Tonometer validation and intraocular pressure reference values in the normal chinchilla (Chinchilla lanigera)

Objectives To determine accuracy and precision of three commonly used tonometers (TonoVet® and TonoLab® (ICare Oy, Finland)—rebound tonometers, and Tono‐Pen VET™ (Reichert, NY)—applanation tonometer) in normal chinchillas, and to establish a normal intraocular pressure (IOP) reference range in this species. Methods The anterior chambers of three chinchilla eyes were cannulated ex vivo and readings obtained at manometric IOPs from 5 to 80 mmHg, using each of the three tonometers in random order. Data were analyzed by linear regression, ANOVA, and Bland–Altman plots. Tonometry was performed in both eyes of 60 chinchillas (age 8 weeks–16.2 years) using the TonoVet® and relationship between age and IOP analyzed using linear regression. For all statistical tests, P < 0.05 was significant. Results Intraocular pressure values obtained using the Tono‐Pen VET™ and TonoVet® (in dog calibration mode;‘d’) showed strong linear correlation with manometry within the physiologic and clinically relevant range of IOP (0–50 mmHg). The TonoVet® ‘d’ setting displayed significantly greater precision over the full range of IOP evaluated than the Tono‐Pen VET™, and both TonoVet and Tono‐Pen VET™ were significantly more accurate than the TonoLab® tonometer. Mean ± SD IOP (TonoVet® ‘d’) in chinchillas was 9.7 ± 2.5 mmHg, and the 95% reference interval was 4.7–14.7 mmHg. Conclusions Both the Tono‐Pen VET™ and TonoVet® provided clinically acceptable estimates of IOP in chinchillas. The TonoVet® provides accurate and precise IOP values, while Tono‐Pen VET™ derived measurements showed greater variability. Values obtained either with the TonoLab® or TonoVet® used in the ‘unspecified’ calibration setting were inaccurate in this species.