Effect of digital image compression on screening for diabetic retinopathy

Background/aims-Digital imaging is widely used for diabetic retinopathy scr eening. The storage and transmission of digital images can be facilitated b y image compression. The authors aimed to assess the effect of image compre ssion on the accuracy of grading diabetic retinopathy. Methods-Forty nine 35 mm transparencies (17 with no retinopathy, eight with background, five with preproliferative, and 19 with proliferative retinopa thy) were digitised and subjected to JPEG compression by 90%, 80%, 70%, and 0%. The 196 images were randomised and graded on a portable computer. Two masked graders assessed the images for grade of retinopathy and image quali ty (0-10). The sensitivity and specificity of retinopathy grading were calc ulated with a weighted kappa for grading agreement between levels of compre ssion. Results-The sensitivity of retinopathy grading was reduced by JPEG compress ion. At 90%, 80%, 70%, and 0% compression the sensitivities were 0.38, 0.50 , 0.65, and 0.72, respectively; the specificity results were 1.00, 1.00, 0. 83, and 0.84, respectively; and the weighted kappa scores were 0.60, 0.75, 0.77, and 0.84, respectively. The quality scores for 90%, 80%, 70%, 0% comp ression were 2.9 (SD 1.1, 95% CI; 2.7-3.2), 4.6 (SD 1.1, 95% CI; 3.0-5.6), 5.8 (SD1.5, 95% CI 5.0-6.6), 6.3 (SD1.4, 95% CI; 5.4-7.2) (p <0.01 for each intergroup comparison). Conclusion-The results demonstrate significant loss of sensitivity to the f eatures of diabetic retinopathy with JPEG compression; this was compounded by the thin film transistor (TFT) screen. The authors found the quality of uncompressed images on TFT screens too poor to give grading sensitivities w hich reach current guidelines for diabetic retinopathy screening.