Quality assessment of a screening algorithm for diabetic retinopathy: prolongation of screening intervals with minimal effect of HbA_1c and arterial hypertension on the length of intervals

Purpose: At Steno Diabetes Center Copenhagen (SDCC), diabetic retinopathy (DR) screening intervals are based on quantification of retinal lesions. Screening intervals are, for the milder forms of DR, prolonged to 2–3 years. The purpose of the present study was to evaluate the effect of the prolongation on developing unexpected events and to evaluate the effect of HbA_1c and arterial hypertension. Methods: We assessed 18 972 screening intervals from 6000 patients from 1/1-2003 to 1/5-2017 for occurrence of unexpected events, defined as: (1) DR progression requiring treatment, at the following screening date, and (2) DR-related hospital contact within the planned interval. We modelled the effect of several risk factors for developing unexpected events in a Cox regression. Furthermore, we assessed the risk of unexpected events in a logistic regression analysis using cubic splines to model the effect of HbA_1c, stratified by arterial hypertension status. Results: 16 283 (86%) intervals followed the planned interval and among those, only 86 (0.5%) experienced unexpected events. Intervals of dysregulated patients (86% of all intervals) did not experience more unexpected events, compared with well-regulated patient intervals (Hazard Ratio: 1.12, 95% CI: 0.55–2.27). We found a nonlinear effect of HbA_1c on the risk of unexpected events which peaked around HbA_1c levels of 80 mmol/mol. Having arterial hypertension slightly increased the risk of unexpected events. Conclusions: The present study supports the validity of the current algorithm. We found no increased risk of unexpected events among dysregulated intervals but a nonlinear effect of HbA_1c. Age, diabetes duration and diabetes type were significantly associated with unexpected events.