ROPtool can now generate clinically meaningful measure of plus disease

The authors conducted this study to determine the best method for combining the tortuosity and width values of the computer program ROPtool to generate a clinically meaningful overall measure of plus disease. ROPtool traces retinal blood vessels and calculates width, expressed as dilation index, and tortuosity, expressed as tortuosity index. The authors tested five methods for combining ROPtool measurements of RetCam images. They found that three of the methods had high overall accuracy: sum of adjusted indices, tortuosity-weighted plus and regression analysis.

The authors used ROPtool to measure vessel tortuosity and width in 184 quadrants of 46 RetCam images of premature infants. The plus index and sum of adjusted indices provide tortuosity and width values using different scales. Tortuosity-weighted plus increases the contribution of width as tortuosity increases. The vessel area index is used to average vessel area per unit length. Regression analysis establishes a mathematical equation to fit expert diagnosis. For each method, receiver operating characteristic curves were generated with a three-expert consensus as the reference standard. Optimal cut points for plus disease were selected and then tested on a second set of 184 quadrants of 46 different images.

When applied to the second data set, areas under the receiver operating characteristic curves were 0.857 for plus index, 0.918 for sum of adjusted indices, 0.941 for tortuosity-weighted plus, 0.868 for vessel area index and 0.934 for regression analysis methods. The highest sensitivities and specificities were for tortuosity-weighted plus (85 percent for both) and regression analysis methods (85 percent and 86 percent, respectively).

The authors are less inclined to include regression analysis in future ROPtool calculations because it is based solely on a statistical "best fit." However, they say the sum of adjusted indices and tortuosity-weighted plus methods have a clinical basis and can now be included in ROPtool results.

They note that by providing more nuanced data than categorical definitions of tortuosity and dilation, ROPtool can describe vascular abnormalities in more detail than a designation of plus versus pre-plus versus neither. This could lead to a better understanding of the evolution of plus disease. These designations could be replaced by a seamless, continuous measure of retinal blood vessel "plusness." Such a measure could be particularly useful as an adjunct to monitor disease progression and response to therapy.