minus 3 mm Hg in 38% of eyes (fig 4). So, we cannot recommend TGDc-01 tonometry as substitute or alternative method for Goldmann applanation tonometry. But as intraindividual deviations between TGDc-01 tonometry and Goldmann tonometry were less compared to those between palpation and Goldmann tonometry (table 2, figs 4 and 6), in cases where Goldmann applanation tonometry is impossible to perform (for example, children without general anaesthesia, corneal pathology) TGDc-01 tonometry seems to be a better choice than palpation, which was the only alternative previously.
Other authors confirm the high variability of intraindividual deviations between TGDc-01 and Goldmann tonometry: Sandner et al
demonstrated a deviation of plus or minus 3 mm Hg between TGDc-01 and Goldmann tonometry in 12% of 150 healthy volunteers, range −11–15 mm Hg,9
and Amm et al
in 29.9% of 80 eyes (40 after keratoplasty).10
We are not aware of any other studies comparing TGDc-01 and palpation.
Some authors give correlation coefficients for TGDc-01 and Goldmann between 0.86 and 0.89.9,10
The correlation coefficient is a measure of the strength of linear association between two variables, which is not the same as a measure of agreement.11
Furthermore, the degree of correlation is difficult if not impossible to define, whereas the limits of agreement using Bland-Altman diagrams provide direct information on clinical agreement.12,13
Worse still, it is possible to obtain a very high correlation while clinical agreement is poor. Correlation analysis is highly sensitive to the choice of subjects. A high value of r
can be obtained because there is large variation between subjects.11
Additionally, in our study we showed that for smaller IOP levels TGDc-01 and palpation values exceeded Goldmann values and with higher IOP levels TGDc-01 and palpation values were smaller (figs 4 and 6). So, both tonometry methods, TGDc-01 and palpation, have less power to discriminate between IOP values compared to the gold standard of Goldmann applanation tonometry. As our study population, like all recently published studies, were normotensive patients (range in our study: 9–20 mm Hg), further studies have to be conducted with a wider range of IOP values to confirm this result.
Furthermore the intraobserver and interobserver variability of TGDc-01 was quite high. In our study the intraobserver deviations were statistically significant (Friedman test: observer 1 p
0.007, observer 2 p
0.002). Rombold et al
performed five replicate IOP measurements by two different observers in 100 eyes using TGDc-01. Intraobserver variability was 29% for observer 1 and 8% for observer 2. Deviations between both observers were statistically significant (p<0.01).7
Because of the high variability of intraindividual deviations between both observers, we could not show any significant interobserver deviations. But deviations between both observers were more than plus or minus 3 mm Hg in 45% of eyes (fig 2), which was clinically relevant. So, TGDc-01 demonstrates an observer dependency. And its high intraobserver variability means that we need to perform at least three replicate measurements per eye.
When measuring intraobserver and interobserver variability of new tonometry methods we have to be aware that the gold standard of Goldmann applanation tonometry also demonstrates a rather high variability. Phelbs et al
found a deviation between two observers of at least 3 mm Hg in 30% of 420 eyes.14
Sudesh et al
found a deviation between two observers of at least 3 mm Hg in 20% of 32 eyes.15
Some previously identified sources of errors for the Goldmann tonometer are incorrect fluorescein concentration,16
and the width of the rings.
Possible sources of errors for the TGDc-01 tonometer are:
- the influence of the eyelid, including tarsus and skin, on the elastic eye surface cannot be calculated;
- the elasticity of the sclera is dependent on its thickness and distance to the limbus;
- the perpendicular fall of the rod is not really controlled.
In summary, we demonstrated that interobserver deviations using TGDc-01 tonometry and intraindividual deviations between TGDc-01 tonometry and Goldmann applanation tonometry and between TGDc-01 tonometry and palpation of IOP were clinically relevant. Thus, we cannot recommend the new transpalpebral tonometry device, TGDc-01, as a substitute or alternative method for Goldmann applanation tonometry. But as deviations between TGDc01 tonometry and Goldmann were less than between palpation of IOP and Goldmann, TGDc-01 seems to be a better choice for tonometry in patients with corneal pathology or in awake children than palpation of IOP, at least in normotensive subjects.