Effectiveness and safety of screening for diabetic retinopathy with two nonmydriatic digital images compared with the seven standard stereoscopic photographic fields.

Background: The use of nonmydriatic cameras, which offer ease of screening and 45 degrees immediate imaging of the fundus, is gaining increasing acceptance for screening programs tailored to diverse conditions. We performed a study to evaluate the effectiveness and safety of screening for diabetic retinopathy with two nonmydriatic camera images compared with the seven standard stereoscopic 30 degrees fields (7SF). We also wished to determine whether safe screening guidelines could be established to identify patients needing referral to an ophthalmologist.

Methods: In this prospective masked cross-sectional study, we evaluated agreement in the assessment of the severity of diabetic retinopathy by means of two 45 degrees images centred on the optic disc and on the macula obtained with the Topcon CRW6 nonmydriatic camera and by means of 7SF photography and ophthalmologic slit-lamp biomicroscopy, both performed with pupil dilation. Between November 2000 and June 2001, 98 adult patients known to have type 1 or 2 diabetes mellitus who presented for the first time to the diabetic retinopathy clinic of a tertiary care centre in Montreal were enrolled consecutively. Thus, patient recruitment was weighted toward more severe retinopathy to ensure sufficient representation of less frequent but more severe levels. Each patient underwent nonmydriatic fundus photography of both eyes, followed by a complete ophthalmologic examination with pupil dilation by a single retina specialist and 7SF photography of both eyes with pupil dilation. The level of retinopathy was graded independently in each eye from the 7SF photographs according to the Early Treatment Diabetic Retinopathy Study (ETDRS) scale by two graders; an independent retina specialist adjudicated the rare instances of interreader disagreement in a masked fashion. Two months later, two graders independently graded the nonmydriatic images in a blinded fashion according to the ETDRS scale; a third observer adjudicated the rare instances of interreader disagreement. We measured concordance between grading results with the various screening techniques using the weighted and unweighted kappa statistic. We used sensitivity and specificity indices to determine safe screening guidelines to identify patients needing referral to an ophthalmologist.

Results: There was substantial agreement in the grading of retinopathy with nonmydriatic camera imaging and with 7SF photography, both for all eyes (kappa = 0.626 [standard deviation (SD) 0.045]) and for the eye with more severe disease (kappa = 0.654 [SD 0.063]). With nonmydriatic camera imaging, screening thresholds for patient referral to an ophthalmologist of very mild retinopathy (ETDRS grade 20), mild retinopathy (ETDRS grade 35) and moderate retinopathy (EDTRS grade 43) had sensitivity values of 97.9%, 97.1% and 53.3% respectively and specificity values of 81.3%, 95.5% and 96.9% respectively. Screening thresholds of very mild or mild retinopathy both correctly identified 100% of eyes with severe nonproliferative or proliferative retinopathy. With a screening threshold of mild retinopathy, screening with the nonmydriatic camera would lead to referral to an ophthalmologist of 37.8% of patients because of detected disease and of an additional 17.3% because of insufficient image quality in at least one eye, for a total of 55.1%. The overall sensitivity and specificity of a two-field nonmydriatic screening strategy with a threshold of mild retinopathy for referral of patients with insufficient image quality in at least one eye are 97.7% and 84.0% respectively.

Interpretation: Our results suggest that two-field nonmydriatic camera imaging is a safe screening strategy that may identify the patients with diabetes most in need of ophthalmologic care.