Tono-Pen agrees with GAT for IOP measurements in children

This study assessed agreement of IOP measured with the Tono-Pen and the Goldmann applanation tonometer (GAT) in normal children. The authors found that while the average differences between IOP measured by Tono-Pen and GAT were small, there was substantial test-retest variability. Younger age was associated with larger average differences between the instruments, as was higher IOP in the office setting.

Given the variability of measurements, the authors advise clinicians to take two or three measurements with the Tono-Pen. In the office setting, a second instrument is recommended if any measurement is 20 mmHg or higher, which is the IOP at which the average difference between instruments is ≥3 mmHg.

The study included 439 subjects ranging in age from birth to < 18 years without anterior segment anomalies or glaucoma. They each had their IOP measured with the two instruments by separate, masked examiners in the office or under general anesthesia.

On average, the Tono-Pen measured values slightly lower than the GAT for IOP < 11 mmHg and slightly higher than the GAT for IOP > 11 mmHg in the office setting. Using the average of GAT and Tono-Pen IOPs to estimate the true IOP, the average difference (GAT - Tono-Pen) was 0.4 mmHg at IOP of 10 mmHg and -3.0 mmHg at IOP of 20 mmHg. The 95 percent limits of agreement of the average difference between instruments were ±6.4 mmHg in the office setting and ±6.8 mmHg under general anesthesia.

Standard error of measurement with the Tono-Pen was 1.44 mmHg in the office and 1.82 mm Hg under general anesthesia. Thicker corneas were associated with higher IOP with both the GAT and Tono-Pen.