Comparing applanation tonometry and rebound tonometry in glaucomatous and ocular hypertensive eyes

To describe Goldmann applanation tonometer (GAT) and rebound tonometer (RT) agreement in measuring intraocular pressure (IOP) in glaucomatous and ocular hypertensive (OH) eyes and to evaluate central corneal thickness (CCT) influence on RT readings.

A total of 347 eyes were enrolled and IOP measured between 9 and 11 am. Rebound tonometry was performed first (RT1), followed by 3 consecutive GAT measurements and by a second RT reading (RT2), within a 5-minute span. Mean IOP (±SD) values were compared by means of paired t-test. Agreement between GAT and RT1 (test 1) and RT2 (test 2) was evaluated with Bland-Altman method, whereas a linear function described the relationship between CCT and IOP taken with RT.

Mean IOP (±SD) taken with RT1, GAT, and RT2 was 18.1±4.3, 15.6±3.3, and 16.3±3.9 mmHg, respectively. Readings were within ±3 mmHg in 63.7% and 86.7% of eyes for test 1 and 2, respectively. A significant (p<0.001) proportional bias was noted on both tests (95% limits of agreement: -2.3/7.4 and -3.6/5.0 mmHg for test 1 and 2, respectively). Agreement between instruments decreased for increasing IOP. Rebound tonometry readings increased by 4.6 and 4.1 mmHg for RT1 and RT2, respectively, for each 100-µm CCT increase.

When used first, RT significantly overestimated IOP compared with GAT. Differences became clinically negligible when RT was used immediately after GAT. Repeated applanation tonometry may explain this observation. Agreement between instruments was acceptable for low IOP, but worsened with increasing IOP values. RT is significantly influenced by CCT. Goldmann applanation tonometer and RT should not be used interchangeably.