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Key Factors in the Subjective and Objective Assessment of Conjunctival Erythema

¹ From the Co-operative Research Centre for Eye Research and Technology, University of New South Wales, Sydney, Australia.

PURPOSE. To establish objectively measurable characteristics of the conjunctival vasculature that correspond with the judgment of erythema by human observers.

METHODS. Color images of bulbar conjunctiva from 21 subjects were digitally analyzed to extract the following variables characteristic of the scene: vessel width (W), number of vessels (V), proportion of area occupied by vessels (PA), relative redness both in vessels (RRV) and in the whole image (RRI), red-green difference both in vessels (RGV) and in the whole image (RGI), red-blue difference both in vessels (RBV) and in the whole image (RBI), and red hue value (RHV). These data were compared with subjective judgments by a panel of seven trained observers who independently rated erythema in the same images, using a 0 to 4 scale with decimal interpolation between grades.

RESULTS. Correlation analysis indicated significant associations (P < 0.05) between the mean response of the human observers and all the objective variables except RHV. Associations with the morphometric variables PA (R² = 0.93) and V (R² = 0.90) were markedly stronger than for the best colorimetric variable RBV (R² = 0.62).

CONCLUSIONS. Judgments of erythema made by human observers do not rely primarily on color but can be closely approximated by a univariate, linear model involving only the proportion of the scene occupied by vessels. Under the conditions of this study, grading of erythema by trained observers can be considered to constitute measurement to at least an interval level.

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