Detection of retinal lesions in diabetic retinopathy: comparative evaluation of 7-field digital color photography versus red-free photography

Red-free light allows better detection of vascular lesions as this wavelength is absorbed by hemoglobin; however, the current gold standard for the detection and grading of diabetic retinopathy remains 7-field color fundus photography. The goal of this study was to compare the ability of 7-field fundus photography using red-free light to detect retinopathy lesions with corresponding images captured using standard 7-field color photography. Non-stereoscopic standard 7-field 30° digital color fundus photography and 7-field 30° digital red-free fundus photography were performed in 200 eyes of 103 patients with various grades of diabetic retinopathy ranging from mild to moderate non-proliferative diabetic retinopathy to proliferative diabetic retinopathy. The color images (n = 1,400) were studied with corresponding red-free images (n = 1,400) by one retina consultant (PV) and two senior residents training in retina. The various retinal lesions [microaneurysms, hemorrhages, hard exudates, soft exudates, intra-retinal microvascular anomalies (IRMA), neovascularization of the retina elsewhere (NVE), and neovascularization of the disc (NVD)] detected by all three observers in each of the photographs were noted followed by determination of agreement scores using κ values (range 0−1). Kappa coefficient was categorized as poor (≤0), slight (0.01–0.20), fair (0.2 –0.40), moderate (0.41–0.60), substantial (0.61–0.80), and almost perfect (0.81–1). The number of lesions detected by red-free images alone was higher for all observers and all abnormalities except hard exudates. Detection of IRMA was especially higher for all observers with red-free images. Between image pairs, there was substantial agreement for detection of hard exudates (average κ = 0.62, range 0.60−0.65) and moderate agreement for detection of hemorrhages (average κ = 0.52, range 0.45−0.58), soft exudates (average κ = 0.51, range 0.42−0.61), NVE (average κ = 0.47, range 0.39−0.53), and NVD (average κ = 0.51, range 0.45−0.54). Fair agreement was noted for detection of microaneurysms (average κ = 0.29, range 0.20−0.39) and IRMA (average κ = 0.23, range 0.23−0.24). Inter-observer agreement with color images was substantial for hemorrhages (average κ = 0.72), soft exudates (average κ = 0.65), and NVD (average κ = 0.65); moderate for microaneurysms (average κ = 0.42), NVE (average κ = 0.44), and hard exudates (average κ = 0.59) and fair for IRMA (average κ = 0.21). Inter-observer agreement with red-free images was substantial for hard exudates (average κ = 0.63) and moderate for detection of hemorrhages (average κ = 0.56), SE (average κ = 0.60), IRMA (average κ = 0.50), NVE (average κ = 0.44), and NVD (average κ = 0.45). Digital red-free photography has a higher level of detection ability for all retinal lesions of diabetic retinopathy. More advanced grades of retinopathy are likely to be detected earlier with red-free imaging because of its better ability to detect IRMA, NVE, and NVD. Red-free monochromatic imaging of the retina is a more effective and less costly alternative for detection of vision-threatening diabetic retinopathy.