© 2003 Diabetes UK. Diabetic Medicine, 20, 727– 733 727 Introduction The efficacy of screening for diabetic retinopathy (DR) is determined by the disease natural history of DR and the effi- cacy of the medical regime, including metabolic control and laser photocoagulation. Earlier studies have shown that laser photocoagulation at the proliferative diabetic retinopathy (PDR) stage could reduce the risk of visual impairment and blindness significantly [1–5]. However, the efficacy of laser photocoagulation is also highly dependent on how early DR is detected. The natural history of DR affects the choice of an interscreening interval for the surveillance of patients because the shorter the duration of DR before visual impairment, the shorter the desirable interscreening interval. Although annual Correspondence to: Professor Tony Hsiu-Hsi Chen, Institute of Preventive Medicine, College of Public Health, National Taiwan University, Taipei, Taiwan. E-mail: stony@episerv.cph.ntu.edu.tw Abstract Aims The natural history and treatment efficacy of diabetic retinopathy (DR) play important roles in the evaluation of screening. Therefore, the natural his- tory of DR and rates of transition after treatment (including metabolic control and laser photocoagulation) from no diabetic retinopathy (NDR) to blindness were quantified. Methods We studied a cohort of 795 patients with diabetes mellitus (DM) receiving fundus examination in the ophthalmology out-patient department of one medical centre between 1 January 1990 and 31 December 1992 in Taiwan. Follow-up data until 31 December 1998 were collected by chart review. Two multistate Markov models were proposed to assess the efficacy of the treatment regime in reducing progression to blindness. Results The average times spent in states (i) no diabetic retinopathy (NDR), (ii) background diabetic retinopathy (BDR), (iii) preproliferative diabetic retinopathy (PPDR), and (iv) proliferative retinopathy (PDR) were 10.86 years, 8.33 years, 1.67 years, and 2.17 years, respectively. Early detection of PPDR may lead to a 60% reduction in PDR and an 83% reduction in blindness. Simulated results based on these parameters show that an annual screening programme, a biennial screening regime and a 4-yearly screening regime can lead to 54% (95% confidence interval (CI): 44 – 62%), 51% (95% CI: 41–59%), and 46% (95% CI: 36 –54%) reductions in blindness, respectively. Conclusions Assessing the progression of DR following the proliferative path- way in this study suggests that screening for DR is worthwhile and that a 4-year interscreening interval for patients as yet without DR may be justified. Diabet. Med. 20, 727– 733 (2003) Keywords diabetic retinopathy, screening, Markov model Blackwell Publishing Ltd. Oxford, UK DME Diabetic Medicine 1464-5491 Blackwell Science Ltd, 2003 20 Original Article Original article Progression of diabetic retinopathy W-J. Liu et al. Assessing progression and efficacy of treatment for diabetic retinopathy following the proliferative pathway to blindness: implication for diabetic retinopathy screening in Taiwan W-J. Liu, L-T. Lee*, M-F. Yen†, T-H. Tung†, R. Williams‡, S. W. Duffy§ and T. H-H. Chen† Department of Family Medicine, National Taiwan University Hospital, *Department of Family Medicine, College of Medicine and †Institute of Preventive Medicine, College of Public Health, National Taiwan University, Taipei, Taiwan, ‡Nuffield Institute for Health, University of Leeds, Leeds and §Cancer Research UK, Department of Epidemiology, Mathematics and Statistics, Wolfson Institute of Preventive Medicine, London, UK Accepted 6 March 2003