Objective Grading of Tonography Tracings for the Measurement of Outflow Facility

Tonography is the standard method for non-invasive measurement of aqueous humor outflow facility. However, assessment of tonography tracing quality is currently subjective, and acceptance of poor-quality data or inappropriately discarding valid data can bias results. The purpose of this study was to develop an objective method for assessing the quality of tonography tracings.

Pneumatonography tracings were obtained from glaucoma and ocular hypertension patients as part of an ongoing multicenter study of aqueous humor dynamics. Intraocular pressure (IOP) was captured digitally at 40 Hz over 2 minutes. Root mean square error (RMSE) of a linear best-fit line was obtained for each tracing. Each tracing was also graded by seven experienced tonographers using a scale of 1 (worst) to 10 (best) for quality (Expert score). A Reference set of 35 tracings was used to determine the relationship between RMSE values and Expert scores using a logarithmic curve. This relationship was used to calculate a predicted score in a second Test set of 20 tracings. A logarithmic curve was used to account for the fixed range of Expert scores and unbounded upper range for RMSE values. The differences between the predicted scores and the Expert scores were evaluated using Bland-Altman analysis.

There was a very strong correlation between predicted and Expert scores (R = 0.94). The mean difference between Expert and predicted scores was -1.01 ± 0.84, and the limits of agreement were between -2.65 and +0.63.

Objective assessment of pneumatonography tracings can be performed using RMSE of a fitted line and calculation of a predicted quality score that closely matches scores given by expert graders.

Tonography tracing quality can now be objectively assessed.