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The Diabetic Retinopathy Study (DRS) Research Group has so far identified four retinopathy factors that increase the two-year risk of developing severe visual loss. The risk grows as the number of risk factors increases. Eyes with three or more risk factors (eyes with "high-risk characteristics") are at a much higher risk than eyes with two or fewer factors. The DRS protocol was changed in 1976 to require consideration of treatment for these "high-risk" eyes.
A 1979 study studied this question.