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Oscar Daniel Albis-Donado, Shibal Barthiya, Veronica Marina Gil-Reyes, Giovanna Casale-Vargas, Nancy Arreguin-Rebollar; Citius, Altius, Fortius: agreement between Perkins and Dynamic Contour Tonometry (Pascal) and the impact of altitude. Invest. Ophthalmol. Vis. Sci. 2014;55(13):127.
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To assess the agreement of measurements between Perkins and Pascal Tonometers, over serial follow up, at the two different altitudes.
A total of 41 eyes of 41 normal consenting subjects were enrolled for this observational, cross-sectional study. Intraocular pressure measurements were performed using the Pascal and Perkins tonometer for both eyes of each subject at Acapulco (sea level) and at Mexico City (2234m from sea level). IOP measurements obtained were compared between tonometers at each of the altitudes, and also for repeatability of each tonometer at different altitudes.
The mean age of patients enrolled in the study was 41.7 (range 28-66 years) with 22 females. Mean IOP as measured by Pascal tonometer was 16.1±2.2 mmHg at sea level and 15.9±2.1 mmHg at 2234m above sea level. Mean IOP as measured by the Perkins tonometer at sea level and 2234m above was 13.1±1.8 and 11.5±1.7mm of Hg, respectively. Pearson’s correlation coefficient, r, indicated a highly statistically significant (p<0.005) moderate positive linear relationship between tonometers. (Table 1) The limits of agreement (LoA) for the Perkins Tonometer IOP measurements at and 2234 metres above sea level was +1.08 to +2.09 mmHg and that for the Pascal Tonometer was -0.44 to +0.83 mmHg. This signifies that the tonometer measurements at each of the altitudes only remain clinically repeatable for the Pascal tonometer, within the acceptable test-retest variability of the two devices. Assessment of the inter-changeability of the two tonometers at sea level and 2234 meters above it revealed similar results regarding a larger difference at the higher altitude. The LoA for the two tonometers at sea level was between +2.31 and +3.55 mm of Hg; and at 2234 m above sea level it was between +3.88 and +4.75 mmHg. (Table 2) Given that the difference is clinically significant, it implies that the Perkins tonometer may not be used interchangeably during routine follow up of patients at both of the altitudes, but that Pascal might.
The repeatability of single-observer measurements with Perkins Tonometer remains clinically acceptable but not at different altitudes. The Pascal tonometer seems to more consistently measure a similar IOP at different altitudes in the same subjects. However, the two tonometers may not be used interchangeably in the serial follow up of patients at any of the altitudes
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