Breed-Specific Relationship Between Rebound Tonometers (TonoVet and TonoVet Plus) in Labrador Retrievers

This study aimed to characterize the relationship between intraocular pressure (IOP) measurements obtained using two rebound tonometers (TonoVet and TonoVet Plus) in Labrador retrievers and to evaluate a regression-based approximation for supporting longitudinal interpretation following device replacement.

Fifty-three clinically normal Labrador retrievers (106 eyes), including guide dog candidates and breeding dogs undergoing routine health screening in Japan, were included.

IOP was measured sequentially using the TonoVet and TonoVet Plus, both set to dog mode. The first of three readings per eye per device was used for analysis. Measurement order effects were assessed in a separate cohort. Statistical analyses included paired t-tests, Pearson's correlation coefficient, and Bland-Altman analysis. Deming regression was used to derive a regression-based approximation accounting for measurement error in both devices.

Measurement order did not significantly influence IOP values. The TonoVet Plus yielded significantly higher IOP values than the TonoVet (p < 0.0001), and a moderate positive correlation was observed between the two devices (r = 0.69). Bland-Altman analysis demonstrated a systematic bias (mean difference + 4.5 mmHg; 95% limits of agreement, 1.3 to 7.7 mmHg). Deming regression yielded a slope of 1.05 and an intercept of 3.52. Age-related effects on IOP were small and not statistically significant.

The TonoVet Plus systematically measures higher IOP values than the TonoVet in Labrador retrievers. The derived breed-specific approximation may support longitudinal interpretation rather than precise individual-level prediction in comparable clinical settings; however, external validation in independent Labrador retriever populations is warranted.